diff --git a/jonesforth.S b/jonesforth.S index 8d23e39..e3b89a7 100644 --- a/jonesforth.S +++ b/jonesforth.S @@ -1,2313 +1,2306 @@ -/* A sometimes minimal FORTH compiler and tutorial for Linux / i386 systems. -*- asm -*- - By Richard W.M. Jones http://annexia.org/forth - This is PUBLIC DOMAIN (see public domain release statement below). - $Id: jonesforth.S,v 1.47 2009-09-11 08:33:13 rich Exp $ +/* A sometimes minimal FORTH compiler and tutorial for Linux / i386 systems. -*- asm -*- + By Richard W.M. Jones http://annexia.org/forth + This is PUBLIC DOMAIN (see public domain release statement below). + $Id: jonesforth.S,v 1.47 2009-09-11 08:33:13 rich Exp $ - gcc -m32 -nostdlib -static -Wl,-Ttext,0 -Wl,--build-id=none -o jonesforth jonesforth.S + gcc -m32 -nostdlib -static -Wl,-Ttext,0 -Wl,--build-id=none -o jonesforth jonesforth.S */ - .set JONES_VERSION,47 + .set JONES_VERSION,47 /* - INTRODUCTION ---------------------------------------------------------------------- + INTRODUCTION ---------------------------------------------------------------------- - FORTH is one of those alien languages which most working programmers regard in the same - way as Haskell, LISP, and so on. Something so strange that they'd rather any thoughts - of it just go away so they can get on with writing this paying code. But that's wrong - and if you care at all about programming then you should at least understand all these - languages, even if you will never use them. + FORTH is one of those alien languages which most working programmers regard in the same + way as Haskell, LISP, and so on. Something so strange that they'd rather any thoughts + of it just go away so they can get on with writing this paying code. But that's wrong + and if you care at all about programming then you should at least understand all these + languages, even if you will never use them. - LISP is the ultimate high-level language, and features from LISP are being added every - decade to the more common languages. But FORTH is in some ways the ultimate in low level - programming. Out of the box it lacks features like dynamic memory management and even - strings. In fact, at its primitive level it lacks even basic concepts like IF-statements - and loops. + LISP is the ultimate high-level language, and features from LISP are being added every + decade to the more common languages. But FORTH is in some ways the ultimate in low level + programming. Out of the box it lacks features like dynamic memory management and even + strings. In fact, at its primitive level it lacks even basic concepts like IF-statements + and loops. - Why then would you want to learn FORTH? There are several very good reasons. First - and foremost, FORTH is minimal. You really can write a complete FORTH in, say, 2000 - lines of code. I don't just mean a FORTH program, I mean a complete FORTH operating - system, environment and language. You could boot such a FORTH on a bare PC and it would - come up with a prompt where you could start doing useful work. The FORTH you have here - isn't minimal and uses a Linux process as its 'base PC' (both for the purposes of making - it a good tutorial). It's possible to completely understand the system. Who can say they - completely understand how Linux works, or gcc? + Why then would you want to learn FORTH? There are several very good reasons. First + and foremost, FORTH is minimal. You really can write a complete FORTH in, say, 2000 + lines of code. I don't just mean a FORTH program, I mean a complete FORTH operating + system, environment and language. You could boot such a FORTH on a bare PC and it would + come up with a prompt where you could start doing useful work. The FORTH you have here + isn't minimal and uses a Linux process as its 'base PC' (both for the purposes of making + it a good tutorial). It's possible to completely understand the system. Who can say they + completely understand how Linux works, or gcc? - Secondly FORTH has a peculiar bootstrapping property. By that I mean that after writing - a little bit of assembly to talk to the hardware and implement a few primitives, all the - rest of the language and compiler is written in FORTH itself. Remember I said before - that FORTH lacked IF-statements and loops? Well of course it doesn't really because - such a lanuage would be useless, but my point was rather that IF-statements and loops are - written in FORTH itself. + Secondly FORTH has a peculiar bootstrapping property. By that I mean that after writing + a little bit of assembly to talk to the hardware and implement a few primitives, all the + rest of the language and compiler is written in FORTH itself. Remember I said before + that FORTH lacked IF-statements and loops? Well of course it doesn't really because + such a lanuage would be useless, but my point was rather that IF-statements and loops are + written in FORTH itself. - Now of course this is common in other languages as well, and in those languages we call - them 'libraries'. For example in C, 'printf' is a library function written in C. But - in FORTH this goes way beyond mere libraries. Can you imagine writing C's 'if' in C? - And that brings me to my third reason: If you can write 'if' in FORTH, then why restrict - yourself to the usual if/while/for/switch constructs? You want a construct that iterates - over every other element in a list of numbers? You can add it to the language. What - about an operator which pulls in variables directly from a configuration file and makes - them available as FORTH variables? Or how about adding Makefile-like dependencies to - the language? No problem in FORTH. How about modifying the FORTH compiler to allow - complex inlining strategies -- simple. This concept isn't common in programming languages, - but it has a name (in fact two names): "macros" (by which I mean LISP-style macros, not - the lame C preprocessor) and "domain specific languages" (DSLs). + Now of course this is common in other languages as well, and in those languages we call + them 'libraries'. For example in C, 'printf' is a library function written in C. But + in FORTH this goes way beyond mere libraries. Can you imagine writing C's 'if' in C? + And that brings me to my third reason: If you can write 'if' in FORTH, then why restrict + yourself to the usual if/while/for/switch constructs? You want a construct that iterates + over every other element in a list of numbers? You can add it to the language. What + about an operator which pulls in variables directly from a configuration file and makes + them available as FORTH variables? Or how about adding Makefile-like dependencies to + the language? No problem in FORTH. How about modifying the FORTH compiler to allow + complex inlining strategies -- simple. This concept isn't common in programming languages, + but it has a name (in fact two names): "macros" (by which I mean LISP-style macros, not + the lame C preprocessor) and "domain specific languages" (DSLs). - This tutorial isn't about learning FORTH as the language. I'll point you to some references - you should read if you're not familiar with using FORTH. This tutorial is about how to - write FORTH. In fact, until you understand how FORTH is written, you'll have only a very - superficial understanding of how to use it. + This tutorial isn't about learning FORTH as the language. I'll point you to some references + you should read if you're not familiar with using FORTH. This tutorial is about how to + write FORTH. In fact, until you understand how FORTH is written, you'll have only a very + superficial understanding of how to use it. - So if you're not familiar with FORTH or want to refresh your memory here are some online - references to read: + So if you're not familiar with FORTH or want to refresh your memory here are some online + references to read: - http://en.wikipedia.org/wiki/Forth_%28programming_language%29 + http://en.wikipedia.org/wiki/Forth_%28programming_language%29 - http://galileo.phys.virginia.edu/classes/551.jvn.fall01/primer.htm + http://galileo.phys.virginia.edu/classes/551.jvn.fall01/primer.htm - http://wiki.laptop.org/go/Forth_Lessons + http://wiki.laptop.org/go/Forth_Lessons - http://www.albany.net/~hello/simple.htm + http://www.albany.net/~hello/simple.htm - Here is another "Why FORTH?" essay: http://www.jwdt.com/~paysan/why-forth.html + Here is another "Why FORTH?" essay: http://www.jwdt.com/~paysan/why-forth.html - Discussion and criticism of this FORTH here: http://lambda-the-ultimate.org/node/2452 + Discussion and criticism of this FORTH here: http://lambda-the-ultimate.org/node/2452 - ACKNOWLEDGEMENTS ---------------------------------------------------------------------- + ACKNOWLEDGEMENTS ---------------------------------------------------------------------- - This code draws heavily on the design of LINA FORTH (http://home.hccnet.nl/a.w.m.van.der.horst/lina.html) - by Albert van der Horst. Any similarities in the code are probably not accidental. + This code draws heavily on the design of LINA FORTH (http://home.hccnet.nl/a.w.m.van.der.horst/lina.html) + by Albert van der Horst. Any similarities in the code are probably not accidental. - Some parts of this FORTH are also based on this IOCCC entry from 1992: - http://ftp.funet.fi/pub/doc/IOCCC/1992/buzzard.2.design. - I was very proud when Sean Barrett, the original author of the IOCCC entry, commented in the LtU thread - http://lambda-the-ultimate.org/node/2452#comment-36818 about this FORTH. + Some parts of this FORTH are also based on this IOCCC entry from 1992: + http://ftp.funet.fi/pub/doc/IOCCC/1992/buzzard.2.design. + I was very proud when Sean Barrett, the original author of the IOCCC entry, commented in the LtU thread + http://lambda-the-ultimate.org/node/2452#comment-36818 about this FORTH. - And finally I'd like to acknowledge the (possibly forgotten?) authors of ARTIC FORTH because their - original program which I still have on original cassette tape kept nagging away at me all these years. - http://en.wikipedia.org/wiki/Artic_Software + And finally I'd like to acknowledge the (possibly forgotten?) authors of ARTIC FORTH because their + original program which I still have on original cassette tape kept nagging away at me all these years. + http://en.wikipedia.org/wiki/Artic_Software - PUBLIC DOMAIN ---------------------------------------------------------------------- + PUBLIC DOMAIN ---------------------------------------------------------------------- - I, the copyright holder of this work, hereby release it into the public domain. This applies worldwide. + I, the copyright holder of this work, hereby release it into the public domain. This applies worldwide. - In case this is not legally possible, I grant any entity the right to use this work for any purpose, - without any conditions, unless such conditions are required by law. + In case this is not legally possible, I grant any entity the right to use this work for any purpose, + without any conditions, unless such conditions are required by law. - SETTING UP ---------------------------------------------------------------------- + SETTING UP ---------------------------------------------------------------------- - Let's get a few housekeeping things out of the way. Firstly because I need to draw lots of - ASCII-art diagrams to explain concepts, the best way to look at this is using a window which - uses a fixed width font and is at least this wide: + Let's get a few housekeeping things out of the way. Firstly because I need to draw lots of + ASCII-art diagrams to explain concepts, the best way to look at this is using a window which + uses a fixed width font and is at least this wide: <------------------------------------------------------------------------------------------------------------------------> - Secondly make sure TABS are set to 8 characters. The following should be a vertical - line. If not, sort out your tabs. + Secondly I assume that your screen is at least 50 characters high. - | - | - | + ASSEMBLING ---------------------------------------------------------------------- - Thirdly I assume that your screen is at least 50 characters high. + If you want to actually run this FORTH, rather than just read it, you will need Linux on an + i386. Linux because instead of programming directly to the hardware on a bare PC which I + could have done, I went for a simpler tutorial by assuming that the 'hardware' is a Linux + process with a few basic system calls (read, write and exit and that's about all). i386 + is needed because I had to write the assembly for a processor, and i386 is by far the most + common. (Of course when I say 'i386', any 32- or 64-bit x86 processor will do. I'm compiling + this on a 64 bit AMD Opteron). - ASSEMBLING ---------------------------------------------------------------------- + Again, to assemble this you will need gcc and gas (the GNU assembler). The commands to + assemble and run the code (save this file as 'jonesforth.S') are: - If you want to actually run this FORTH, rather than just read it, you will need Linux on an - i386. Linux because instead of programming directly to the hardware on a bare PC which I - could have done, I went for a simpler tutorial by assuming that the 'hardware' is a Linux - process with a few basic system calls (read, write and exit and that's about all). i386 - is needed because I had to write the assembly for a processor, and i386 is by far the most - common. (Of course when I say 'i386', any 32- or 64-bit x86 processor will do. I'm compiling - this on a 64 bit AMD Opteron). + gcc -m32 -nostdlib -static -Wl,-Ttext,0 -Wl,--build-id=none -o jonesforth jonesforth.S + cat jonesforth.f - | ./jonesforth - Again, to assemble this you will need gcc and gas (the GNU assembler). The commands to - assemble and run the code (save this file as 'jonesforth.S') are: + If you want to run your own FORTH programs you can do: - gcc -m32 -nostdlib -static -Wl,-Ttext,0 -Wl,--build-id=none -o jonesforth jonesforth.S - cat jonesforth.f - | ./jonesforth + cat jonesforth.f myprog.f | ./jonesforth - If you want to run your own FORTH programs you can do: + If you want to load your own FORTH code and then continue reading user commands, you can do: - cat jonesforth.f myprog.f | ./jonesforth + cat jonesforth.f myfunctions.f - | ./jonesforth - If you want to load your own FORTH code and then continue reading user commands, you can do: + ASSEMBLER ---------------------------------------------------------------------- - cat jonesforth.f myfunctions.f - | ./jonesforth + (You can just skip to the next section -- you don't need to be able to read assembler to + follow this tutorial). - ASSEMBLER ---------------------------------------------------------------------- + However if you do want to read the assembly code here are a few notes about gas (the GNU assembler): - (You can just skip to the next section -- you don't need to be able to read assembler to - follow this tutorial). + (1) Register names are prefixed with '%', so %eax is the 32 bit i386 accumulator. The registers + available on i386 are: %eax, %ebx, %ecx, %edx, %esi, %edi, %ebp and %esp, and most of them + have special purposes. - However if you do want to read the assembly code here are a few notes about gas (the GNU assembler): + (2) Add, mov, etc. take arguments in the form SRC,DEST. So mov %eax,%ecx moves %eax -> %ecx - (1) Register names are prefixed with '%', so %eax is the 32 bit i386 accumulator. The registers - available on i386 are: %eax, %ebx, %ecx, %edx, %esi, %edi, %ebp and %esp, and most of them - have special purposes. + (3) Constants are prefixed with '$', and you mustn't forget it! If you forget it then it + causes a read from memory instead, so: + mov $2,%eax moves number 2 into %eax + mov 2,%eax reads the 32 bit word from address 2 into %eax (ie. most likely a mistake) - (2) Add, mov, etc. take arguments in the form SRC,DEST. So mov %eax,%ecx moves %eax -> %ecx + (4) gas has a funky syntax for local labels, where '1f' (etc.) means label '1:' "forwards" + and '1b' (etc.) means label '1:' "backwards". Notice that these labels might be mistaken + for hex numbers (eg. you might confuse 1b with $0x1b). - (3) Constants are prefixed with '$', and you mustn't forget it! If you forget it then it - causes a read from memory instead, so: - mov $2,%eax moves number 2 into %eax - mov 2,%eax reads the 32 bit word from address 2 into %eax (ie. most likely a mistake) + (5) 'ja' is "jump if above", 'jb' for "jump if below", 'je' "jump if equal" etc. - (4) gas has a funky syntax for local labels, where '1f' (etc.) means label '1:' "forwards" - and '1b' (etc.) means label '1:' "backwards". Notice that these labels might be mistaken - for hex numbers (eg. you might confuse 1b with $0x1b). + (6) gas has a reasonably nice .macro syntax, and I use them a lot to make the code shorter and + less repetitive. - (5) 'ja' is "jump if above", 'jb' for "jump if below", 'je' "jump if equal" etc. + For more help reading the assembler, do "info gas" at the Linux prompt. - (6) gas has a reasonably nice .macro syntax, and I use them a lot to make the code shorter and - less repetitive. + Now the tutorial starts in earnest. - For more help reading the assembler, do "info gas" at the Linux prompt. + THE DICTIONARY ---------------------------------------------------------------------- - Now the tutorial starts in earnest. + In FORTH as you will know, functions are called "words", and just as in other languages they + have a name and a definition. Here are two FORTH words: - THE DICTIONARY ---------------------------------------------------------------------- + : DOUBLE DUP + ; \ name is "DOUBLE", definition is "DUP +" + : QUADRUPLE DOUBLE DOUBLE ; \ name is "QUADRUPLE", definition is "DOUBLE DOUBLE" - In FORTH as you will know, functions are called "words", and just as in other languages they - have a name and a definition. Here are two FORTH words: + Words, both built-in ones and ones which the programmer defines later, are stored in a dictionary + which is just a linked list of dictionary entries. - : DOUBLE DUP + ; \ name is "DOUBLE", definition is "DUP +" - : QUADRUPLE DOUBLE DOUBLE ; \ name is "QUADRUPLE", definition is "DOUBLE DOUBLE" + <--- DICTIONARY ENTRY (HEADER) -----------------------> + +------------------------+--------+---------- - - - - +----------- - - - - + | LINK POINTER | LENGTH/| NAME | DEFINITION + | | FLAGS | | + +--- (4 bytes) ----------+- byte -+- n bytes - - - - +----------- - - - - - Words, both built-in ones and ones which the programmer defines later, are stored in a dictionary - which is just a linked list of dictionary entries. + I'll come to the definition of the word later. For now just look at the header. The first + 4 bytes are the link pointer. This points back to the previous word in the dictionary, or, for + the first word in the dictionary it is just a NULL pointer. Then comes a length/flags byte. + The length of the word can be up to 31 characters (5 bits used) and the top three bits are used + for various flags which I'll come to later. This is followed by the name itself, and in this + implementation the name is rounded up to a multiple of 4 bytes by padding it with zero bytes. + That's just to ensure that the definition starts on a 32 bit boundary. - <--- DICTIONARY ENTRY (HEADER) -----------------------> - +------------------------+--------+---------- - - - - +----------- - - - - - | LINK POINTER | LENGTH/| NAME | DEFINITION - | | FLAGS | | - +--- (4 bytes) ----------+- byte -+- n bytes - - - - +----------- - - - - + A FORTH variable called LATEST contains a pointer to the most recently defined word, in + other words, the head of this linked list. - I'll come to the definition of the word later. For now just look at the header. The first - 4 bytes are the link pointer. This points back to the previous word in the dictionary, or, for - the first word in the dictionary it is just a NULL