XMOS

An extension to the MOS (Machine Operating System) for the BBC Master. By Richard Talbot-Watkins and Matt Godbolt, circa 1992.

Found on an old disc by Rich, now being reverse engineered back to health by Matt (and Claude).

What is XMOS?

XMOS is a sideways ROM for the BBC Master that adds a collection of utility commands and an extended line editing mode:

• Extended input (*XON) — cursor-based line editing with insert/delete, input history via SHIFT-Up/Down, and TAB to recall BASIC lines by number
• Command aliases (*ALIAS) — define shorthand names that type out their expansion at the prompt, with %0-%9 parameter substitution
• Key redefinition (*KEYON/*DEFKEYS) — remap cursor and fire keys
• BASIC utilities — *S saves with an incore filename from REM >, *BAU splits multi-statement lines, *SPACE adds keyword spacing, *LVAR lists variable names
• Development tools — *DIS disassembles 6502 code, *MEM is a hex/ASCII memory editor

See USAGE.md for the full command reference and user guide.

Quick start

Load the ROM into sideways RAM and press CTRL+BREAK:

*SRLOAD XMOS 8000 7Q

Then type *HELP XMOS to see all commands, or *XON to enable extended input mode.

Building

Requires beebasm on your PATH.

beebasm -i xmos.asm -o build.rom   # Assemble the ROM
npm test                            # Run automated tests (requires Node.js)

The original byte-identical disassembly is preserved at git tag original. Since then, bug fixes and improvements mean the output no longer matches the original ROM — correctness is verified by the automated test suite.

Project structure

Assembly source (split by subsystem)

File	Description
xmos.asm	Main: ROM header, service dispatch, help, command table, XON/XOFF
input.asm	Extended input: handle_reset, keyboard intercept, cursor editing
util.asm	Utilities: print_inline, copy_inline_to_stack, compare_string
basic.asm	BASIC save: *S (save with incore name), *L (mode setup)
keys.asm	Key system: remap handler, *KEYON/OFF, *KSTATUS, *DEFKEYS
alias.asm	Aliases: *ALIAS, *ALIASES, *ALICLR, *ALILD, *ALISV, *STORE
mem.asm	Memory editor: *MEM command
dis.asm	Disassembler: *DIS command, addressing mode format tables
bau.asm	BASIC utilities: *BAU (split lines), *SPACE (insert spaces)
lvar.asm	Variable lister: *LVAR, token classifier, decimal printer
data.asm	ROM data: features text, opcode table, keyword table, padding
constants.asm	System constants: MOS vectors, hardware registers, zero page
macros.asm	Assembly macros: STROUT, OP, NOOP, KW

Other files

File	Description
original.ssd	Original BBC Micro disc image
original.rom	Extracted 16KB ROM binary
USAGE.md	Full command reference and user guide
JOURNAL.md	Reverse engineering notes and discoveries
TESTING_PLAN.md	Test infrastructure and jsbeeb improvement notes

Status

The ROM is fully disassembled and annotated. We're now fixing bugs and making improvements — the original byte-identical version is at git tag original. See JOURNAL.md for detailed notes.