CHIP-8 Interpreter for Epson HX-20

This project builds a CHIP-8 interpreter for the Epson HX-20 portable computer.

Implementation Notes

• Behaves like the original CHIP-8 implementation without any "quirks".
• Passes relevant tests in the chip8-test-suite.
• Graphics uses 64x32 pixels on the left side of the (120x32 pixel) LCD display.
• Keyboard layout same as many PC based CHIP-8 interpreters, using WASD, etc.
• Speaker beep works, but temporarily blocks execution.
• Timer (OCF) interrupt is not used since it interferes with keyboard scanning.
• The interpreter and CHIP-8 programs are loaded into memory with the built-in MONITOR.

Additional Information

Video:

• Epson HX-20 CHIP-8 Interpreter

Blog:

• CHIP-8 Interpreter for Epson HX-20
• Epson HX-20 Binary WAV File Creator

Memory Map

Start	End	Usage
0x1000	0x18FF	Interpreter Code
0x1900	0x1AFF	Interpreter Memory
0x1B00	0x28FF	CHIP-8 Program

The memory area from 0x1900 to 0x28FF is seen as 0x000 to 0xFFF from within the CHIP-8 execution.

Build

The dasm macro assembler is required. The SRecord software is used to convert files to S-records. Just run make:

make

Testing in an Emulator

Use the hex20 emulator to test. Only a single S-record can be loaded, so just concatenate the interpreter with the CHIP-8 program, e.g.:

cat chip8.srec hx20-logo.srec > chip8-hx20-logo.srec
hex20 -s chip8-hx20-logo.srec

Start execution after loading the S-record into the MONITOR by typing:

G1000

Emulator Keyboard Issue

The default time in hex20 that the keys are "held down" on the keyboard is too short for the interpreter to properly detect. This can be somewhat alleviated by increasing this constant from 500 to 10000 and recompiling hex20:

#define CONSOLE_KEYBOARD_RELEASE 10000

Transfer to a Real Epson HX-20

Preferred method is to use the external cassette interface by playing audio WAV files from a PC. The included "bin2hxwav" C program converts binary files to HX-20 compatible audio files.

Loading on an Epson HX-20

Connect an audio cable from a PC to the "Ear" input. Enter the MONITOR and type:

A
1000
18FF
/
R C,*.*

Play back the "chip8.wav" file from the PC to load the interpreter. Once "Ok" appears, type:

A
1B00
1CFF
/
R C,*.*

Play back the "hx20-logo.wav" file from the PC to load that CHIP-8 program. Note that the second parameter to the "A" command is the end location, this can be replaced with FFFF for other programs if that address is not known. Once "Ok" appears, start execution:

G1000

It is possible to abort the ongoing CHIP-8 execution with the "MENU" button. At that point another CHIP-8 program can be loaded from the MONITOR again. It is not necessary to re-load the interpreter since the HX-20 retains the memory.