pointer. Then comes a length/flags byte. - The length of the word can be up to 31 characters (5 bits used) and the top three bits are used - for various flags which I'll come to later. This is followed by the name itself, and in this - implementation the name is rounded up to a multiple of 4 bytes by padding it with zero bytes. - That's just to ensure that the definition starts on a 32 bit boundary. + DOUBLE and QUADRUPLE might look like this: - A FORTH variable called LATEST contains a pointer to the most recently defined word, in - other words, the head of this linked list. - - DOUBLE and QUADRUPLE might look like this: - - pointer to previous word - ^ - | - +--|------+---+---+---+---+---+---+---+---+------------- - - - - - | LINK | 6 | D | O | U | B | L | E | 0 | (definition ...) - +---------+---+---+---+---+---+---+---+---+------------- - - - - + pointer to previous word + ^ + | + +--|------+---+---+---+---+---+---+---+---+------------- - - - - + | LINK | 6 | D | O | U | B | L | E | 0 | (definition ...) + +---------+---+---+---+---+---+---+---+---+------------- - - - - ^ len padding - | - +--|------+---+---+---+---+---+---+---+---+---+---+---+---+------------- - - - - - | LINK | 9 | Q | U | A | D | R | U | P | L | E | 0 | 0 | (definition ...) - +---------+---+---+---+---+---+---+---+---+---+---+---+---+------------- - - - - + | + +--|------+---+---+---+---+---+---+---+---+---+---+---+---+------------- - - - - + | LINK | 9 | Q | U | A | D | R | U | P | L | E | 0 | 0 | (definition ...) + +---------+---+---+---+---+---+---+---+---+---+---+---+---+------------- - - - - ^ len padding | | - LATEST - - You should be able to see from this how you might implement functions to find a word in - the dictionary (just walk along the dictionary entries starting at LATEST and matching - the names until you either find a match or hit the NULL pointer at the end of the dictionary); - and add a word to the dictionary (create a new definition, set its LINK to LATEST, and set - LATEST to point to the new word). We'll see precisely these functions implemented in - assembly code later on. - - One interesting consequence of using a linked list is that you can redefine words, and - a newer definition of a word overrides an older one. This is an important concept in - FORTH because it means that any word (even "built-in" or "standard" words) can be - overridden with a new definition, either to enhance it, to make it faster or even to - disable it. However because of the way that FORTH words get compiled, which you'll - understand below, words defined using the old definition of a word continue to use - the old definition. Only words defined after the new definition use the new definition. - - DIRECT THREADED CODE ---------------------------------------------------------------------- - - Now we'll get to the really crucial bit in understanding FORTH, so go and get a cup of tea - or coffee and settle down. It's fair to say that if you don't understand this section, then you - won't "get" how FORTH works, and that would be a failure on my part for not explaining it well. - So if after reading this section a few times you don't understand it, please email me - (rich@annexia.org). - - Let's talk first about what "threaded code" means. Imagine a peculiar version of C where - you are only allowed to call functions without arguments. (Don't worry for now that such a - language would be completely useless!) So in our peculiar C, code would look like this: - - f () - { - a (); - b (); - c (); - } - - and so on. How would a function, say 'f' above, be compiled by a standard C compiler? - Probably into assembly code like this. On the right hand side I've written the actual - i386 machine code. - - f: - CALL a E8 08 00 00 00 - CALL b E8 1C 00 00 00 - CALL c E8 2C 00 00 00 - ; ignore the return from the function for now - - "E8" is the x86 machine code to "CALL" a function. In the first 20 years of computing - memory was hideously expensive and we might have worried about the wasted space being used - by the repeated "E8" bytes. We can save 20% in code size (and therefore, in expensive memory) - by compressing this into just: - - 08 00 00 00 Just the function addresses, without - 1C 00 00 00 the CALL prefix. - 2C 00 00 00 - - On a 16-bit machine like the ones which originally ran FORTH the savings are even greater - 33%. - - [Historical note: If the execution model that FORTH uses looks strange from the following - paragraphs, then it was motivated entirely by the need to save memory on early computers. - This code compression isn't so important now when our machines have more memory in their L1 - caches than those early computers had in total, but the execution model still has some - useful properties]. - - Of course this code won't run directly on the CPU any more. Instead we need to write an - interpreter which takes each set of bytes and calls it. - - On an i386 machine it turns out that we can write this interpreter rather easily, in just - two assembly instructions which turn into just 3 bytes of machine code. Let's store the - pointer to the next word to execute in the %esi register: - - 08 00 00 00 <- We're executing this one now. %esi is the _next_ one to execute. - %esi -> 1C 00 00 00 - 2C 00 00 00 - - The all-important i386 instruction is called LODSL (or in Intel manuals, LODSW). It does - two things. Firstly it reads the memory at %esi into the accumulator (%eax). Secondly it - increments %esi by 4 bytes. So after LODSL, the situation now looks like this: - - 08 00 00 00 <- We're still executing this one - 1C 00 00 00 <- %eax now contains this address (0x0000001C) - %esi -> 2C 00 00 00 - - Now we just need to jump to the address in %eax. This is again just a single x86 instruction - written JMP *(%eax). And after doing the jump, the situation looks like: - - 08 00 00 00 - 1C 00 00 00 <- Now we're executing this subroutine. - %esi -> 2C 00 00 00 - - To make this work, each subroutine is followed by the two instructions 'LODSL; JMP *(%eax)' - which literally make the jump to the next subroutine. - - And that brings us to our first piece of actual code! Well, it's a macro. + LATEST + + You should be able to see from this how you might implement functions to find a word in + the dictionary (just walk along the dictionary entries starting at LATEST and matching + the names until you either find a match or hit the NULL pointer at the end of the dictionary); + and add a word to the dictionary (create a new definition, set its LINK to LATEST, and set + LATEST to point to the new word). We'll see precisely these functions implemented in + assembly code later on. + + One interesting consequence of using a linked list is that you can redefine words, and + a newer definition of a word overrides an older one. This is an important concept in + FORTH because it means that any word (even "built-in" or "standard" words) can be + overridden with a new definition, either to enhance it, to make it faster or even to + disable it. However because of the way that FORTH words get compiled, which you'll + understand below, words defined using the old definition of a word continue to use + the old definition. Only words defined after the new definition use the new definition. + + DIRECT THREADED CODE ---------------------------------------------------------------------- + + Now we'll get to the really crucial bit in understanding FORTH, so go and get a cup of tea + or coffee and settle down. It's fair to say that if you don't understand this section, then you + won't "get" how FORTH works, and that would be a failure on my part for not explaining it well. + So if after reading this section a few times you don't understand it, please email me + (rich@annexia.org). + + Let's talk first about what "threaded code" means. Imagine a peculiar version of C where + you are only allowed to call functions without arguments. (Don't worry for now that such a + language would be completely useless!) So in our peculiar C, code would look like this: + + f () + { + a (); + b (); + c (); + } + + and so on. How would a function, say 'f' above, be compiled by a standard C compiler? + Probably into assembly code like this. On the right hand side I've written the actual + i386 machine code. + + f: + CALL a E8 08 00 00 00 + CALL b E8 1C 00 00 00 + CALL c E8 2C 00 00 00 + ; ignore the return from the function for now + + "E8" is the x86 machine code to "CALL" a function. In the first 20 years of computing + memory was hideously expensive and we might have worried about the wasted space being used + by the repeated "E8" bytes. We can save 20% in code size (and therefore, in expensive memory) + by compressing this into just: + + 08 00 00 00 Just the function addresses, without + 1C 00 00 00 the CALL prefix. + 2C 00 00 00 + + On a 16-bit machine like the ones which originally ran FORTH the savings are even greater - 33%. + + [Historical note: If the execution model that FORTH uses looks strange from the following + paragraphs, then it was motivated entirely by the need to save memory on early computers. + This code compression isn't so important now when our machines have more memory in their L1 + caches than those early computers had in total, but the execution model still has some + useful properties]. + + Of course this code won't run directly on the CPU any more. Instead we need to write an + interpreter which takes each set of bytes and calls it. + + On an i386 machine it turns out that we can write this interpreter rather easily, in just + two assembly instructions which turn into just 3 bytes of machine code. Let's store the + pointer to the next word to execute in the %esi register: + + 08 00 00 00 <- We're executing this one now. %esi is the _next_ one to execute. + %esi -> 1C 00 00 00 + 2C 00 00 00 + + The all-important i386 instruction is called LODSL (or in Intel manuals, LODSW). It does + two things. Firstly it reads the memory at %esi into the accumulator (%eax). Secondly it + increments %esi by 4 bytes. So after LODSL, the situation now looks like this: + + 08 00 00 00 <- We're still executing this one + 1C 00 00 00 <- %eax now contains this address (0x0000001C) + %esi -> 2C 00 00 00 + + Now we just need to jump to the address in %eax. This is again just a single x86 instruction + written JMP *(%eax). And after doing the jump, the situation looks like: + + 08 00 00 00 + 1C 00 00 00 <- Now we're executing this subroutine. + %esi -> 2C 00 00 00 + + To make this work, each subroutine is followed by the two instructions 'LODSL; JMP *(%eax)' + which literally make the jump to the next subroutine. + + And that brings us to our first piece of actual code! Well, it's a macro. */ /* NEXT macro. */ - .macro NEXT - lodsl - jmp *(%eax) - .endm - -/* The macro is called NEXT. That's a FORTH-ism. It expands to those two instructions. - - Every FORTH primitive that we write has to be ended by NEXT. Think of it kind of like - a return. - - The above describes what is known as direct threaded code. - - To sum up: We compress our function calls down to a list of addresses and use a somewhat - magical macro to act as a "jump to next function in the list". We also use one register (%esi) - to act as a kind of instruction pointer, pointing to the next function in the list. - - I'll just give you a hint of what is to come by saying that a FORTH definition such as: - - : QUADRUPLE DOUBLE DOUBLE ; - - actually compiles (almost, not precisely but we'll see why in a moment) to a list of - function addresses for DOUBLE, DOUBLE and a special function called EXIT to finish off. - - At this point, REALLY EAGLE-EYED ASSEMBLY EXPERTS are saying "JONES, YOU'VE MADE A MISTAKE!". - - I lied about JMP *(%eax). - - INDIRECT THREADED CODE ---------------------------------------------------------------------- - - It turns out that direct threaded code is interesting but only if you want to just execute - a list of functions written in assembly language. So QUADRUPLE would work only if DOUBLE - was an assembly language function. In the direct threaded code, QUADRUPLE would look like: - - +------------------+ - | addr of DOUBLE --------------------> (assembly code to do the double) - +------------------+ NEXT - %esi -> | addr of DOUBLE | - +------------------+ - - We can add an extra indirection to allow us to run both words written in assembly language - (primitives written for speed) and words written in FORTH themselves as lists of addresses. - - The extra indirection is the reason for the brackets in JMP *(%eax). - - Let's have a look at how QUADRUPLE and DOUBLE really look in FORTH: - - : QUADRUPLE DOUBLE DOUBLE ; - - +------------------+ - | codeword | : DOUBLE DUP + ; - +------------------+ - | addr of DOUBLE ---------------> +------------------+ - +------------------+ | codeword | - | addr of DOUBLE | +------------------+ - +------------------+ | addr of DUP --------------> +------------------+ - | addr of EXIT | +------------------+ | codeword -------+ - +------------------+ %esi -> | addr of + --------+ +------------------+ | - +------------------+ | | assembly to <-----+ - | addr of EXIT | | | implement DUP | - +------------------+ | | .. | - | | .. | - | | NEXT | - | +------------------+ - | - +-----> +------------------+ - | codeword -------+ - +------------------+ | - | assembly to <------+ - | implement + | - | .. | - | .. | - | NEXT | - +------------------+ - - This is the part where you may need an extra cup of tea/coffee/favourite caffeinated - beverage. What has changed is that I've added an extra pointer to the beginning of - the definitions. In FORTH this is sometimes called the "codeword". The codeword is - a pointer to the interpreter to run the function. For primitives written in - assembly language, the "interpreter" just points to the actual assembly code itself. - They don't need interpreting, they just run. - - In words written in FORTH (like QUADRUPLE and DOUBLE), the codeword points to an interpreter - function. - - I'll show you the interpreter function shortly, but let's recall our indirect - JMP *(%eax) with the "extra" brackets. Take the case where we're executing DOUBLE - as shown, and DUP has been called. Note that %esi is pointing to the address of + - - The assembly code for DUP eventually does a NEXT. That: - - (1) reads the address of + into %eax %eax points to the codeword of + - (2) increments %esi by 4 - (3) jumps to the indirect %eax jumps to the address in the codeword of +, - ie. the assembly code to implement + - - +------------------+ - | codeword | - +------------------+ - | addr of DOUBLE ---------------> +------------------+ - +------------------+ | codeword | - | addr of DOUBLE | +------------------+ - +------------------+ | addr of DUP --------------> +------------------+ - | addr of EXIT | +------------------+ | codeword -------+ - +------------------+ | addr of + --------+ +------------------+ | - +------------------+ | | assembly to <-----+ - %esi -> | addr of EXIT | | | implement DUP | - +------------------+ | | .. | - | | .. | - | | NEXT | - | +------------------+ - | - +-----> +------------------+ - | codeword -------+ - +------------------+ | - now we're | assembly to <-----+ - executing | implement + | - this | .. | - function | .. | - | NEXT | - +------------------+ - - So I hope that I've convinced you that NEXT does roughly what you'd expect. This is - indirect threaded code. - - I've glossed over four things. I wonder if you can guess without reading on what they are? - - . - . - . - - My list of four things are: (1) What does "EXIT" do? (2) which is related to (1) is how do - you call into a function, ie. how does %esi start off pointing at part of QUADRUPLE, but - then point at part of DOUBLE. (3) What goes in the codeword for the words which are written - in FORTH? (4) How do you compile a function which does anything except call other functions - ie. a function which contains a number like : DOUBLE 2 * ; ? - - THE INTERPRETER AND RETURN STACK ------------------------------------------------------------ - - Going at these in no particular order, let's talk about issues (3) and (2), the interpreter - and the return stack. - - Words which are defined in FORTH need a codeword which points to a little bit of code to - give them a "helping hand" in life. They don't need much, but they do need what is known - as an "interpreter", although it doesn't really "interpret" in the same way that, say, - Java bytecode used to be interpreted (ie. slowly). This interpreter just sets up a few - machine registers so that the word can then execute at full speed using the indirect - threaded model above. - - One of the things that needs to happen when QUADRUPLE calls DOUBLE is that we save the old - %esi ("instruction pointer") and create a new one pointing to the first word in DOUBLE. - Because we will need to restore the old %esi at the end of DOUBLE (this is, after all, like - a function call), we will need a stack to store these "return addresses" (old values of %esi). - - As you will have seen in the background documentation, FORTH has two stacks, an ordinary - stack for parameters, and a return stack which is a bit more mysterious. But our return - stack is just the stack I talked about in the previous paragraph, used to save %esi when - calling from a FORTH word into another FORTH word. - - In this FORTH, we are using the normal stack pointer (%esp) for the parameter stack. - We will use the i386's "other" stack pointer (%ebp, usually called the "frame pointer") - for our return stack. - - I've got two macros which just wrap up the details of using %ebp for the return stack. - You use them as for example "PUSHRSP %eax" (push %eax on the return stack) or "POPRSP %ebx" - (pop top of return stack into %ebx). + .macro NEXT + lodsl + jmp *(%eax) + .endm + +/* The macro is called NEXT. That's a FORTH-ism. It expands to those two instructions. + + Every FORTH primitive that we write has to be ended by NEXT. Think of it kind of like + a return. + + The above describes what is known as direct threaded code. + + To sum up: We compress our function calls down to a list of addresses and use a somewhat + magical macro to act as a "jump to next function in the list". We also use one register (%esi) + to act as a kind of instruction pointer, pointing to the next function in the list. + + I'll just give you a hint of what is to come by saying that a FORTH definition such as: + + : QUADRUPLE DOUBLE DOUBLE ; + + actually compiles (almost, not precisely but we'll see why in a moment) to a list of + function addresses for DOUBLE, DOUBLE and a special function called EXIT to finish off. + + At this point, REALLY EAGLE-EYED ASSEMBLY EXPERTS are saying "JONES, YOU'VE MADE A MISTAKE!". + + I lied about JMP *(%eax). + + INDIRECT THREADED CODE ---------------------------------------------------------------------- + + It turns out that direct threaded code is interesting but only if you want to just execute + a list of functions written in assembly language. So QUADRUPLE would work only if DOUBLE + was an assembly language function. In the direct threaded code, QUADRUPLE would look like: + + +------------------+ + | addr of DOUBLE --------------------> (assembly code to do the double) + +------------------+ NEXT + %esi -> | addr of DOUBLE | + +------------------+ + + We can add an extra indirection to allow us to run both words written in assembly language + (primitives written for speed) and words written in FORTH themselves as lists of addresses. + + The extra indirection is the reason for the brackets in JMP *(%eax). + + Let's have a look at how QUADRUPLE and DOUBLE really look in FORTH: + + : QUADRUPLE DOUBLE DOUBLE ; + + +------------------+ + | codeword | : DOUBLE DUP + ; + +------------------+ + | addr of DOUBLE ---------------> +------------------+ + +------------------+ | codeword | + | addr of DOUBLE | +------------------+ + +------------------+ | addr of DUP --------------> +------------------+ + | addr of EXIT | +------------------+ | codeword -------+ + +------------------+ %esi -> | addr of + --------+ +------------------+ | + +------------------+ | | assembly to <-----+ + | addr of EXIT | | | implement DUP | + +------------------+ | | .. | + | | .. | + | | NEXT | + | +------------------+ + | + +-----> +------------------+ + | codeword -------+ + +------------------+ | + | assembly to <------+ + | implement + | + | .. | + | .. | + | NEXT | + +------------------+ + + This is the part where you may need an extra cup of tea/coffee/favourite caffeinated + beverage. What has changed is that I've added an extra pointer to the beginning of + the definitions. In FORTH this is sometimes called the "codeword". The codeword is + a pointer to the interpreter to run the function. For primitives written in + assembly language, the "interpreter" just points to the actual assembly code itself. + They don't need interpreting, they just run. + + In words written in FORTH (like QUADRUPLE and DOUBLE), the codeword points to an interpreter + function. + + I'll show you the interpreter function shortly, but let's recall our indirect + JMP *(%eax) with the "extra" brackets. Take the case where we're executing DOUBLE + as shown, and DUP has been called. Note that %esi is pointing to the address of + + + The assembly code for DUP eventually does a NEXT. That: + + (1) reads the address of + into %eax %eax points to the codeword of + + (2) increments %esi by 4 + (3) jumps to the indirect %eax jumps to the address in the codeword of +, + ie. the assembly code to implement + + + +------------------+ + | codeword | + +------------------+ + | addr of DOUBLE ---------------> +------------------+ + +------------------+ | codeword | + | addr of DOUBLE | +------------------+ + +------------------+ | addr of DUP --------------> +------------------+ + | addr of EXIT | +------------------+ | codeword -------+ + +------------------+ | addr of + --------+ +------------------+ | + +------------------+ | | assembly to <-----+ + %esi -> | addr of EXIT | | | implement DUP | + +------------------+ | | .. | + | | .. | + | | NEXT | + | +------------------+ + | + +-----> +------------------+ + | codeword -------+ + +------------------+ | + now we're | assembly to <-----+ + executing | implement + | + this | .. | + function | .. | + | NEXT | + +------------------+ + + So I hope that I've convinced you that NEXT does roughly what you'd expect. This is + indirect threaded code. + + I've glossed over four things. I wonder if you can guess without reading on what they are? + + . + . + . + + My list of four things are: (1) What does "EXIT" do? (2) which is related to (1) is how do + you call into a function, ie. how does %esi start off pointing at part of QUADRUPLE, but + then point at part of DOUBLE. (3) What goes in the codeword for the words which are written + in FORTH? (4) How do you compile a function which does anything except call other functions + ie. a function which contains a number like : DOUBLE 2 * ; ? + + THE INTERPRETER AND RETURN STACK ------------------------------------------------------------ + + Going at these in no particular order, let's talk about issues (3) and (2), the interpreter + and the return stack. + + Words which are defined in FORTH need a codeword which points to a little bit of code to + give them a "helping hand" in life. They don't need much, but they do need what is known + as an "interpreter", although it doesn't really "interpret" in the same way that, say, + Java bytecode used to be interpreted (ie. slowly). This interpreter just sets up a few + machine registers so that the word can then execute at full speed using the indirect + threaded model above. + + One of the things that needs to happen when QUADRUPLE calls DOUBLE is that we save the old + %esi ("instruction pointer") and create a new one pointing to the first word in DOUBLE. + Because we will need to restore the old %esi at the end of DOUBLE (this is, after all, like + a function call), we will need a stack to store these "return addresses" (old values of %esi). + + As you will have seen in the background documentation, FORTH has two stacks, an ordinary + stack for parameters, and a return stack which is a bit more mysterious. But our return + stack is just the stack I talked about in the previous paragraph, used to save %esi when + calling from a FORTH word into another FORTH word. + + In this FORTH, we are using the normal stack pointer (%esp) for the parameter stack. + We will use the i386's "other" stack pointer (%ebp, usually called the "frame pointer") + for our return stack. + + I've got two macros which just wrap up the details of using %ebp for the return stack. + You use them as for example "PUSHRSP %eax" (push %eax on the return stack) or "POPRSP %ebx" + (pop top of return stack into %ebx). */ /* Macros to deal with the return stack. */ - .macro PUSHRSP reg - lea -4(%ebp),%ebp // push reg on to return stack - movl \reg,(%ebp) - .endm + .macro PUSHRSP reg + lea -4(%ebp),%ebp // push reg on to return stack + movl \reg,(%ebp) + .endm - .macro POPRSP reg - mov (%ebp),\reg // pop top of return stack to reg - lea 4(%ebp),%ebp - .endm + .macro POPRSP reg + mov (%ebp),\reg // pop top of return stack to reg + lea 4(%ebp),%ebp + .endm /* - And with that we can now talk about the interpreter. + And with that we can now talk about the interpreter. - In FORTH the interpreter function is often called DOCOL (I think it means "DO COLON" because - all FORTH definitions start with a colon, as in : DOUBLE DUP + ; + In FORTH the interpreter function is often called DOCOL (I think it means "DO COLON" because + all FORTH definitions start with a colon, as in : DOUBLE DUP + ; - The "interpreter" (it's not really "interpreting") just needs to push the old %esi on the - stack and set %esi to the first word in the definition. Remember that we jumped to the - function using JMP *(%eax)? Well a consequence of that is that conveniently %eax contains - the address of this codeword, so just by adding 4 to it we get the address of the first - data word. Finally after setting up %esi, it just does NEXT which causes that first word - to run. + The "interpreter" (it's not really "interpreting") just needs to push the old %esi on the + stack and set %esi to the first word in the definition. Remember that we jumped to the + function using JMP *(%eax)? Well a consequence of that is that conveniently %eax contains + the address of this codeword, so just by adding 4 to it we get the address of the first + data word. Finally after setting up %esi, it just does NEXT which causes that first word + to run. */ /* DOCOL - the interpreter! */ - .text - .align 4 + .text + .align 4 DOCOL: - PUSHRSP %esi // push %esi on to the return stack - addl $4,%eax // %eax points to codeword, so make - movl %eax,%esi // %esi point to first data word - NEXT + PUSHRSP %esi // push %esi on to the return stack + addl $4,%eax // %eax points to codeword, so make + movl %eax,%esi // %esi point to first data word + NEXT /* - Just to make this absolutely clear, let's see how DOCOL works when jumping from QUADRUPLE - into DOUBLE: - - QUADRUPLE: - +------------------+ - | codeword | - +------------------+ DOUBLE: - | addr of DOUBLE ---------------> +------------------+ - +------------------+ %eax -> | addr of DOCOL | - %esi -> | addr of DOUBLE | +------------------+ - +------------------+ | addr of DUP | - | addr of EXIT | +------------------+ - +------------------+ | etc. | - - First, the call to DOUBLE calls DOCOL (the codeword of DOUBLE). DOCOL does this: It - pushes the old %esi on the return stack. %eax points to the codeword of DOUBLE, so we - just add 4 on to it to get our new %esi: - - QUADRUPLE: - +------------------+ - | codeword | - +------------------+ DOUBLE: - | addr of DOUBLE ---------------> +------------------+ -top of return +------------------+ %eax -> | addr of DOCOL | -stack points -> | addr of DOUBLE | + 4 = +------------------+ - +------------------+ %esi -> | addr of DUP | - | addr of EXIT | +------------------+ - +------------------+ | etc. | - - Then we do NEXT, and because of the magic of threaded code that increments %esi again - and calls DUP. - - Well, it seems to work. - - One minor point here. Because DOCOL is the first bit of assembly actually to be defined - in this file (the others were just macros), and because I usually compile this code with the - text segment starting at address 0, DOCOL has address 0. So if you are disassembling the - code and see a word with a codeword of 0, you will immediately know that the word is - written in FORTH (it's not an assembler primitive) and so uses DOCOL as the interpreter. - - STARTING UP ---------------------------------------------------------------------- - - Now let's get down to nuts and bolts. When we start the program we need to set up - a few things like the return stack. But as soon as we can, we want to jump into FORTH - code (albeit much of the "early" FORTH code will still need to be written as - assembly language primitives). - - This is what the set up code does. Does a tiny bit of house-keeping, sets up the - separate return stack (NB: Linux gives us the ordinary parameter stack already), then - immediately jumps to a FORTH word called QUIT. Despite its name, QUIT doesn't quit - anything. It resets some internal state and starts reading and interpreting commands. - (The reason it is called QUIT is because you can call QUIT from your own FORTH code - to "quit" your program and go back to interpreting). + Just to make this absolutely clear, let's see how DOCOL works when jumping from QUADRUPLE + into DOUBLE: + + QUADRUPLE: + +------------------+ + | codeword | + +------------------+ DOUBLE: + | addr of DOUBLE ---------------> +------------------+ + +------------------+ %eax -> | addr of DOCOL | + %esi -> | addr of DOUBLE | +------------------+ + +------------------+ | addr of DUP | + | addr of EXIT | +------------------+ + +------------------+ | etc. | + + First, the call to DOUBLE calls DOCOL (the codeword of DOUBLE). DOCOL does this: It + pushes the old %esi on the return stack. %eax points to the codeword of DOUBLE, so we + just add 4 on to it to get our new %esi: + + QUADRUPLE: + +------------------+ + | codeword | + +------------------+ DOUBLE: + | addr of DOUBLE ---------------> +------------------+ +top of return +------------------+ %eax -> | addr of DOCOL | +stack points -> | addr of DOUBLE | + 4 = +------------------+ + +------------------+ %esi -> | addr of DUP | + | addr of EXIT | +------------------+ + +------------------+ | etc. | + + Then we do NEXT, and because of the magic of threaded code that increments %esi again + and calls DUP. + + Well, it seems to work. + + One minor point here. Because DOCOL is the first bit of assembly actually to be defined + in this file (the others were just macros), and because I usually compile this code with the + text segment starting at address 0, DOCOL has address 0. So if you are disassembling the + code and see a word with a codeword of 0, you will immediately know that the word is + written in FORTH (it's not an assembler primitive) and so uses DOCOL as the interpreter. + + STARTING UP ---------------------------------------------------------------------- + + Now let's get down to nuts and bolts. When we start the program we need to set up + a few things like the return stack. But as soon as we can, we want to jump into FORTH + code (albeit much of the "early" FORTH code will still need to be written as + assembly language primitives). + + This is what the set up code does. Does a tiny bit of house-keeping, sets up the + separate return stack (NB: Linux gives us the ordinary parameter stack already), then + immediately jumps to a FORTH word called QUIT. Despite its name, QUIT doesn't quit + anything. It resets some internal state and starts reading and interpreting commands. + (The reason it is called QUIT is because you can call QUIT from your own FORTH code + to "quit" your program and go back to interpreting). */ /* Assembler entry point. */ - .text - .globl _start + .text + .globl _start _start: - cld - mov %esp,var_S0 // Save the initial data stack pointer in FORTH variable S0. - mov $return_stack_top,%ebp // Initialise the return stack. - call set_up_data_segment + cld + mov %esp,var_S0 // Save the initial data stack pointer in FORTH variable S0. + mov $return_stack_top,%ebp // Initialise the return stack. + call set_up_data_segment - mov $cold_start,%esi // Initialise interpreter. - NEXT // Run interpreter! + mov $cold_start,%esi // Initialise interpreter. + NEXT // Run interpreter! - .section .rodata -cold_start: // High-level code without a codeword. - .int QUIT + .section .rodata +cold_start: // High-level code without a codeword. + .int QUIT /* - BUILT-IN WORDS ---------------------------------------------------------------------- + BUILT-IN WORDS ---------------------------------------------------------------------- - Remember our dictionary entries (headers)? Let's bring those together with the codeword - and data words to see how : DOUBLE DUP + ; really looks in memory. + Remember our dictionary entries (headers)? Let's bring those together with the codeword + and data words to see how : DOUBLE DUP + ; really looks in memory. - pointer to previous word - ^ - | - +--|------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ - | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | - +---------+---+---+---+---+---+---+---+---+------------+--|---------+------------+------------+ + pointer to previous word + ^ + | + +--|------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ + | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | + +---------+---+---+---+---+---+---+---+---+------------+--|---------+------------+------------+ ^ len pad codeword | - | V - LINK in next word points to codeword of DUP - - Initially we can't just write ": DOUBLE DUP + ;" (ie. that literal string) here because we - don't yet have anything to read the string, break it up at spaces, parse each word, etc. etc. - So instead we will have to define built-in words using the GNU assembler data constructors - (like .int, .byte, .string, .ascii and so on -- look them up in the gas info page if you are - unsure of them). - - The long way would be: - - .int - .byte 6 // len - .ascii "DOUBLE" // string - .byte 0 // padding -DOUBLE: .int DOCOL // codeword - .int DUP // pointer to codeword of DUP - .int PLUS // pointer to codeword of + - .int EXIT // pointer to codeword of EXIT - - That's going to get quite tedious rather quickly, so here I define an assembler macro - so that I can just write: - - defword "DOUBLE",6,,DOUBLE - .int DUP,PLUS,EXIT - - and I'll get exactly the same effect. - - Don't worry too much about the exact implementation details of this macro - it's complicated! + | V + LINK in next word points to codeword of DUP + + Initially we can't just write ": DOUBLE DUP + ;" (ie. that literal string) here because we + don't yet have anything to read the string, break it up at spaces, parse each word, etc. etc. + So instead we will have to define built-in words using the GNU assembler data constructors + (like .int, .byte, .string, .ascii and so on -- look them up in the gas info page if you are + unsure of them). + + The long way would be: + + .int + .byte 6 // len + .ascii "DOUBLE" // string + .byte 0 // padding +DOUBLE: .int DOCOL // codeword + .int DUP // pointer to codeword of DUP + .int PLUS // pointer to codeword of + + .int EXIT // pointer to codeword of EXIT + + That's going to get quite tedious rather quickly, so here I define an assembler macro + so that I can just write: + + defword "DOUBLE",6,,DOUBLE + .int DUP,PLUS,EXIT + + and I'll get exactly the same effect. + + Don't worry too much about the exact implementation details of this macro - it's complicated! */ /* Flags - these are discussed later. */ - .set F_IMMED,0x80 - .set F_HIDDEN,0x20 - .set F_LENMASK,0x1f // length mask + .set F_IMMED,0x80 + .set F_HIDDEN,0x20 + .set F_LENMASK,0x1f // length mask - // Store the chain of links. - .set link,0 + // Store the chain of links. + .set link,0 - .macro defword name, namelen, flags=0, label - .section .rodata - .align 4 - .globl name_\label + .macro defword name, namelen, flags=0, label + .section .rodata + .align 4 + .globl name_\label name_\label : - .int link // link - .set link,name_\label - .byte \flags+\namelen // flags + length byte - .ascii "\name" // the name - .align 4 // padding to next 4 byte boundary - .globl \label + .int link // link + .set link,name_\label + .byte \flags+\namelen // flags + length byte + .ascii "\name" // the name + .align 4 // padding to next 4 byte boundary + .globl \label \label : - .int DOCOL // codeword - the interpreter - // list of word pointers follow - .endm + .int DOCOL // codeword - the interpreter + // list of word pointers follow + .endm /* - Similarly I want a way to write words written in assembly language. There will be quite a few - of these to start with because, well, everything has to start in assembly before there's - enough "infrastructure" to be able to start writing FORTH words, but also I want to define - some common FORTH words in assembly language for speed, even though I could write them in FORTH. - - This is what DUP looks like in memory: - - pointer to previous word - ^ - | - +--|------+---+---+---+---+------------+ - | LINK | 3 | D | U | P | code_DUP ---------------------> points to the assembly - +---------+---+---+---+---+------------+ code used to write DUP, - ^ len codeword which ends with NEXT. - | - LINK in next word - - Again, for brevity in writing the header I'm going to write an assembler macro called defcode. - As with defword above, don't worry about the complicated details of the macro. + Similarly I want a way to write words written in assembly language. There will be quite a few + of these to start with because, well, everything has to start in assembly before there's + enough "infrastructure" to be able to start writing FORTH words, but also I want to define + some common FORTH words in assembly language for speed, even though I could write them in FORTH. + + This is what DUP looks like in memory: + + pointer to previous word + ^ + | + +--|------+---+---+---+---+------------+ + | LINK | 3 | D | U | P | code_DUP ---------------------> points to the assembly + +---------+---+---+---+---+------------+ code used to write DUP, + ^ len codeword which ends with NEXT. + | + LINK in next word + + Again, for brevity in writing the header I'm going to write an assembler macro called defcode. + As with defword above, don't worry about the complicated details of the macro. */ - .macro defcode name, namelen, flags=0, label - .section .rodata - .align 4 - .globl name_\label + .macro defcode name, namelen, flags=0, label + .section .rodata + .align 4 + .globl name_\label name_\label : - .int link // link - .set link,name_\label - .byte \flags+\namelen // flags + length byte - .ascii "\name" // the name - .align 4 // padding to next 4 byte boundary - .globl \label + .int link // link + .set link,name_\label + .byte \flags+\namelen // flags + length byte + .ascii "\name" // the name + .align 4 // padding to next 4 byte boundary + .globl \label \label : - .int code_\label // codeword - .text - //.align 4 - .globl code_\label -code_\label : // assembler code follows - .endm + .int code_\label // codeword + .text + //.align 4 + .globl code_\label +code_\label : // assembler code follows + .endm /* - Now some easy FORTH primitives. These are written in assembly for speed. If you understand - i386 assembly language then it is worth reading these. However if you don't understand assembly - you can skip the details. + Now some easy FORTH primitives. These are written in assembly for speed. If you understand + i386 assembly language then it is worth reading these. However if you don't understand assembly + you can skip the details. */ - defcode "DROP",4,,DROP - pop %eax // drop top of stack - NEXT - - defcode "SWAP",4,,SWAP - pop %eax // swap top two elements on stack - pop %ebx - push %eax - push %ebx - NEXT - - defcode "DUP",3,,DUP - mov (%esp),%eax // duplicate top of stack - push %eax - NEXT - - defcode "OVER",4,,OVER - mov 4(%esp),%eax // get the second element of stack - push %eax // and push it on top - NEXT - - defcode "ROT",3,,ROT - pop %eax - pop %ebx - pop %ecx - push %ebx - push %eax - push %ecx - NEXT - - defcode "-ROT",4,,NROT - pop %eax - pop %ebx - pop %ecx - push %eax - push %ecx - push %ebx - NEXT - - defcode "2DROP",5,,TWODROP // drop top two elements of stack - pop %eax - pop %eax - NEXT - - defcode "2DUP",4,,TWODUP // duplicate top two elements of stack - mov (%esp),%eax - mov 4(%esp),%ebx - push %ebx - push %eax - NEXT - - defcode "2SWAP",5,,TWOSWAP // swap top two pairs of elements of stack - pop %eax - pop %ebx - pop %ecx - pop %edx - push %ebx - push %eax - push %edx - push %ecx - NEXT - - defcode "?DUP",4,,QDUP // duplicate top of stack if non-zero - movl (%esp),%eax - test %eax,%eax - jz 1f - push %eax -1: NEXT - - defcode "1+",2,,INCR - incl (%esp) // increment top of stack - NEXT - - defcode "1-",2,,DECR - decl (%esp) // decrement top of stack - NEXT - - defcode "4+",2,,INCR4 - addl $4,(%esp) // add 4 to top of stack - NEXT - - defcode "4-",2,,DECR4 - subl $4,(%esp) // subtract 4 from top of stack - NEXT - - defcode "+",1,,ADD - pop %eax // get top of stack - addl %eax,(%esp) // and add it to next word on stack - NEXT - - defcode "-",1,,SUB - pop %eax // get top of stack - subl %eax,(%esp) // and subtract it from next word on stack - NEXT - - defcode "*",1,,MUL - pop %eax - pop %ebx - imull %ebx,%eax - push %eax // ignore overflow - NEXT + defcode "DROP",4,,DROP + pop %eax // drop top of stack + NEXT + + defcode "SWAP",4,,SWAP + pop %eax // swap top two elements on stack + pop %ebx + push %eax + push %ebx + NEXT + + defcode "DUP",3,,DUP + mov (%esp),%eax // duplicate top of stack + push %eax + NEXT + + defcode "OVER",4,,OVER + mov 4(%esp),%eax // get the second element of stack + push %eax // and push it on top + NEXT + + defcode "ROT",3,,ROT + pop %eax + pop %ebx + pop %ecx + push %ebx + push %eax + push %ecx + NEXT + + defcode "-ROT",4,,NROT + pop %eax + pop %ebx + pop %ecx + push %eax + push %ecx + push %ebx + NEXT + + defcode "2DROP",5,,TWODROP // drop top two elements of stack + pop %eax + pop %eax + NEXT + + defcode "2DUP",4,,TWODUP // duplicate top two elements of stack + mov (%esp),%eax + mov 4(%esp),%ebx + push %ebx + push %eax + NEXT + + defcode "2SWAP",5,,TWOSWAP // swap top two pairs of elements of stack + pop %eax + pop %ebx + pop %ecx + pop %edx + push %ebx + push %eax + push %edx + push %ecx + NEXT + + defcode "?DUP",4,,QDUP // duplicate top of stack if non-zero + movl (%esp),%eax + test %eax,%eax + jz 1f + push %eax +1: NEXT + + defcode "1+",2,,INCR + incl (%esp) // increment top of stack + NEXT + + defcode "1-",2,,DECR + decl (%esp) // decrement top of stack + NEXT + + defcode "4+",2,,INCR4 + addl $4,(%esp) // add 4 to top of stack + NEXT + + defcode "4-",2,,DECR4 + subl $4,(%esp) // subtract 4 from top of stack + NEXT + + defcode "+",1,,ADD + pop %eax // get top of stack + addl %eax,(%esp) // and add it to next word on stack + NEXT + + defcode "-",1,,SUB + pop %eax // get top of stack + subl %eax,(%esp) // and subtract it from next word on stack + NEXT + + defcode "*",1,,MUL + pop %eax + pop %ebx + imull %ebx,%eax + push %eax // ignore overflow + NEXT /* - In this FORTH, only /MOD is primitive. Later we will define the / and MOD words in - terms of the primitive /MOD. The design of the i386 assembly instruction idiv which - leaves both quotient and remainder makes this the obvious choice. + In this FORTH, only /MOD is primitive. Later we will define the / and MOD words in + terms of the primitive /MOD. The design of the i386 assembly instruction idiv which + leaves both quotient and remainder makes this the obvious choice. */ - defcode "/MOD",4,,DIVMOD - xor %edx,%edx - pop %ebx - pop %eax - idivl %ebx - push %edx // push remainder - push %eax // push quotient - NEXT + defcode "/MOD",4,,DIVMOD + xor %edx,%edx + pop %ebx + pop %eax + idivl %ebx + push %edx // push remainder + push %eax // push quotient + NEXT /* - Lots of comparison operations like =, <, >, etc.. + Lots of comparison operations like =, <, >, etc.. - ANS FORTH says that the comparison words should return all (binary) 1's for - TRUE and all 0's for FALSE. However this is a bit of a strange convention - so this FORTH breaks it and returns the more normal (for C programmers ...) - 1 meaning TRUE and 0 meaning FALSE. + ANS FORTH says that the comparison words should return all (binary) 1's for + TRUE and all 0's for FALSE. However this is a bit of a strange convention + so this FORTH breaks it and returns the more normal (for C programmers ...) + 1 meaning TRUE and 0 meaning FALSE. */ - defcode "=",1,,EQU // top two words are equal? - pop %eax - pop %ebx - cmp %ebx,%eax - sete %al - movzbl %al,%eax - pushl %eax - NEXT - - defcode "<>",2,,NEQU // top two words are not equal? - pop %eax - pop %ebx - cmp %ebx,%eax - setne %al - movzbl %al,%eax - pushl %eax - NEXT - - defcode "<",1,,LT - pop %eax - pop %ebx - cmp %eax,%ebx - setl %al - movzbl %al,%eax - pushl %eax - NEXT - - defcode ">",1,,GT - pop %eax - pop %ebx - cmp %eax,%ebx - setg %al - movzbl %al,%eax - pushl %eax - NEXT - - defcode "<=",2,,LE - pop %eax - pop %ebx - cmp %eax,%ebx - setle %al - movzbl %al,%eax - pushl %eax - NEXT - - defcode ">=",2,,GE - pop %eax - pop %ebx - cmp %eax,%ebx - setge %al - movzbl %al,%eax - pushl %eax - NEXT - - defcode "0=",2,,ZEQU // top of stack equals 0? - pop %eax - test %eax,%eax - setz %al - movzbl %al,%eax - pushl %eax - NEXT - - defcode "0<>",3,,ZNEQU // top of stack not 0? - pop %eax - test %eax,%eax - setnz %al - movzbl %al,%eax - pushl %eax - NEXT - - defcode "0<",2,,ZLT // comparisons with 0 - pop %eax - test %eax,%eax - setl %al - movzbl %al,%eax - pushl %eax - NEXT - - defcode "0>",2,,ZGT - pop %eax - test %eax,%eax - setg %al - movzbl %al,%eax - pushl %eax - NEXT - - defcode "0<=",3,,ZLE - pop %eax - test %eax,%eax - setle %al - movzbl %al,%eax - pushl %eax - NEXT - - defcode "0>=",3,,ZGE - pop %eax - test %eax,%eax - setge %al - movzbl %al,%eax - pushl %eax - NEXT - - defcode "AND",3,,AND // bitwise AND - pop %eax - andl %eax,(%esp) - NEXT - - defcode "OR",2,,OR // bitwise OR - pop %eax - orl %eax,(%esp) - NEXT - - defcode "XOR",3,,XOR // bitwise XOR - pop %eax - xorl %eax,(%esp) - NEXT - - defcode "INVERT",6,,INVERT // this is the FORTH bitwise "NOT" function (cf. NEGATE and NOT) - notl (%esp) - NEXT + defcode "=",1,,EQU // top two words are equal? + pop %eax + pop %ebx + cmp %ebx,%eax + sete %al + movzbl %al,%eax + pushl %eax + NEXT + + defcode "<>",2,,NEQU // top two words are not equal? + pop %eax + pop %ebx + cmp %ebx,%eax + setne %al + movzbl %al,%eax + pushl %eax + NEXT + + defcode "<",1,,LT + pop %eax + pop %ebx + cmp %eax,%ebx + setl %al + movzbl %al,%eax + pushl %eax + NEXT + + defcode ">",1,,GT + pop %eax + pop %ebx + cmp %eax,%ebx + setg %al + movzbl %al,%eax + pushl %eax + NEXT + + defcode "<=",2,,LE + pop %eax + pop %ebx + cmp %eax,%ebx + setle %al + movzbl %al,%eax + pushl %eax + NEXT + + defcode ">=",2,,GE + pop %eax + pop %ebx + cmp %eax,%ebx + setge %al + movzbl %al,%eax + pushl %eax + NEXT + + defcode "0=",2,,ZEQU // top of stack equals 0? + pop %eax + test %eax,%eax + setz %al + movzbl %al,%eax + pushl %eax + NEXT + + defcode "0<>",3,,ZNEQU // top of stack not 0? + pop %eax + test %eax,%eax + setnz %al + movzbl %al,%eax + pushl %eax + NEXT + + defcode "0<",2,,ZLT // comparisons with 0 + pop %eax + test %eax,%eax + setl %al + movzbl %al,%eax + pushl %eax + NEXT + + defcode "0>",2,,ZGT + pop %eax + test %eax,%eax + setg %al + movzbl %al,%eax + pushl %eax + NEXT + + defcode "0<=",3,,ZLE + pop %eax + test %eax,%eax + setle %al + movzbl %al,%eax + pushl %eax + NEXT + + defcode "0>=",3,,ZGE + pop %eax + test %eax,%eax + setge %al + movzbl %al,%eax + pushl %eax + NEXT + + defcode "AND",3,,AND // bitwise AND + pop %eax + andl %eax,(%esp) + NEXT + + defcode "OR",2,,OR // bitwise OR + pop %eax + orl %eax,(%esp) + NEXT + + defcode "XOR",3,,XOR // bitwise XOR + pop %eax + xorl %eax,(%esp) + NEXT + + defcode "INVERT",6,,INVERT // this is the FORTH bitwise "NOT" function (cf. NEGATE and NOT) + notl (%esp) + NEXT /* - RETURNING FROM FORTH WORDS ---------------------------------------------------------------------- - - Time to talk about what happens when we EXIT a function. In this diagram QUADRUPLE has called - DOUBLE, and DOUBLE is about to exit (look at where %esi is pointing): - - QUADRUPLE - +------------------+ - | codeword | - +------------------+ DOUBLE - | addr of DOUBLE ---------------> +------------------+ - +------------------+ | codeword | - | addr of DOUBLE | +------------------+ - +------------------+ | addr of DUP | - | addr of EXIT | +------------------+ - +------------------+ | addr of + | - +------------------+ - %esi -> | addr of EXIT | - +------------------+ - - What happens when the + function does NEXT? Well, the following code is executed. + RETURNING FROM FORTH WORDS ---------------------------------------------------------------------- + + Time to talk about what happens when we EXIT a function. In this diagram QUADRUPLE has called + DOUBLE, and DOUBLE is about to exit (look at where %esi is pointing): + + QUADRUPLE + +------------------+ + | codeword | + +------------------+ DOUBLE + | addr of DOUBLE ---------------> +------------------+ + +------------------+ | codeword | + | addr of DOUBLE | +------------------+ + +------------------+ | addr of DUP | + | addr of EXIT | +------------------+ + +------------------+ | addr of + | + +------------------+ + %esi -> | addr of EXIT | + +------------------+ + + What happens when the + function does NEXT? Well, the following code is executed. */ - defcode "EXIT",4,,EXIT - POPRSP %esi // pop return stack into %esi - NEXT + defcode "EXIT",4,,EXIT + POPRSP %esi // pop return stack into %esi + NEXT /* - EXIT gets the old %esi which we saved from before on the return stack, and puts it in %esi. - So after this (but just before NEXT) we get: + EXIT gets the old %esi which we saved from before on the return stack, and puts it in %esi. + So after this (but just before NEXT) we get: - QUADRUPLE - +------------------+ - | codeword | - +------------------+ DOUBLE - | addr of DOUBLE ---------------> +------------------+ - +------------------+ | codeword | - %esi -> | addr of DOUBLE | +------------------+ - +------------------+ | addr of DUP | - | addr of EXIT | +------------------+ - +------------------+ | addr of + | - +------------------+ - | addr of EXIT | - +------------------+ + QUADRUPLE + +------------------+ + | codeword | + +------------------+ DOUBLE + | addr of DOUBLE ---------------> +------------------+ + +------------------+ | codeword | + %esi -> | addr of DOUBLE | +------------------+ + +------------------+ | addr of DUP | + | addr of EXIT | +------------------+ + +------------------+ | addr of + | + +------------------+ + | addr of EXIT | + +------------------+ - And NEXT just completes the job by, well, in this case just by calling DOUBLE again :-) + And NEXT just completes the job by, well, in this case just by calling DOUBLE again :-) - LITERALS ---------------------------------------------------------------------- + LITERALS ---------------------------------------------------------------------- - The final point I "glossed over" before was how to deal with functions that do anything - apart from calling other functions. For example, suppose that DOUBLE was defined like this: + The final point I "glossed over" before was how to deal with functions that do anything + apart from calling other functions. For example, suppose that DOUBLE was defined like this: - : DOUBLE 2 * ; + : DOUBLE 2 * ; - It does the same thing, but how do we compile it since it contains the literal 2? One way - would be to have a function called "2" (which you'd have to write in assembler), but you'd need - a function for every single literal that you wanted to use. + It does the same thing, but how do we compile it since it contains the literal 2? One way + would be to have a function called "2" (which you'd have to write in assembler), but you'd need + a function for every single literal that you wanted to use. - FORTH solves this by compiling the function using a special word called LIT: + FORTH solves this by compiling the function using a special word called LIT: - +---------------------------+-------+-------+-------+-------+-------+ - | (usual header of DOUBLE) | DOCOL | LIT | 2 | * | EXIT | - +---------------------------+-------+-------+-------+-------+-------+ + +---------------------------+-------+-------+-------+-------+-------+ + | (usual header of DOUBLE) | DOCOL | LIT | 2 | * | EXIT | + +---------------------------+-------+-------+-------+-------+-------+ - LIT is executed in the normal way, but what it does next is definitely not normal. It - looks at %esi (which now points to the number 2), grabs it, pushes it on the stack, then - manipulates %esi in order to skip the number as if it had never been there. + LIT is executed in the normal way, but what it does next is definitely not normal. It + looks at %esi (which now points to the number 2), grabs it, pushes it on the stack, then + manipulates %esi in order to skip the number as if it had never been there. - What's neat is that the whole grab/manipulate can be done using a single byte single - i386 instruction, our old friend LODSL. Rather than me drawing more ASCII-art diagrams, - see if you can find out how LIT works: + What's neat is that the whole grab/manipulate can be done using a single byte single + i386 instruction, our old friend LODSL. Rather than me drawing more ASCII-art diagrams, + see if you can find out how LIT works: */ - defcode "LIT",3,,LIT - // %esi points to the next command, but in this case it points to the next - // literal 32 bit integer. Get that literal into %eax and increment %esi. - // On x86, it's a convenient single byte instruction! (cf. NEXT macro) - lodsl - push %eax // push the literal number on to stack - NEXT + defcode "LIT",3,,LIT + // %esi points to the next command, but in this case it points to the next + // literal 32 bit integer. Get that literal into %eax and increment %esi. + // On x86, it's a convenient single byte instruction! (cf. NEXT macro) + lodsl + push %eax // push the literal number on to stack + NEXT /* - MEMORY ---------------------------------------------------------------------- + MEMORY ---------------------------------------------------------------------- - An important point about FORTH is that it gives you direct access to the lowest levels - of the machine. Manipulating memory directly is done frequently in FORTH, and these are - the primitive words for doing it. + An important point about FORTH is that it gives you direct access to the lowest levels + of the machine. Manipulating memory directly is done frequently in FORTH, and these are + the primitive words for doing it. */ - defcode "!",1,,STORE - pop %ebx // address to store at - pop %eax // data to store there - mov %eax,(%ebx) // store it - NEXT - - defcode "@",1,,FETCH - pop %ebx // address to fetch - mov (%ebx),%eax // fetch it - push %eax // push value onto stack - NEXT - - defcode "+!",2,,ADDSTORE - pop %ebx // address - pop %eax // the amount to add - addl %eax,(%ebx) // add it - NEXT - - defcode "-!",2,,SUBSTORE - pop %ebx // address - pop %eax // the amount to subtract - subl %eax,(%ebx) // add it - NEXT + defcode "!",1,,STORE + pop %ebx // address to store at + pop %eax // data to store there + mov %eax,(%ebx) // store it + NEXT + + defcode "@",1,,FETCH + pop %ebx // address to fetch + mov (%ebx),%eax // fetch it + push %eax // push value onto stack + NEXT + + defcode "+!",2,,ADDSTORE + pop %ebx // address + pop %eax // the amount to add + addl %eax,(%ebx) // add it + NEXT + + defcode "-!",2,,SUBSTORE + pop %ebx // address + pop %eax // the amount to subtract + subl %eax,(%ebx) // add it + NEXT /* - ! and @ (STORE and FETCH) store 32-bit words. It's also useful to be able to read and write bytes - so we also define standard words C@ and C!. + ! and @ (STORE and FETCH) store 32-bit words. It's also useful to be able to read and write bytes + so we also define standard words C@ and C!. - Byte-oriented operations only work on architectures which permit them (i386 is one of those). + Byte-oriented operations only work on architectures which permit them (i386 is one of those). */ - defcode "C!",2,,STOREBYTE - pop %ebx // address to store at - pop %eax // data to store there - movb %al,(%ebx) // store it - NEXT + defcode "C!",2,,STOREBYTE + pop %ebx // address to store at + pop %eax // data to store there + movb %al,(%ebx) // store it + NEXT - defcode "C@",2,,FETCHBYTE - pop %ebx // address to fetch - xor %eax,%eax - movb (%ebx),%al // fetch it - push %eax // push value onto stack - NEXT + defcode "C@",2,,FETCHBYTE + pop %ebx // address to fetch + xor %eax,%eax + movb (%ebx),%al // fetch it + push %eax // push value onto stack + NEXT /* C@C! is a useful byte copy primitive. */ - defcode "C@C!",4,,CCOPY - movl 4(%esp),%ebx // source address - movb (%ebx),%al // get source character - pop %edi // destination address - stosb // copy to destination - push %edi // increment destination address - incl 4(%esp) // increment source address - NEXT + defcode "C@C!",4,,CCOPY + movl 4(%esp),%ebx // source address + movb (%ebx),%al // get source character + pop %edi // destination address + stosb // copy to destination + push %edi // increment destination address + incl 4(%esp) // increment source address + NEXT /* and CMOVE is a block copy operation. */ - defcode "CMOVE",5,,CMOVE - mov %esi,%edx // preserve %esi - pop %ecx // length - pop %edi // destination address - pop %esi // source address - rep movsb // copy source to destination - mov %edx,%esi // restore %esi - NEXT + defcode "CMOVE",5,,CMOVE + mov %esi,%edx // preserve %esi + pop %ecx // length + pop %edi // destination address + pop %esi // source address + rep movsb // copy source to destination + mov %edx,%esi // restore %esi + NEXT /* - BUILT-IN VARIABLES ---------------------------------------------------------------------- + BUILT-IN VARIABLES ---------------------------------------------------------------------- - These are some built-in variables and related standard FORTH words. Of these, the only one that we - have discussed so far was LATEST, which points to the last (most recently defined) word in the - FORTH dictionary. LATEST is also a FORTH word which pushes the address of LATEST (the variable) - on to the stack, so you can read or write it using @ and ! operators. For example, to print - the current value of LATEST (and this can apply to any FORTH variable) you would do: + These are some built-in variables and related standard FORTH words. Of these, the only one that we + have discussed so far was LATEST, which points to the last (most recently defined) word in the + FORTH dictionary. LATEST is also a FORTH word which pushes the address of LATEST (the variable) + on to the stack, so you can read or write it using @ and ! operators. For example, to print + the current value of LATEST (and this can apply to any FORTH variable) you would do: - LATEST @ . CR + LATEST @ . CR - To make defining variables shorter, I'm using a macro called defvar, similar to defword and - defcode above. (In fact the defvar macro uses defcode to do the dictionary header). + To make defining variables shorter, I'm using a macro called defvar, similar to defword and + defcode above. (In fact the defvar macro uses defcode to do the dictionary header). */ - .macro defvar name, namelen, flags=0, label, initial=0 - defcode \name,\namelen,\flags,\label - push $var_\name - NEXT - .data - .align 4 + .macro defvar name, namelen, flags=0, label, initial=0 + defcode \name,\namelen,\flags,\label + push $var_\name + NEXT + .data + .align 4 var_\name : - .int \initial - .endm + .int \initial + .endm /* - The built-in variables are: + The built-in variables are: - STATE Is the interpreter executing code (0) or compiling a word (non-zero)? - LATEST Points to the latest (most recently defined) word in the dictionary. - HERE Points to the next free byte of memory. When compiling, compiled words go here. - S0 Stores the address of the top of the parameter stack. - BASE The current base for printing and reading numbers. + STATE Is the interpreter executing code (0) or compiling a word (non-zero)? + LATEST Points to the latest (most recently defined) word in the dictionary. + HERE Points to the next free byte of memory. When compiling, compiled words go here. + S0 Stores the address of the top of the parameter stack. + BASE The current base for printing and reading numbers. */ - defvar "STATE",5,,STATE - defvar "HERE",4,,HERE - defvar "LATEST",6,,LATEST,name_SYSCALL0 // SYSCALL0 must be last in built-in dictionary - defvar "S0",2,,SZ - defvar "BASE",4,,BASE,10 + defvar "STATE",5,,STATE + defvar "HERE",4,,HERE + defvar "LATEST",6,,LATEST,name_SYSCALL0 // SYSCALL0 must be last in built-in dictionary + defvar "S0",2,,SZ + defvar "BASE",4,,BASE,10 /* - BUILT-IN CONSTANTS ---------------------------------------------------------------------- + BUILT-IN CONSTANTS ---------------------------------------------------------------------- - It's also useful to expose a few constants to FORTH. When the word is executed it pushes a - constant value on the stack. + It's also useful to expose a few constants to FORTH. When the word is executed it pushes a + constant value on the stack. - The built-in constants are: + The built-in constants are: - VERSION Is the current version of this FORTH. - R0 The address of the top of the return stack. - DOCOL Pointer to DOCOL. - F_IMMED The IMMEDIATE flag's actual value. - F_HIDDEN The HIDDEN flag's actual value. - F_LENMASK The length mask in the flags/len byte. + VERSION Is the current version of this FORTH. + R0 The address of the top of the return stack. + DOCOL Pointer to DOCOL. + F_IMMED The IMMEDIATE flag's actual value. + F_HIDDEN The HIDDEN flag's actual value. + F_LENMASK The length mask in the flags/len byte. - SYS_* and the numeric codes of various Linux syscalls (from ) + SYS_* and the numeric codes of various Linux syscalls (from ) */ -//#include // you might need this instead +//#include // you might need this instead #include - .macro defconst name, namelen, flags=0, label, value - defcode \name,\namelen,\flags,\label - push $\value - NEXT - .endm - - defconst "VERSION",7,,VERSION,JONES_VERSION - defconst "R0",2,,RZ,return_stack_top - defconst "DOCOL",5,,__DOCOL,DOCOL - defconst "F_IMMED",7,,__F_IMMED,F_IMMED - defconst "F_HIDDEN",8,,__F_HIDDEN,F_HIDDEN - defconst "F_LENMASK",9,,__F_LENMASK,F_LENMASK - - defconst "SYS_EXIT",8,,SYS_EXIT,__NR_exit - defconst "SYS_OPEN",8,,SYS_OPEN,__NR_open - defconst "SYS_CLOSE",9,,SYS_CLOSE,__NR_close - defconst "SYS_READ",8,,SYS_READ,__NR_read - defconst "SYS_WRITE",9,,SYS_WRITE,__NR_write - defconst "SYS_CREAT",9,,SYS_CREAT,__NR_creat - defconst "SYS_BRK",7,,SYS_BRK,__NR_brk - - defconst "O_RDONLY",8,,__O_RDONLY,0 - defconst "O_WRONLY",8,,__O_WRONLY,1 - defconst "O_RDWR",6,,__O_RDWR,2 - defconst "O_CREAT",7,,__O_CREAT,0100 - defconst "O_EXCL",6,,__O_EXCL,0200 - defconst "O_TRUNC",7,,__O_TRUNC,01000 - defconst "O_APPEND",8,,__O_APPEND,02000 - defconst "O_NONBLOCK",10,,__O_NONBLOCK,04000 + .macro defconst name, namelen, flags=0, label, value + defcode \name,\namelen,\flags,\label + push $\value + NEXT + .endm + + defconst "VERSION",7,,VERSION,JONES_VERSION + defconst "R0",2,,RZ,return_stack_top + defconst "DOCOL",5,,__DOCOL,DOCOL + defconst "F_IMMED",7,,__F_IMMED,F_IMMED + defconst "F_HIDDEN",8,,__F_HIDDEN,F_HIDDEN + defconst "F_LENMASK",9,,__F_LENMASK,F_LENMASK + + defconst "SYS_EXIT",8,,SYS_EXIT,__NR_exit + defconst "SYS_OPEN",8,,SYS_OPEN,__NR_open + defconst "SYS_CLOSE",9,,SYS_CLOSE,__NR_close + defconst "SYS_READ",8,,SYS_READ,__NR_read + defconst "SYS_WRITE",9,,SYS_WRITE,__NR_write + defconst "SYS_CREAT",9,,SYS_CREAT,__NR_creat + defconst "SYS_BRK",7,,SYS_BRK,__NR_brk + + defconst "O_RDONLY",8,,__O_RDONLY,0 + defconst "O_WRONLY",8,,__O_WRONLY,1 + defconst "O_RDWR",6,,__O_RDWR,2 + defconst "O_CREAT",7,,__O_CREAT,0100 + defconst "O_EXCL",6,,__O_EXCL,0200 + defconst "O_TRUNC",7,,__O_TRUNC,01000 + defconst "O_APPEND",8,,__O_APPEND,02000 + defconst "O_NONBLOCK",10,,__O_NONBLOCK,04000 /* - RETURN STACK ---------------------------------------------------------------------- + RETURN STACK ---------------------------------------------------------------------- - These words allow you to access the return stack. Recall that the register %ebp always points to - the top of the return stack. + These words allow you to access the return stack. Recall that the register %ebp always points to + the top of the return stack. */ - defcode ">R",2,,TOR - pop %eax // pop parameter stack into %eax - PUSHRSP %eax // push it on to the return stack - NEXT + defcode ">R",2,,TOR + pop %eax // pop parameter stack into %eax + PUSHRSP %eax // push it on to the return stack + NEXT - defcode "R>",2,,FROMR - POPRSP %eax // pop return stack on to %eax - push %eax // and push on to parameter stack - NEXT + defcode "R>",2,,FROMR + POPRSP %eax // pop return stack on to %eax + push %eax // and push on to parameter stack + NEXT - defcode "RSP@",4,,RSPFETCH - push %ebp - NEXT + defcode "RSP@",4,,RSPFETCH + push %ebp + NEXT - defcode "RSP!",4,,RSPSTORE - pop %ebp - NEXT + defcode "RSP!",4,,RSPSTORE + pop %ebp + NEXT - defcode "RDROP",5,,RDROP - addl $4,%ebp // pop return stack and throw away - NEXT + defcode "RDROP",5,,RDROP + addl $4,%ebp // pop return stack and throw away + NEXT /* - PARAMETER (DATA) STACK ---------------------------------------------------------------------- + PARAMETER (DATA) STACK ---------------------------------------------------------------------- - These functions allow you to manipulate the parameter stack. Recall that Linux sets up the parameter - stack for us, and it is accessed through %esp. + These functions allow you to manipulate the parameter stack. Recall that Linux sets up the parameter + stack for us, and it is accessed through %esp. */ - defcode "DSP@",4,,DSPFETCH - mov %esp,%eax - push %eax - NEXT + defcode "DSP@",4,,DSPFETCH + mov %esp,%eax + push %eax + NEXT - defcode "DSP!",4,,DSPSTORE - pop %esp - NEXT + defcode "DSP!",4,,DSPSTORE + pop %esp + NEXT /* - INPUT AND OUTPUT ---------------------------------------------------------------------- - - These are our first really meaty/complicated FORTH primitives. I have chosen to write them in - assembler, but surprisingly in "real" FORTH implementations these are often written in terms - of more fundamental FORTH primitives. I chose to avoid that because I think that just obscures - the implementation. After all, you may not understand assembler but you can just think of it - as an opaque block of code that does what it says. - - Let's discuss input first. - - The FORTH word KEY reads the next byte from stdin (and pushes it on the parameter stack). - So if KEY is called and someone hits the space key, then the number 32 (ASCII code of space) - is pushed on the stack. - - In FORTH there is no distinction between reading code and reading input. We might be reading - and compiling code, we might be reading words to execute, we might be asking for the user - to type their name -- ultimately it all comes in through KEY. - - The implementation of KEY uses an input buffer of a certain size (defined at the end of this - file). It calls the Linux read(2) system call to fill this buffer and tracks its position - in the buffer using a couple of variables, and if it runs out of input buffer then it refills - it automatically. The other thing that KEY does is if it detects that stdin has closed, it - exits the program, which is why when you hit ^D the FORTH system cleanly exits. - - buffer bufftop - | | - V V - +-------------------------------+--------------------------------------+ - | INPUT READ FROM STDIN ....... | unused part of the buffer | - +-------------------------------+--------------------------------------+ - ^ - | - currkey (next character to read) - - <---------------------- BUFFER_SIZE (4096 bytes) ----------------------> + INPUT AND OUTPUT ---------------------------------------------------------------------- + + These are our first really meaty/complicated FORTH primitives. I have chosen to write them in + assembler, but surprisingly in "real" FORTH implementations these are often written in terms + of more fundamental FORTH primitives. I chose to avoid that because I think that just obscures + the implementation. After all, you may not understand assembler but you can just think of it + as an opaque block of code that does what it says. + + Let's discuss input first. + + The FORTH word KEY reads the next byte from stdin (and pushes it on the parameter stack). + So if KEY is called and someone hits the space key, then the number 32 (ASCII code of space) + is pushed on the stack. + + In FORTH there is no distinction between reading code and reading input. We might be reading + and compiling code, we might be reading words to execute, we might be asking for the user + to type their name -- ultimately it all comes in through KEY. + + The implementation of KEY uses an input buffer of a certain size (defined at the end of this + file). It calls the Linux read(2) system call to fill this buffer and tracks its position + in the buffer using a couple of variables, and if it runs out of input buffer then it refills + it automatically. The other thing that KEY does is if it detects that stdin has closed, it + exits the program, which is why when you hit ^D the FORTH system cleanly exits. + + buffer bufftop + | | + V V + +-------------------------------+--------------------------------------+ + | INPUT READ FROM STDIN ....... | unused part of the buffer | + +-------------------------------+--------------------------------------+ + ^ + | + currkey (next character to read) + + <---------------------- BUFFER_SIZE (4096 bytes) ----------------------> */ - defcode "KEY",3,,KEY - call _KEY - push %eax // push return value on stack - NEXT + defcode "KEY",3,,KEY + call _KEY + push %eax // push return value on stack + NEXT _KEY: - mov (currkey),%ebx - cmp (bufftop),%ebx - jge 1f // exhausted the input buffer? - xor %eax,%eax - mov (%ebx),%al // get next key from input buffer - inc %ebx - mov %ebx,(currkey) // increment currkey - ret - -1: // Out of input; use read(2) to fetch more input from stdin. - xor %ebx,%ebx // 1st param: stdin - mov $buffer,%ecx // 2nd param: buffer - mov %ecx,currkey - mov $BUFFER_SIZE,%edx // 3rd param: max length - mov $__NR_read,%eax // syscall: read - int $0x80 - test %eax,%eax // If %eax <= 0, then exit. - jbe 2f - addl %eax,%ecx // buffer+%eax = bufftop - mov %ecx,bufftop - jmp _KEY - -2: // Error or end of input: exit the program. - xor %ebx,%ebx - mov $__NR_exit,%eax // syscall: exit - int $0x80 - - .data - .align 4 + mov (currkey),%ebx + cmp (bufftop),%ebx + jge 1f // exhausted the input buffer? + xor %eax,%eax + mov (%ebx),%al // get next key from input buffer + inc %ebx + mov %ebx,(currkey) // increment currkey + ret + +1: // Out of input; use read(2) to fetch more input from stdin. + xor %ebx,%ebx // 1st param: stdin + mov $buffer,%ecx // 2nd param: buffer + mov %ecx,currkey + mov $BUFFER_SIZE,%edx // 3rd param: max length + mov $__NR_read,%eax // syscall: read + int $0x80 + test %eax,%eax // If %eax <= 0, then exit. + jbe 2f + addl %eax,%ecx // buffer+%eax = bufftop + mov %ecx,bufftop + jmp _KEY + +2: // Error or end of input: exit the program. + xor %ebx,%ebx + mov $__NR_exit,%eax // syscall: exit + int $0x80 + + .data + .align 4 currkey: - .int buffer // Current place in input buffer (next character to read). + .int buffer // Current place in input buffer (next character to read). bufftop: - .int buffer // Last valid data in input buffer + 1. + .int buffer // Last valid data in input buffer + 1. /* - By contrast, output is much simpler. The FORTH word EMIT writes out a single byte to stdout. - This implementation just uses the write system call. No attempt is made to buffer output, but - it would be a good exercise to add it. + By contrast, output is much simpler. The FORTH word EMIT writes out a single byte to stdout. + This implementation just uses the write system call. No attempt is made to buffer output, but + it would be a good exercise to add it. */ - defcode "EMIT",4,,EMIT - pop %eax - call _EMIT - NEXT + defcode "EMIT",4,,EMIT + pop %eax + call _EMIT + NEXT _EMIT: - mov $1,%ebx // 1st param: stdout + mov $1,%ebx // 1st param: stdout - // write needs the address of the byte to write - mov %al,emit_scratch - mov $emit_scratch,%ecx // 2nd param: address + // write needs the address of the byte to write + mov %al,emit_scratch + mov $emit_scratch,%ecx // 2nd param: address - mov $1,%edx // 3rd param: nbytes = 1 + mov $1,%edx // 3rd param: nbytes = 1 - mov $__NR_write,%eax // write syscall - int $0x80 - ret + mov $__NR_write,%eax // write syscall + int $0x80 + ret - .data // NB: easier to fit in the .data section + .data // NB: easier to fit in the .data section emit_scratch: - .space 1 // scratch used by EMIT + .space 1 // scratch used by EMIT /* - Back to input, WORD is a FORTH word which reads the next full word of input. - - What it does in detail is that it first skips any blanks (spaces, tabs, newlines and so on). - Then it calls KEY to read characters into an internal buffer until it hits a blank. Then it - calculates the length of the word it read and returns the address and the length as - two words on the stack (with the length at the top of stack). - - Notice that WORD has a single internal buffer which it overwrites each time (rather like - a static C string). Also notice that WORD's internal buffer is just 32 bytes long and - there is NO checking for overflow. 31 bytes happens to be the maximum length of a - FORTH word that we support, and that is what WORD is used for: to read FORTH words when - we are compiling and executing code. The returned strings are not NUL-terminated. - - Start address+length is the normal way to represent strings in FORTH (not ending in an - ASCII NUL character as in C), and so FORTH strings can contain any character including NULs - and can be any length. - - WORD is not suitable for just reading strings (eg. user input) because of all the above - peculiarities and limitations. - - Note that when executing, you'll see: - WORD FOO - which puts "FOO" and length 3 on the stack, but when compiling: - : BAR WORD FOO ; - is an error (or at least it doesn't do what you might expect). Later we'll talk about compiling - and immediate mode, and you'll understand why. + Back to input, WORD is a FORTH word which reads the next full word of input. + + What it does in detail is that it first skips any blanks (spaces, tabs, newlines and so on). + Then it calls KEY to read characters into an internal buffer until it hits a blank. Then it + calculates the length of the word it read and returns the address and the length as + two words on the stack (with the length at the top of stack). + + Notice that WORD has a single internal buffer which it overwrites each time (rather like + a static C string). Also notice that WORD's internal buffer is just 32 bytes long and + there is NO checking for overflow. 31 bytes happens to be the maximum length of a + FORTH word that we support, and that is what WORD is used for: to read FORTH words when + we are compiling and executing code. The returned strings are not NUL-terminated. + + Start address+length is the normal way to represent strings in FORTH (not ending in an + ASCII NUL character as in C), and so FORTH strings can contain any character including NULs + and can be any length. + + WORD is not suitable for just reading strings (eg. user input) because of all the above + peculiarities and limitations. + + Note that when executing, you'll see: + WORD FOO + which puts "FOO" and length 3 on the stack, but when compiling: + : BAR WORD FOO ; + is an error (or at least it doesn't do what you might expect). Later we'll talk about compiling + and immediate mode, and you'll understand why. */ - defcode "WORD",4,,WORD - call _WORD - push %edi // push base address - push %ecx // push length - NEXT + defcode "WORD",4,,WORD + call _WORD + push %edi // push base address + push %ecx // push length + NEXT _WORD: - /* Search for first non-blank character. Also skip \ comments. */ + /* Search for first non-blank character. Also skip \ comments. */ 1: - call _KEY // get next key, returned in %eax - cmpb $'\\',%al // start of a comment? - je 3f // if so, skip the comment - cmpb $' ',%al - jbe 1b // if so, keep looking - - /* Search for the end of the word, storing chars as we go. */ - mov $word_buffer,%edi // pointer to return buffer + call _KEY // get next key, returned in %eax + cmpb $'\\',%al // start of a comment? + je 3f // if so, skip the comment + cmpb $' ',%al + jbe 1b // if so, keep looking + + /* Search for the end of the word, storing chars as we go. */ + mov $word_buffer,%edi // pointer to return buffer 2: - stosb // add character to return buffer - call _KEY // get next key, returned in %al - cmpb $' ',%al // is blank? - ja 2b // if not, keep looping - - /* Return the word (well, the static buffer) and length. */ - sub $word_buffer,%edi - mov %edi,%ecx // return length of the word - mov $word_buffer,%edi // return address of the word - ret - - /* Code to skip \ comments to end of the current line. */ + stosb // add character to return buffer + call _KEY // get next key, returned in %al + cmpb $' ',%al // is blank? + ja 2b // if not, keep looping + + /* Return the word (well, the static buffer) and length. */ + sub $word_buffer,%edi + mov %edi,%ecx // return length of the word + mov $word_buffer,%edi // return address of the word + ret + + /* Code to skip \ comments to end of the current line. */ 3: - call _KEY - cmpb $'\n',%al // end of line yet? - jne 3b - jmp 1b - - .data // NB: easier to fit in the .data section - // A static buffer where WORD returns. Subsequent calls - // overwrite this buffer. Maximum word length is 32 chars. + call _KEY + cmpb $'\n',%al // end of line yet? + jne 3b + jmp 1b + + .data // NB: easier to fit in the .data section + // A static buffer where WORD returns. Subsequent calls + // overwrite this buffer. Maximum word length is 32 chars. word_buffer: - .space 32 + .space 32 /* - As well as reading in words we'll need to read in numbers and for that we are using a function - called NUMBER. This parses a numeric string such as one returned by WORD and pushes the - number on the parameter stack. + As well as reading in words we'll need to read in numbers and for that we are using a function + called NUMBER. This parses a numeric string such as one returned by WORD and pushes the + number on the parameter stack. - The function uses the variable BASE as the base (radix) for conversion, so for example if - BASE is 2 then we expect a binary number. Normally BASE is 10. + The function uses the variable BASE as the base (radix) for conversion, so for example if + BASE is 2 then we expect a binary number. Normally BASE is 10. - If the word starts with a '-' character then the returned value is negative. + If the word starts with a '-' character then the returned value is negative. - If the string can't be parsed as a number (or contains characters outside the current BASE) - then we need to return an error indication. So NUMBER actually returns two items on the stack. - At the top of stack we return the number of unconverted characters (ie. if 0 then all characters - were converted, so there is no error). Second from top of stack is the parsed number or a - partial value if there was an error. + If the string can't be parsed as a number (or contains characters outside the current BASE) + then we need to return an error indication. So NUMBER actually returns two items on the stack. + At the top of stack we return the number of unconverted characters (ie. if 0 then all characters + were converted, so there is no error). Second from top of stack is the parsed number or a + partial value if there was an error. */ - defcode "NUMBER",6,,NUMBER - pop %ecx // length of string - pop %edi // start address of string - call _NUMBER - push %eax // parsed number - push %ecx // number of unparsed characters (0 = no error) - NEXT + defcode "NUMBER",6,,NUMBER + pop %ecx // length of string + pop %edi // start address of string + call _NUMBER + push %eax // parsed number + push %ecx // number of unparsed characters (0 = no error) + NEXT _NUMBER: - xor %eax,%eax - xor %ebx,%ebx - - test %ecx,%ecx // trying to parse a zero-length string is an error, but will return 0. - jz 5f - - movl var_BASE,%edx // get BASE (in %dl) - - // Check if first character is '-'. - movb (%edi),%bl // %bl = first character in string - inc %edi - push %eax // push 0 on stack - cmpb $'-',%bl // negative number? - jnz 2f - pop %eax - push %ebx // push <> 0 on stack, indicating negative - dec %ecx - jnz 1f - pop %ebx // error: string is only '-'. - movl $1,%ecx - ret - - // Loop reading digits. -1: imull %edx,%eax // %eax *= BASE - movb (%edi),%bl // %bl = next character in string - inc %edi - - // Convert 0-9, A-Z to a number 0-35. -2: subb $'0',%bl // < '0'? - jb 4f - cmp $10,%bl // <= '9'? - jb 3f - subb $17,%bl // < 'A'? (17 is 'A'-'0') - jb 4f - addb $10,%bl - -3: cmp %dl,%bl // >= BASE? - jge 4f - - // OK, so add it to %eax and loop. - add %ebx,%eax - dec %ecx - jnz 1b - - // Negate the result if first character was '-' (saved on the stack). -4: pop %ebx - test %ebx,%ebx - jz 5f - neg %eax - -5: ret + xor %eax,%eax + xor %ebx,%ebx + + test %ecx,%ecx // trying to parse a zero-length string is an error, but will return 0. + jz 5f + + movl var_BASE,%edx // get BASE (in %dl) + + // Check if first character is '-'. + movb (%edi),%bl // %bl = first character in string + inc %edi + push %eax // push 0 on stack + cmpb $'-',%bl // negative number? + jnz 2f + pop %eax + push %ebx // push <> 0 on stack, indicating negative + dec %ecx + jnz 1f + pop %ebx // error: string is only '-'. + movl $1,%ecx + ret + + // Loop reading digits. +1: imull %edx,%eax // %eax *= BASE + movb (%edi),%bl // %bl = next character in string + inc %edi + + // Convert 0-9, A-Z to a number 0-35. +2: subb $'0',%bl // < '0'? + jb 4f + cmp $10,%bl // <= '9'? + jb 3f + subb $17,%bl // < 'A'? (17 is 'A'-'0') + jb 4f + addb $10,%bl + +3: cmp %dl,%bl // >= BASE? + jge 4f + + // OK, so add it to %eax and loop. + add %ebx,%eax + dec %ecx + jnz 1b + + // Negate the result if first character was '-' (saved on the stack). +4: pop %ebx + test %ebx,%ebx + jz 5f + neg %eax + +5: ret /* - DICTIONARY LOOK UPS ---------------------------------------------------------------------- + DICTIONARY LOOK UPS ---------------------------------------------------------------------- - We're building up to our prelude on how FORTH code is compiled, but first we need yet more infrastructure. + We're building up to our prelude on how FORTH code is compiled, but first we need yet more infrastructure. - The FORTH word FIND takes a string (a word as parsed by WORD -- see above) and looks it up in the - dictionary. What it actually returns is the address of the dictionary header, if it finds it, - or 0 if it didn't. + The FORTH word FIND takes a string (a word as parsed by WORD -- see above) and looks it up in the + dictionary. What it actually returns is the address of the dictionary header, if it finds it, + or 0 if it didn't. - So if DOUBLE is defined in the dictionary, then WORD DOUBLE FIND returns the following pointer: + So if DOUBLE is defined in the dictionary, then WORD DOUBLE FIND returns the following pointer: pointer to this - | - | - V - +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ - | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | - +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ + | + | + V + +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ + | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | + +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ - See also >CFA and >DFA. + See also >CFA and >DFA. - FIND doesn't find dictionary entries which are flagged as HIDDEN. See below for why. + FIND doesn't find dictionary entries which are flagged as HIDDEN. See below for why. */ - defcode "FIND",4,,FIND - pop %ecx // %ecx = length - pop %edi // %edi = address - call _FIND - push %eax // %eax = address of dictionary entry (or NULL) - NEXT + defcode "FIND",4,,FIND + pop %ecx // %ecx = length + pop %edi // %edi = address + call _FIND + push %eax // %eax = address of dictionary entry (or NULL) + NEXT _FIND: - push %esi // Save %esi so we can use it in string comparison. - - // Now we start searching backwards through the dictionary for this word. - mov var_LATEST,%edx // LATEST points to name header of the latest word in the dictionary -1: test %edx,%edx // NULL pointer? (end of the linked list) - je 4f - - // Compare the length expected and the length of the word. - // Note that if the F_HIDDEN flag is set on the word, then by a bit of trickery - // this won't pick the word (the length will appear to be wrong). - xor %eax,%eax - movb 4(%edx),%al // %al = flags+length field - andb $(F_HIDDEN|F_LENMASK),%al // %al = name length - cmpb %cl,%al // Length is the same? - jne 2f - - // Compare the strings in detail. - push %ecx // Save the length - push %edi // Save the address (repe cmpsb will move this pointer) - lea 5(%edx),%esi // Dictionary string we are checking against. - repe cmpsb // Compare the strings. - pop %edi - pop %ecx - jne 2f // Not the same. - - // The strings are the same - return the header pointer in %eax - pop %esi - mov %edx,%eax - ret - -2: mov (%edx),%edx // Move back through the link field to the previous word - jmp 1b // .. and loop. - -4: // Not found. - pop %esi - xor %eax,%eax // Return zero to indicate not found. - ret + push %esi // Save %esi so we can use it in string comparison. + + // Now we start searching backwards through the dictionary for this word. + mov var_LATEST,%edx // LATEST points to name header of the latest word in the dictionary +1: test %edx,%edx // NULL pointer? (end of the linked list) + je 4f + + // Compare the length expected and the length of the word. + // Note that if the F_HIDDEN flag is set on the word, then by a bit of trickery + // this won't pick the word (the length will appear to be wrong). + xor %eax,%eax + movb 4(%edx),%al // %al = flags+length field + andb $(F_HIDDEN|F_LENMASK),%al // %al = name length + cmpb %cl,%al // Length is the same? + jne 2f + + // Compare the strings in detail. + push %ecx // Save the length + push %edi // Save the address (repe cmpsb will move this pointer) + lea 5(%edx),%esi // Dictionary string we are checking against. + repe cmpsb // Compare the strings. + pop %edi + pop %ecx + jne 2f // Not the same. + + // The strings are the same - return the header pointer in %eax + pop %esi + mov %edx,%eax + ret + +2: mov (%edx),%edx // Move back through the link field to the previous word + jmp 1b // .. and loop. + +4: // Not found. + pop %esi + xor %eax,%eax // Return zero to indicate not found. + ret /* - FIND returns the dictionary pointer, but when compiling we need the codeword pointer (recall - that FORTH definitions are compiled into lists of codeword pointers). The standard FORTH - word >CFA turns a dictionary pointer into a codeword pointer. + FIND returns the dictionary pointer, but when compiling we need the codeword pointer (recall + that FORTH definitions are compiled into lists of codeword pointers). The standard FORTH + word >CFA turns a dictionary pointer into a codeword pointer. - The example below shows the result of: + The example below shows the result of: - WORD DOUBLE FIND >CFA + WORD DOUBLE FIND >CFA - FIND returns a pointer to this - | >CFA converts it to a pointer to this - | | - V V - +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ - | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | - +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ - codeword + FIND returns a pointer to this + | >CFA converts it to a pointer to this + | | + V V + +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ + | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | + +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ + codeword - Notes: + Notes: - Because names vary in length, this isn't just a simple increment. + Because names vary in length, this isn't just a simple increment. - In this FORTH you cannot easily turn a codeword pointer back into a dictionary entry pointer, but - that is not true in most FORTH implementations where they store a back pointer in the definition - (with an obvious memory/complexity cost). The reason they do this is that it is useful to be - able to go backwards (codeword -> dictionary entry) in order to decompile FORTH definitions - quickly. + In this FORTH you cannot easily turn a codeword pointer back into a dictionary entry pointer, but + that is not true in most FORTH implementations where they store a back pointer in the definition + (with an obvious memory/complexity cost). The reason they do this is that it is useful to be + able to go backwards (codeword -> dictionary entry) in order to decompile FORTH definitions + quickly. - What does CFA stand for? My best guess is "Code Field Address". + What does CFA stand for? My best guess is "Code Field Address". */ - defcode ">CFA",4,,TCFA - pop %edi - call _TCFA - push %edi - NEXT + defcode ">CFA",4,,TCFA + pop %edi + call _TCFA + push %edi + NEXT _TCFA: - xor %eax,%eax - add $4,%edi // Skip link pointer. - movb (%edi),%al // Load flags+len into %al. - inc %edi // Skip flags+len byte. - andb $F_LENMASK,%al // Just the length, not the flags. - add %eax,%edi // Skip the name. - addl $3,%edi // The codeword is 4-byte aligned. - andl $~3,%edi - ret + xor %eax,%eax + add $4,%edi // Skip link pointer. + movb (%edi),%al // Load flags+len into %al. + inc %edi // Skip flags+len byte. + andb $F_LENMASK,%al // Just the length, not the flags. + add %eax,%edi // Skip the name. + addl $3,%edi // The codeword is 4-byte aligned. + andl $~3,%edi + ret /* - Related to >CFA is >DFA which takes a dictionary entry address as returned by FIND and - returns a pointer to the first data field. - - FIND returns a pointer to this - | >CFA converts it to a pointer to this - | | - | | >DFA converts it to a pointer to this - | | | - V V V - +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ - | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | - +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ - codeword - - (Note to those following the source of FIG-FORTH / ciforth: My >DFA definition is - different from theirs, because they have an extra indirection). - - You can see that >DFA is easily defined in FORTH just by adding 4 to the result of >CFA. + Related to >CFA is >DFA which takes a dictionary entry address as returned by FIND and + returns a pointer to the first data field. + + FIND returns a pointer to this + | >CFA converts it to a pointer to this + | | + | | >DFA converts it to a pointer to this + | | | + V V V + +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ + | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | + +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ + codeword + + (Note to those following the source of FIG-FORTH / ciforth: My >DFA definition is + different from theirs, because they have an extra indirection). + + You can see that >DFA is easily defined in FORTH just by adding 4 to the result of >CFA. */ - defword ">DFA",4,,TDFA - .int TCFA // >CFA (get code field address) - .int INCR4 // 4+ (add 4 to it to get to next word) - .int EXIT // EXIT (return from FORTH word) + defword ">DFA",4,,TDFA + .int TCFA // >CFA (get code field address) + .int INCR4 // 4+ (add 4 to it to get to next word) + .int EXIT // EXIT (return from FORTH word) /* - COMPILING ---------------------------------------------------------------------- + COMPILING ---------------------------------------------------------------------- - Now we'll talk about how FORTH compiles words. Recall that a word definition looks like this: + Now we'll talk about how FORTH compiles words. Recall that a word definition looks like this: - : DOUBLE DUP + ; + : DOUBLE DUP + ; - and we have to turn this into: + and we have to turn this into: - pointer to previous word - ^ - | - +--|------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ - | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | - +---------+---+---+---+---+---+---+---+---+------------+--|---------+------------+------------+ + pointer to previous word + ^ + | + +--|------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ + | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | + +---------+---+---+---+---+---+---+---+---+------------+--|---------+------------+------------+ ^ len pad codeword | - | V - LATEST points here points to codeword of DUP + | V + LATEST points here points to codeword of DUP - There are several problems to solve. Where to put the new word? How do we read words? How - do we define the words : (COLON) and ; (SEMICOLON)? + There are several problems to solve. Where to put the new word? How do we read words? How + do we define the words : (COLON) and ; (SEMICOLON)? - FORTH solves this rather elegantly and as you might expect in a very low-level way which - allows you to change how the compiler works on your own code. + FORTH solves this rather elegantly and as you might expect in a very low-level way which + allows you to change how the compiler works on your own code. - FORTH has an INTERPRET function (a true interpreter this time, not DOCOL) which runs in a - loop, reading words (using WORD), looking them up (using FIND), turning them into codeword - pointers (using >CFA) and deciding what to do with them. + FORTH has an INTERPRET function (a true interpreter this time, not DOCOL) which runs in a + loop, reading words (using WORD), looking them up (using FIND), turning them into codeword + pointers (using >CFA) and deciding what to do with them. - What it does depends on the mode of the interpreter (in variable STATE). + What it does depends on the mode of the interpreter (in variable STATE). - When STATE is zero, the interpreter just runs each word as it looks them up. This is known as - immediate mode. + When STATE is zero, the interpreter just runs each word as it looks them up. This is known as + immediate mode. - The interesting stuff happens when STATE is non-zero -- compiling mode. In this mode the - interpreter appends the codeword pointer to user memory (the HERE variable points to the next - free byte of user memory -- see DATA SEGMENT section below). + The interesting stuff happens when STATE is non-zero -- compiling mode. In this mode the + interpreter appends the codeword pointer to user memory (the HERE variable points to the next + free byte of user memory -- see DATA SEGMENT section below). - So you may be able to see how we could define : (COLON). The general plan is: + So you may be able to see how we could define : (COLON). The general plan is: - (1) Use WORD to read the name of the function being defined. + (1) Use WORD to read the name of the function being defined. - (2) Construct the dictionary entry -- just the header part -- in user memory: + (2) Construct the dictionary entry -- just the header part -- in user memory: - pointer to previous word (from LATEST) +-- Afterwards, HERE points here, where - ^ | the interpreter will start appending - | V codewords. - +--|------+---+---+---+---+---+---+---+---+------------+ - | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | - +---------+---+---+---+---+---+---+---+---+------------+ + pointer to previous word (from LATEST) +-- Afterwards, HERE points here, where + ^ | the interpreter will start appending + | V codewords. + +--|------+---+---+---+---+---+---+---+---+------------+ + | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | + +---------+---+---+---+---+---+---+---+---+------------+ len pad codeword - (3) Set LATEST to point to the newly defined word, ... + (3) Set LATEST to point to the newly defined word, ... - (4) .. and most importantly leave HERE pointing just after the new codeword. This is where - the interpreter will append codewords. + (4) .. and most importantly leave HERE pointing just after the new codeword. This is where + the interpreter will append codewords. - (5) Set STATE to 1. This goes into compile mode so the interpreter starts appending codewords to - our partially-formed header. + (5) Set STATE to 1. This goes into compile mode so the interpreter starts appending codewords to + our partially-formed header. - After : has run, our input is here: + After : has run, our input is here: - : DOUBLE DUP + ; - ^ - | - Next byte returned by KEY will be the 'D' character of DUP + : DOUBLE DUP + ; + ^ + | + Next byte returned by KEY will be the 'D' character of DUP - so the interpreter (now it's in compile mode, so I guess it's really the compiler) reads "DUP", - looks it up in the dictionary, gets its codeword pointer, and appends it: + so the interpreter (now it's in compile mode, so I guess it's really the compiler) reads "DUP", + looks it up in the dictionary, gets its codeword pointer, and appends it: - +-- HERE updated to point here. - | - V - +---------+---+---+---+---+---+---+---+---+------------+------------+ - | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | - +---------+---+---+---+---+---+---+---+---+------------+------------+ + +-- HERE updated to point here. + | + V + +---------+---+---+---+---+---+---+---+---+------------+------------+ + | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + +---------+---+---+---+---+---+---+---+---+------------+------------+ len pad codeword - Next we read +, get the codeword pointer, and append it: + Next we read +, get the codeword pointer, and append it: - +-- HERE updated to point here. - | - V - +---------+---+---+---+---+---+---+---+---+------------+------------+------------+ - | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | - +---------+---+---+---+---+---+---+---+---+------------+------------+------------+ + +-- HERE updated to point here. + | + V + +---------+---+---+---+---+---+---+---+---+------------+------------+------------+ + | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | + +---------+---+---+---+---+---+---+---+---+------------+------------+------------+ len pad codeword - The issue is what happens next. Obviously what we _don't_ want to happen is that we - read ";" and compile it and go on compiling everything afterwards. - - At this point, FORTH uses a trick. Remember the length byte in the dictionary definition - isn't just a plain length byte, but can also contain flags. One flag is called the - IMMEDIATE flag (F_IMMED in this code). If a word in the dictionary is flagged as - IMMEDIATE then the interpreter runs it immediately _even if it's in compile mode_. - - This is how the word ; (SEMICOLON) works -- as a word flagged in the dictionary as IMMEDIATE. - - And all it does is append the codeword for EXIT on to the current definition and switch - back to immediate mode (set STATE back to 0). Shortly we'll see the actual definition - of ; and we'll see that it's really a very simple definition, declared IMMEDIATE. - - After the interpreter reads ; and executes it 'immediately', we get this: - - +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ - | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | - +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ - len pad codeword ^ - | - HERE - STATE is set to 0. - - And that's it, job done, our new definition is compiled, and we're back in immediate mode - just reading and executing words, perhaps including a call to test our new word DOUBLE. - - The only last wrinkle in this is that while our word was being compiled, it was in a - half-finished state. We certainly wouldn't want DOUBLE to be called somehow during - this time. There are several ways to stop this from happening, but in FORTH what we - do is flag the word with the HIDDEN flag (F_HIDDEN in this code) just while it is - being compiled. This prevents FIND from finding it, and thus in theory stops any - chance of it being called. - - The above explains how compiling, : (COLON) and ; (SEMICOLON) works and in a moment I'm - going to define them. The : (COLON) function can be made a little bit more general by writing - it in two parts. The first part, called CREATE, makes just the header: - - +-- Afterwards, HERE points here. - | - V - +---------+---+---+---+---+---+---+---+---+ - | LINK | 6 | D | O | U | B | L | E | 0 | - +---------+---+---+---+---+---+---+---+---+ + The issue is what happens next. Obviously what we _don't_ want to happen is that we + read ";" and compile it and go on compiling everything afterwards. + + At this point, FORTH uses a trick. Remember the length byte in the dictionary definition + isn't just a plain length byte, but can also contain flags. One flag is called the + IMMEDIATE flag (F_IMMED in this code). If a word in the dictionary is flagged as + IMMEDIATE then the interpreter runs it immediately _even if it's in compile mode_. + + This is how the word ; (SEMICOLON) works -- as a word flagged in the dictionary as IMMEDIATE. + + And all it does is append the codeword for EXIT on to the current definition and switch + back to immediate mode (set STATE back to 0). Shortly we'll see the actual definition + of ; and we'll see that it's really a very simple definition, declared IMMEDIATE. + + After the interpreter reads ; and executes it 'immediately', we get this: + + +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ + | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | + +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ + len pad codeword ^ + | + HERE + STATE is set to 0. + + And that's it, job done, our new definition is compiled, and we're back in immediate mode + just reading and executing words, perhaps including a call to test our new word DOUBLE. + + The only last wrinkle in this is that while our word was being compiled, it was in a + half-finished state. We certainly wouldn't want DOUBLE to be called somehow during + this time. There are several ways to stop this from happening, but in FORTH what we + do is flag the word with the HIDDEN flag (F_HIDDEN in this code) just while it is + being compiled. This prevents FIND from finding it, and thus in theory stops any + chance of it being called. + + The above explains how compiling, : (COLON) and ; (SEMICOLON) works and in a moment I'm + going to define them. The : (COLON) function can be made a little bit more general by writing + it in two parts. The first part, called CREATE, makes just the header: + + +-- Afterwards, HERE points here. + | + V + +---------+---+---+---+---+---+---+---+---+ + | LINK | 6 | D | O | U | B | L | E | 0 | + +---------+---+---+---+---+---+---+---+---+ len pad - and the second part, the actual definition of : (COLON), calls CREATE and appends the - DOCOL codeword, so leaving: + and the second part, the actual definition of : (COLON), calls CREATE and appends the + DOCOL codeword, so leaving: - +-- Afterwards, HERE points here. - | - V - +---------+---+---+---+---+---+---+---+---+------------+ - | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | - +---------+---+---+---+---+---+---+---+---+------------+ + +-- Afterwards, HERE points here. + | + V + +---------+---+---+---+---+---+---+---+---+------------+ + | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | + +---------+---+---+---+---+---+---+---+---+------------+ len pad codeword - CREATE is a standard FORTH word and the advantage of this split is that we can reuse it to - create other types of words (not just ones which contain code, but words which contain variables, - constants and other data). + CREATE is a standard FORTH word and the advantage of this split is that we can reuse it to + create other types of words (not just ones which contain code, but words which contain variables, + constants and other data). */ - defcode "CREATE",6,,CREATE - - // Get the name length and address. - pop %ecx // %ecx = length - pop %ebx // %ebx = address of name - - // Link pointer. - movl var_HERE,%edi // %edi is the address of the header - movl var_LATEST,%eax // Get link pointer - stosl // and store it in the header. - - // Length byte and the word itself. - mov %cl,%al // Get the length. - stosb // Store the length/flags byte. - push %esi - mov %ebx,%esi // %esi = word - rep movsb // Copy the word - pop %esi - addl $3,%edi // Align to next 4 byte boundary. - andl $~3,%edi - - // Update LATEST and HERE. - movl var_HERE,%eax - movl %eax,var_LATEST - movl %edi,var_HERE - NEXT + defcode "CREATE",6,,CREATE + + // Get the name length and address. + pop %ecx // %ecx = length + pop %ebx // %ebx = address of name + + // Link pointer. + movl var_HERE,%edi // %edi is the address of the header + movl var_LATEST,%eax // Get link pointer + stosl // and store it in the header. + + // Length byte and the word itself. + mov %cl,%al // Get the length. + stosb // Store the length/flags byte. + push %esi + mov %ebx,%esi // %esi = word + rep movsb // Copy the word + pop %esi + addl $3,%edi // Align to next 4 byte boundary. + andl $~3,%edi + + // Update LATEST and HERE. + movl var_HERE,%eax + movl %eax,var_LATEST + movl %edi,var_HERE + NEXT /* - Because I want to define : (COLON) in FORTH, not assembler, we need a few more FORTH words - to use. - - The first is , (COMMA) which is a standard FORTH word which appends a 32 bit integer to the user - memory pointed to by HERE, and adds 4 to HERE. So the action of , (COMMA) is: - - previous value of HERE - | - V - +---------+---+---+---+---+---+---+---+---+-- - - - - --+------------+ - | LINK | 6 | D | O | U | B | L | E | 0 | | | - +---------+---+---+---+---+---+---+---+---+-- - - - - --+------------+ - len pad ^ - | - new value of HERE - - and is whatever 32 bit integer was at the top of the stack. - - , (COMMA) is quite a fundamental operation when compiling. It is used to append codewords - to the current word that is being compiled. + Because I want to define : (COLON) in FORTH, not assembler, we need a few more FORTH words + to use. + + The first is , (COMMA) which is a standard FORTH word which appends a 32 bit integer to the user + memory pointed to by HERE, and adds 4 to HERE. So the action of , (COMMA) is: + + previous value of HERE + | + V + +---------+---+---+---+---+---+---+---+---+-- - - - - --+------------+ + | LINK | 6 | D | O | U | B | L | E | 0 | | | + +---------+---+---+---+---+---+---+---+---+-- - - - - --+------------+ + len pad ^ + | + new value of HERE + + and is whatever 32 bit integer was at the top of the stack. + + , (COMMA) is quite a fundamental operation when compiling. It is used to append codewords + to the current word that is being compiled. */ - defcode ",",1,,COMMA - pop %eax // Code pointer to store. - call _COMMA - NEXT + defcode ",",1,,COMMA + pop %eax // Code pointer to store. + call _COMMA + NEXT _COMMA: - movl var_HERE,%edi // HERE - stosl // Store it. - movl %edi,var_HERE // Update HERE (incremented) - ret + movl var_HERE,%edi // HERE + stosl // Store it. + movl %edi,var_HERE // Update HERE (incremented) + ret /* - Our definitions of : (COLON) and ; (SEMICOLON) will need to switch to and from compile mode. + Our definitions of : (COLON) and ; (SEMICOLON) will need to switch to and from compile mode. - Immediate mode vs. compile mode is stored in the global variable STATE, and by updating this - variable we can switch between the two modes. + Immediate mode vs. compile mode is stored in the global variable STATE, and by updating this + variable we can switch between the two modes. - For various reasons which may become apparent later, FORTH defines two standard words called - [ and ] (LBRAC and RBRAC) which switch between modes: + For various reasons which may become apparent later, FORTH defines two standard words called + [ and ] (LBRAC and RBRAC) which switch between modes: - Word Assembler Action Effect - [ LBRAC STATE := 0 Switch to immediate mode. - ] RBRAC STATE := 1 Switch to compile mode. + Word Assembler Action Effect + [ LBRAC STATE := 0 Switch to immediate mode. + ] RBRAC STATE := 1 Switch to compile mode. - [ (LBRAC) is an IMMEDIATE word. The reason is as follows: If we are in compile mode and the - interpreter saw [ then it would compile it rather than running it. We would never be able to - switch back to immediate mode! So we flag the word as IMMEDIATE so that even in compile mode - the word runs immediately, switching us back to immediate mode. + [ (LBRAC) is an IMMEDIATE word. The reason is as follows: If we are in compile mode and the + interpreter saw [ then it would compile it rather than running it. We would never be able to + switch back to immediate mode! So we flag the word as IMMEDIATE so that even in compile mode + the word runs immediately, switching us back to immediate mode. */ - defcode "[",1,F_IMMED,LBRAC - xor %eax,%eax - movl %eax,var_STATE // Set STATE to 0. - NEXT + defcode "[",1,F_IMMED,LBRAC + xor %eax,%eax + movl %eax,var_STATE // Set STATE to 0. + NEXT - defcode "]",1,,RBRAC - movl $1,var_STATE // Set STATE to 1. - NEXT + defcode "]",1,,RBRAC + movl $1,var_STATE // Set STATE to 1. + NEXT /* - Now we can define : (COLON) using CREATE. It just calls CREATE, appends DOCOL (the codeword), sets - the word HIDDEN and goes into compile mode. + Now we can define : (COLON) using CREATE. It just calls CREATE, appends DOCOL (the codeword), sets + the word HIDDEN and goes into compile mode. */ - defword ":",1,,COLON - .int WORD // Get the name of the new word - .int CREATE // CREATE the dictionary entry / header - .int LIT, DOCOL, COMMA // Append DOCOL (the codeword). - .int LATEST, FETCH, HIDDEN // Make the word hidden (see below for definition). - .int RBRAC // Go into compile mode. - .int EXIT // Return from the function. + defword ":",1,,COLON + .int WORD // Get the name of the new word + .int CREATE // CREATE the dictionary entry / header + .int LIT, DOCOL, COMMA // Append DOCOL (the codeword). + .int LATEST, FETCH, HIDDEN // Make the word hidden (see below for definition). + .int RBRAC // Go into compile mode. + .int EXIT // Return from the function. /* - ; (SEMICOLON) is also elegantly simple. Notice the F_IMMED flag. + ; (SEMICOLON) is also elegantly simple. Notice the F_IMMED flag. */ - defword ";",1,F_IMMED,SEMICOLON - .int LIT, EXIT, COMMA // Append EXIT (so the word will return). - .int LATEST, FETCH, HIDDEN // Toggle hidden flag -- unhide the word (see below for definition). - .int LBRAC // Go back to IMMEDIATE mode. - .int EXIT // Return from the function. + defword ";",1,F_IMMED,SEMICOLON + .int LIT, EXIT, COMMA // Append EXIT (so the word will return). + .int LATEST, FETCH, HIDDEN // Toggle hidden flag -- unhide the word (see below for definition). + .int LBRAC // Go back to IMMEDIATE mode. + .int EXIT // Return from the function. /* - EXTENDING THE COMPILER ---------------------------------------------------------------------- + EXTENDING THE COMPILER ---------------------------------------------------------------------- - Words flagged with IMMEDIATE (F_IMMED) aren't just for the FORTH compiler to use. You can define - your own IMMEDIATE words too, and this is a crucial aspect when extending basic FORTH, because - it allows you in effect to extend the compiler itself. Does gcc let you do that? + Words flagged with IMMEDIATE (F_IMMED) aren't just for the FORTH compiler to use. You can define + your own IMMEDIATE words too, and this is a crucial aspect when extending basic FORTH, because + it allows you in effect to extend the compiler itself. Does gcc let you do that? - Standard FORTH words like IF, WHILE, ." and so on are all written as extensions to the basic - compiler, and are all IMMEDIATE words. + Standard FORTH words like IF, WHILE, ." and so on are all written as extensions to the basic + compiler, and are all IMMEDIATE words. - The IMMEDIATE word toggles the F_IMMED (IMMEDIATE flag) on the most recently defined word, - or on the current word if you call it in the middle of a definition. + The IMMEDIATE word toggles the F_IMMED (IMMEDIATE flag) on the most recently defined word, + or on the current word if you call it in the middle of a definition. - Typical usage is: + Typical usage is: - : MYIMMEDWORD IMMEDIATE - ...definition... - ; + : MYIMMEDWORD IMMEDIATE + ...definition... + ; - but some FORTH programmers write this instead: + but some FORTH programmers write this instead: - : MYIMMEDWORD - ...definition... - ; IMMEDIATE + : MYIMMEDWORD + ...definition... + ; IMMEDIATE - The two usages are equivalent, to a first approximation. + The two usages are equivalent, to a first approximation. */ - defcode "IMMEDIATE",9,F_IMMED,IMMEDIATE - movl var_LATEST,%edi // LATEST word. - addl $4,%edi // Point to name/flags byte. - xorb $F_IMMED,(%edi) // Toggle the IMMED bit. - NEXT + defcode "IMMEDIATE",9,F_IMMED,IMMEDIATE + movl var_LATEST,%edi // LATEST word. + addl $4,%edi // Point to name/flags byte. + xorb $F_IMMED,(%edi) // Toggle the IMMED bit. + NEXT /* - 'addr HIDDEN' toggles the hidden flag (F_HIDDEN) of the word defined at addr. To hide the - most recently defined word (used above in : and ; definitions) you would do: + 'addr HIDDEN' toggles the hidden flag (F_HIDDEN) of the word defined at addr. To hide the + most recently defined word (used above in : and ; definitions) you would do: - LATEST @ HIDDEN + LATEST @ HIDDEN - 'HIDE word' toggles the flag on a named 'word'. + 'HIDE word' toggles the flag on a named 'word'. - Setting this flag stops the word from being found by FIND, and so can be used to make 'private' - words. For example, to break up a large word into smaller parts you might do: + Setting this flag stops the word from being found by FIND, and so can be used to make 'private' + words. For example, to break up a large word into smaller parts you might do: - : SUB1 ... subword ... ; - : SUB2 ... subword ... ; - : SUB3 ... subword ... ; - : MAIN ... defined in terms of SUB1, SUB2, SUB3 ... ; - HIDE SUB1 - HIDE SUB2 - HIDE SUB3 + : SUB1 ... subword ... ; + : SUB2 ... subword ... ; + : SUB3 ... subword ... ; + : MAIN ... defined in terms of SUB1, SUB2, SUB3 ... ; + HIDE SUB1 + HIDE SUB2 + HIDE SUB3 - After this, only MAIN is 'exported' or seen by the rest of the program. + After this, only MAIN is 'exported' or seen by the rest of the program. */ - defcode "HIDDEN",6,,HIDDEN - pop %edi // Dictionary entry. - addl $4,%edi // Point to name/flags byte. - xorb $F_HIDDEN,(%edi) // Toggle the HIDDEN bit. - NEXT + defcode "HIDDEN",6,,HIDDEN + pop %edi // Dictionary entry. + addl $4,%edi // Point to name/flags byte. + xorb $F_HIDDEN,(%edi) // Toggle the HIDDEN bit. + NEXT - defword "HIDE",4,,HIDE - .int WORD // Get the word (after HIDE). - .int FIND // Look up in the dictionary. - .int HIDDEN // Set F_HIDDEN flag. - .int EXIT // Return. + defword "HIDE",4,,HIDE + .int WORD // Get the word (after HIDE). + .int FIND // Look up in the dictionary. + .int HIDDEN // Set F_HIDDEN flag. + .int EXIT // Return. /* - ' (TICK) is a standard FORTH word which returns the codeword pointer of the next word. + ' (TICK) is a standard FORTH word which returns the codeword pointer of the next word. - The common usage is: + The common usage is: - ' FOO , + ' FOO , - which appends the codeword of FOO to the current word we are defining (this only works in compiled code). + which appends the codeword of FOO to the current word we are defining (this only works in compiled code). - You tend to use ' in IMMEDIATE words. For example an alternate (and rather useless) way to define - a literal 2 might be: + You tend to use ' in IMMEDIATE words. For example an alternate (and rather useless) way to define + a literal 2 might be: - : LIT2 IMMEDIATE - ' LIT , \ Appends LIT to the currently-being-defined word - 2 , \ Appends the number 2 to the currently-being-defined word - ; + : LIT2 IMMEDIATE + ' LIT , \ Appends LIT to the currently-being-defined word + 2 , \ Appends the number 2 to the currently-being-defined word + ; - So you could do: + So you could do: - : DOUBLE LIT2 * ; + : DOUBLE LIT2 * ; - (If you don't understand how LIT2 works, then you should review the material about compiling words - and immediate mode). + (If you don't understand how LIT2 works, then you should review the material about compiling words + and immediate mode). - This definition of ' uses a cheat which I copied from buzzard92. As a result it only works in - compiled code. It is possible to write a version of ' based on WORD, FIND, >CFA which works in - immediate mode too. + This definition of ' uses a cheat which I copied from buzzard92. As a result it only works in + compiled code. It is possible to write a version of ' based on WORD, FIND, >CFA which works in + immediate mode too. */ - defcode "'",1,,TICK - lodsl // Get the address of the next word and skip it. - pushl %eax // Push it on the stack. - NEXT + defcode "'",1,,TICK + lodsl // Get the address of the next word and skip it. + pushl %eax // Push it on the stack. + NEXT /* - BRANCHING ---------------------------------------------------------------------- + BRANCHING ---------------------------------------------------------------------- - It turns out that all you need in order to define looping constructs, IF-statements, etc. - are two primitives. + It turns out that all you need in order to define looping constructs, IF-statements, etc. + are two primitives. - BRANCH is an unconditional branch. 0BRANCH is a conditional branch (it only branches if the - top of stack is zero). + BRANCH is an unconditional branch. 0BRANCH is a conditional branch (it only branches if the + top of stack is zero). - The diagram below shows how BRANCH works in some imaginary compiled word. When BRANCH executes, - %esi starts by pointing to the offset field (compare to LIT above): + The diagram below shows how BRANCH works in some imaginary compiled word. When BRANCH executes, + %esi starts by pointing to the offset field (compare to LIT above): - +---------------------+-------+---- - - ---+------------+------------+---- - - - ----+------------+ - | (Dictionary header) | DOCOL | | BRANCH | offset | (skipped) | word | - +---------------------+-------+---- - - ---+------------+-----|------+---- - - - ----+------------+ - ^ | ^ - | | | - | +-----------------------+ - %esi added to offset + +---------------------+-------+---- - - ---+------------+------------+---- - - - ----+------------+ + | (Dictionary header) | DOCOL | | BRANCH | offset | (skipped) | word | + +---------------------+-------+---- - - ---+------------+-----|------+---- - - - ----+------------+ + ^ | ^ + | | | + | +-----------------------+ + %esi added to offset - The offset is added to %esi to make the new %esi, and the result is that when NEXT runs, execution - continues at the branch target. Negative offsets work as expected. + The offset is added to %esi to make the new %esi, and the result is that when NEXT runs, execution + continues at the branch target. Negative offsets work as expected. - 0BRANCH is the same except the branch happens conditionally. + 0BRANCH is the same except the branch happens conditionally. - Now standard FORTH words such as IF, THEN, ELSE, WHILE, REPEAT, etc. can be implemented entirely - in FORTH. They are IMMEDIATE words which append various combinations of BRANCH or 0BRANCH - into the word currently being compiled. + Now standard FORTH words such as IF, THEN, ELSE, WHILE, REPEAT, etc. can be implemented entirely + in FORTH. They are IMMEDIATE words which append various combinations of BRANCH or 0BRANCH + into the word currently being compiled. - As an example, code written like this: + As an example, code written like this: - condition-code IF true-part THEN rest-code + condition-code IF true-part THEN rest-code - compiles to: + compiles to: - condition-code 0BRANCH OFFSET true-part rest-code - | ^ - | | - +-------------+ + condition-code 0BRANCH OFFSET true-part rest-code + | ^ + | | + +-------------+ */ - defcode "BRANCH",6,,BRANCH - add (%esi),%esi // add the offset to the instruction pointer - NEXT + defcode "BRANCH",6,,BRANCH + add (%esi),%esi // add the offset to the instruction pointer + NEXT - defcode "0BRANCH",7,,ZBRANCH - pop %eax - test %eax,%eax // top of stack is zero? - jz code_BRANCH // if so, jump back to the branch function above - lodsl // otherwise we need to skip the offset - NEXT + defcode "0BRANCH",7,,ZBRANCH + pop %eax + test %eax,%eax // top of stack is zero? + jz code_BRANCH // if so, jump back to the branch function above + lodsl // otherwise we need to skip the offset + NEXT /* - LITERAL STRINGS ---------------------------------------------------------------------- + LITERAL STRINGS ---------------------------------------------------------------------- - LITSTRING is a primitive used to implement the ." and S" operators (which are written in - FORTH). See the definition of those operators later. + LITSTRING is a primitive used to implement the ." and S" operators (which are written in + FORTH). See the definition of those operators later. - TELL just prints a string. It's more efficient to define this in assembly because we - can make it a single Linux syscall. + TELL just prints a string. It's more efficient to define this in assembly because we + can make it a single Linux syscall. */ - defcode "LITSTRING",9,,LITSTRING - lodsl // get the length of the string - push %esi // push the address of the start of the string - push %eax // push it on the stack - addl %eax,%esi // skip past the string - addl $3,%esi // but round up to next 4 byte boundary - andl $~3,%esi - NEXT - - defcode "TELL",4,,TELL - mov $1,%ebx // 1st param: stdout - pop %edx // 3rd param: length of string - pop %ecx // 2nd param: address of string - mov $__NR_write,%eax // write syscall - int $0x80 - NEXT + defcode "LITSTRING",9,,LITSTRING + lodsl // get the length of the string + push %esi // push the address of the start of the string + push %eax // push it on the stack + addl %eax,%esi // skip past the string + addl $3,%esi // but round up to next 4 byte boundary + andl $~3,%esi + NEXT + + defcode "TELL",4,,TELL + mov $1,%ebx // 1st param: stdout + pop %edx // 3rd param: length of string + pop %ecx // 2nd param: address of string + mov $__NR_write,%eax // write syscall + int $0x80 + NEXT /* - QUIT AND INTERPRET ---------------------------------------------------------------------- + QUIT AND INTERPRET ---------------------------------------------------------------------- - QUIT is the first FORTH function called, almost immediately after the FORTH system "boots". - As explained before, QUIT doesn't "quit" anything. It does some initialisation (in particular - it clears the return stack) and it calls INTERPRET in a loop to interpret commands. The - reason it is called QUIT is because you can call it from your own FORTH words in order to - "quit" your program and start again at the user prompt. + QUIT is the first FORTH function called, almost immediately after the FORTH system "boots". + As explained before, QUIT doesn't "quit" anything. It does some initialisation (in particular + it clears the return stack) and it calls INTERPRET in a loop to interpret commands. The + reason it is called QUIT is because you can call it from your own FORTH words in order to + "quit" your program and start again at the user prompt. - INTERPRET is the FORTH interpreter ("toploop", "toplevel" or "REPL" might be a more accurate - description -- see: http://en.wikipedia.org/wiki/REPL). + INTERPRET is the FORTH interpreter ("toploop", "toplevel" or "REPL" might be a more accurate + description -- see: http://en.wikipedia.org/wiki/REPL). */ - // QUIT must not return (ie. must not call EXIT). - defword "QUIT",4,,QUIT - .int RZ,RSPSTORE // R0 RSP!, clear the return stack - .int INTERPRET // interpret the next word - .int BRANCH,-8 // and loop (indefinitely) + // QUIT must not return (ie. must not call EXIT). + defword "QUIT",4,,QUIT + .int RZ,RSPSTORE // R0 RSP!, clear the return stack + .int INTERPRET // interpret the next word + .int BRANCH,-8 // and loop (indefinitely) /* - This interpreter is pretty simple, but remember that in FORTH you can always override - it later with a more powerful one! + This interpreter is pretty simple, but remember that in FORTH you can always override + it later with a more powerful one! */ - defcode "INTERPRET",9,,INTERPRET - call _WORD // Returns %ecx = length, %edi = pointer to word. - - // Is it in the dictionary? - xor %eax,%eax - movl %eax,interpret_is_lit // Not a literal number (not yet anyway ...) - call _FIND // Returns %eax = pointer to header or 0 if not found. - test %eax,%eax // Found? - jz 1f - - // In the dictionary. Is it an IMMEDIATE codeword? - mov %eax,%edi // %edi = dictionary entry - movb 4(%edi),%al // Get name+flags. - push %ax // Just save it for now. - call _TCFA // Convert dictionary entry (in %edi) to codeword pointer. - pop %ax - andb $F_IMMED,%al // Is IMMED flag set? - mov %edi,%eax - jnz 4f // If IMMED, jump straight to executing. - - jmp 2f - -1: // Not in the dictionary (not a word) so assume it's a literal number. - incl interpret_is_lit - call _NUMBER // Returns the parsed number in %eax, %ecx > 0 if error - test %ecx,%ecx - jnz 6f - mov %eax,%ebx - mov $LIT,%eax // The word is LIT - -2: // Are we compiling or executing? - movl var_STATE,%edx - test %edx,%edx - jz 4f // Jump if executing. - - // Compiling - just append the word to the current dictionary definition. - call _COMMA - mov interpret_is_lit,%ecx // Was it a literal? - test %ecx,%ecx - jz 3f - mov %ebx,%eax // Yes, so LIT is followed by a number. - call _COMMA -3: NEXT - -4: // Executing - run it! - mov interpret_is_lit,%ecx // Literal? - test %ecx,%ecx // Literal? - jnz 5f - - // Not a literal, execute it now. This never returns, but the codeword will - // eventually call NEXT which will reenter the loop in QUIT. - jmp *(%eax) - -5: // Executing a literal, which means push it on the stack. - push %ebx - NEXT - -6: // Parse error (not a known word or a number in the current BASE). - // Print an error message followed by up to 40 characters of context. - mov $2,%ebx // 1st param: stderr - mov $errmsg,%ecx // 2nd param: error message - mov $errmsgend-errmsg,%edx // 3rd param: length of string - mov $__NR_write,%eax // write syscall - int $0x80 - - mov (currkey),%ecx // the error occurred just before currkey position - mov %ecx,%edx - sub $buffer,%edx // %edx = currkey - buffer (length in buffer before currkey) - cmp $40,%edx // if > 40, then print only 40 characters - jle 7f - mov $40,%edx -7: sub %edx,%ecx // %ecx = start of area to print, %edx = length - mov $__NR_write,%eax // write syscall - int $0x80 - - mov $errmsgnl,%ecx // newline - mov $1,%edx - mov $__NR_write,%eax // write syscall - int $0x80 - - NEXT - - .section .rodata + defcode "INTERPRET",9,,INTERPRET + call _WORD // Returns %ecx = length, %edi = pointer to word. + + // Is it in the dictionary? + xor %eax,%eax + movl %eax,interpret_is_lit // Not a literal number (not yet anyway ...) + call _FIND // Returns %eax = pointer to header or 0 if not found. + test %eax,%eax // Found? + jz 1f + + // In the dictionary. Is it an IMMEDIATE codeword? + mov %eax,%edi // %edi = dictionary entry + movb 4(%edi),%al // Get name+flags. + push %ax // Just save it for now. + call _TCFA // Convert dictionary entry (in %edi) to codeword pointer. + pop %ax + andb $F_IMMED,%al // Is IMMED flag set? + mov %edi,%eax + jnz 4f // If IMMED, jump straight to executing. + + jmp 2f + +1: // Not in the dictionary (not a word) so assume it's a literal number. + incl interpret_is_lit + call _NUMBER // Returns the parsed number in %eax, %ecx > 0 if error + test %ecx,%ecx + jnz 6f + mov %eax,%ebx + mov $LIT,%eax // The word is LIT + +2: // Are we compiling or executing? + movl var_STATE,%edx + test %edx,%edx + jz 4f // Jump if executing. + + // Compiling - just append the word to the current dictionary definition. + call _COMMA + mov interpret_is_lit,%ecx // Was it a literal? + test %ecx,%ecx + jz 3f + mov %ebx,%eax // Yes, so LIT is followed by a number. + call _COMMA +3: NEXT + +4: // Executing - run it! + mov interpret_is_lit,%ecx // Literal? + test %ecx,%ecx // Literal? + jnz 5f + + // Not a literal, execute it now. This never returns, but the codeword will + // eventually call NEXT which will reenter the loop in QUIT. + jmp *(%eax) + +5: // Executing a literal, which means push it on the stack. + push %ebx + NEXT + +6: // Parse error (not a known word or a number in the current BASE). + // Print an error message followed by up to 40 characters of context. + mov $2,%ebx // 1st param: stderr + mov $errmsg,%ecx // 2nd param: error message + mov $errmsgend-errmsg,%edx // 3rd param: length of string + mov $__NR_write,%eax // write syscall + int $0x80 + + mov (currkey),%ecx // the error occurred just before currkey position + mov %ecx,%edx + sub $buffer,%edx // %edx = currkey - buffer (length in buffer before currkey) + cmp $40,%edx // if > 40, then print only 40 characters + jle 7f + mov $40,%edx +7: sub %edx,%ecx // %ecx = start of area to print, %edx = length + mov $__NR_write,%eax // write syscall + int $0x80 + + mov $errmsgnl,%ecx // newline + mov $1,%edx + mov $__NR_write,%eax // write syscall + int $0x80 + + NEXT + + .section .rodata errmsg: .ascii "PARSE ERROR: " errmsgend: errmsgnl: .ascii "\n" - .data // NB: easier to fit in the .data section - .align 4 + .data // NB: easier to fit in the .data section + .align 4 interpret_is_lit: - .int 0 // Flag used to record if reading a literal + .int 0 // Flag used to record if reading a literal /* - ODDS AND ENDS ---------------------------------------------------------------------- + ODDS AND ENDS ---------------------------------------------------------------------- - CHAR puts the ASCII code of the first character of the following word on the stack. For example - CHAR A puts 65 on the stack. + CHAR puts the ASCII code of the first character of the following word on the stack. For example + CHAR A puts 65 on the stack. - EXECUTE is used to run execution tokens. See the discussion of execution tokens in the - FORTH code for more details. + EXECUTE is used to run execution tokens. See the discussion of execution tokens in the + FORTH code for more details. - SYSCALL0, SYSCALL1, SYSCALL2, SYSCALL3 make a standard Linux system call. (See - for a list of system call numbers). As their name suggests these forms take between 0 and 3 - syscall parameters, plus the system call number. + SYSCALL0, SYSCALL1, SYSCALL2, SYSCALL3 make a standard Linux system call. (See + for a list of system call numbers). As their name suggests these forms take between 0 and 3 + syscall parameters, plus the system call number. - In this FORTH, SYSCALL0 must be the last word in the built-in (assembler) dictionary because we - initialise the LATEST variable to point to it. This means that if you want to extend the assembler - part, you must put new words before SYSCALL0, or else change how LATEST is initialised. + In this FORTH, SYSCALL0 must be the last word in the built-in (assembler) dictionary because we + initialise the LATEST variable to point to it. This means that if you want to extend the assembler + part, you must put new words before SYSCALL0, or else change how LATEST is initialised. */ - defcode "CHAR",4,,CHAR - call _WORD // Returns %ecx = length, %edi = pointer to word. - xor %eax,%eax - movb (%edi),%al // Get the first character of the word. - push %eax // Push it onto the stack. - NEXT - - defcode "EXECUTE",7,,EXECUTE - pop %eax // Get xt into %eax - jmp *(%eax) // and jump to it. - // After xt runs its NEXT will continue executing the current word. - - defcode "SYSCALL3",8,,SYSCALL3 - pop %eax // System call number (see ) - pop %ebx // First parameter. - pop %ecx // Second parameter - pop %edx // Third parameter - int $0x80 - push %eax // Result (negative for -errno) - NEXT - - defcode "SYSCALL2",8,,SYSCALL2 - pop %eax // System call number (see ) - pop %ebx // First parameter. - pop %ecx // Second parameter - int $0x80 - push %eax // Result (negative for -errno) - NEXT - - defcode "SYSCALL1",8,,SYSCALL1 - pop %eax // System call number (see ) - pop %ebx // First parameter. - int $0x80 - push %eax // Result (negative for -errno) - NEXT - - defcode "SYSCALL0",8,,SYSCALL0 - pop %eax // System call number (see ) - int $0x80 - push %eax // Result (negative for -errno) - NEXT + defcode "CHAR",4,,CHAR + call _WORD // Returns %ecx = length, %edi = pointer to word. + xor %eax,%eax + movb (%edi),%al // Get the first character of the word. + push %eax // Push it onto the stack. + NEXT + + defcode "EXECUTE",7,,EXECUTE + pop %eax // Get xt into %eax + jmp *(%eax) // and jump to it. + // After xt runs its NEXT will continue executing the current word. + + defcode "SYSCALL3",8,,SYSCALL3 + pop %eax // System call number (see ) + pop %ebx // First parameter. + pop %ecx // Second parameter + pop %edx // Third parameter + int $0x80 + push %eax // Result (negative for -errno) + NEXT + + defcode "SYSCALL2",8,,SYSCALL2 + pop %eax // System call number (see ) + pop %ebx // First parameter. + pop %ecx // Second parameter + int $0x80 + push %eax // Result (negative for -errno) + NEXT + + defcode "SYSCALL1",8,,SYSCALL1 + pop %eax // System call number (see ) + pop %ebx // First parameter. + int $0x80 + push %eax // Result (negative for -errno) + NEXT + + defcode "SYSCALL0",8,,SYSCALL0 + pop %eax // System call number (see ) + int $0x80 + push %eax // Result (negative for -errno) + NEXT /* - DATA SEGMENT ---------------------------------------------------------------------- + DATA SEGMENT ---------------------------------------------------------------------- - Here we set up the Linux data segment, used for user definitions and variously known as just - the 'data segment', 'user memory' or 'user definitions area'. It is an area of memory which - grows upwards and stores both newly-defined FORTH words and global variables of various - sorts. + Here we set up the Linux data segment, used for user definitions and variously known as just + the 'data segment', 'user memory' or 'user definitions area'. It is an area of memory which + grows upwards and stores both newly-defined FORTH words and global variables of various + sorts. - It is completely analogous to the C heap, except there is no generalised 'malloc' and 'free' - (but as with everything in FORTH, writing such functions would just be a Simple Matter - Of Programming). Instead in normal use the data segment just grows upwards as new FORTH - words are defined/appended to it. + It is completely analogous to the C heap, except there is no generalised 'malloc' and 'free' + (but as with everything in FORTH, writing such functions would just be a Simple Matter + Of Programming). Instead in normal use the data segment just grows upwards as new FORTH + words are defined/appended to it. - There are various "features" of the GNU toolchain which make setting up the data segment - more complicated than it really needs to be. One is the GNU linker which inserts a random - "build ID" segment. Another is Address Space Randomization which means we can't tell - where the kernel will choose to place the data segment (or the stack for that matter). + There are various "features" of the GNU toolchain which make setting up the data segment + more complicated than it really needs to be. One is the GNU linker which inserts a random + "build ID" segment. Another is Address Space Randomization which means we can't tell + where the kernel will choose to place the data segment (or the stack for that matter). - Therefore writing this set_up_data_segment assembler routine is a little more complicated - than it really needs to be. We ask the Linux kernel where it thinks the data segment starts - using the brk(2) system call, then ask it to reserve some initial space (also using brk(2)). + Therefore writing this set_up_data_segment assembler routine is a little more complicated + than it really needs to be. We ask the Linux kernel where it thinks the data segment starts + using the brk(2) system call, then ask it to reserve some initial space (also using brk(2)). - You don't need to worry about this code. + You don't need to worry about this code. */ - .text - .set INITIAL_DATA_SEGMENT_SIZE,65536 + .text + .set INITIAL_DATA_SEGMENT_SIZE,65536 set_up_data_segment: - xor %ebx,%ebx // Call brk(0) - movl $__NR_brk,%eax - int $0x80 - movl %eax,var_HERE // Initialise HERE to point at beginning of data segment. - addl $INITIAL_DATA_SEGMENT_SIZE,%eax // Reserve nn bytes of memory for initial data segment. - movl %eax,%ebx // Call brk(HERE+INITIAL_DATA_SEGMENT_SIZE) - movl $__NR_brk,%eax - int $0x80 - ret + xor %ebx,%ebx // Call brk(0) + movl $__NR_brk,%eax + int $0x80 + movl %eax,var_HERE // Initialise HERE to point at beginning of data segment. + addl $INITIAL_DATA_SEGMENT_SIZE,%eax // Reserve nn bytes of memory for initial data segment. + movl %eax,%ebx // Call brk(HERE+INITIAL_DATA_SEGMENT_SIZE) + movl $__NR_brk,%eax + int $0x80 + ret /* - We allocate static buffers for the return static and input buffer (used when - reading in files and text that the user types in). + We allocate static buffers for the return static and input buffer (used when + reading in files and text that the user types in). */ - .set RETURN_STACK_SIZE,8192 - .set BUFFER_SIZE,4096 + .set RETURN_STACK_SIZE,8192 + .set BUFFER_SIZE,4096 - .bss + .bss /* FORTH return stack. */ - .align 4096 + .align 4096 return_stack: - .space RETURN_STACK_SIZE -return_stack_top: // Initial top of return stack. + .space RETURN_STACK_SIZE +return_stack_top: // Initial top of return stack. /* This is used as a temporary input buffer when reading from files or the terminal. */ - .align 4096 + .align 4096 buffer: - .space BUFFER_SIZE + .space BUFFER_SIZE /* - START OF FORTH CODE ---------------------------------------------------------------------- + START OF FORTH CODE ---------------------------------------------------------------------- - We've now reached the stage where the FORTH system is running and self-hosting. All further - words can be written as FORTH itself, including words like IF, THEN, .", etc which in most - languages would be considered rather fundamental. + We've now reached the stage where the FORTH system is running and self-hosting. All further + words can be written as FORTH itself, including words like IF, THEN, .", etc which in most + languages would be considered rather fundamental. - I used to append this here in the assembly file, but I got sick of fighting against gas's - crack-smoking (lack of) multiline string syntax. So now that is in a separate file called - jonesforth.f + I used to append this here in the assembly file, but I got sick of fighting against gas's + crack-smoking (lack of) multiline string syntax. So now that is in a separate file called + jonesforth.f - If you don't already have that file, download it from http://annexia.org/forth in order - to continue the tutorial. + If you don't already have that file, download it from http://annexia.org/forth in order + to continue the tutorial. */