CHIP-8

I made a CHIP-8 virtual machine in C!

This is a cross-platform implementation, meaning that it can compile to both run on your PC as well as cross-compile to run on an Apple //e. It expects the //e to have an 80-column card.

What is CHIP-8?

CHIP-8 is an interpreted programming language, developed by Joseph Weisbecker made on his 1802 Microprocessor. It was initially used on the COSMAC VIP and Telmac 1800 8-bit microcomputers in the mid-1970s. CHIP-8 programs are run on a CHIP-8 virtual machine. — Wikipedia

Cross Compilation

Compiling to the //e target is accomplished via the use of the CC65 tool chain, a cross development package for 6502 systems.

Apple //e

The Apple //e is the third model in the Apple II series of personal computers produced by Apple Computer. It runs on a MOS-6502 processor, the mighty 8-bit processor that helped sparked the home computing revolution of the early 1980s.

The //e's screen memory buffers are rather peculiar. I make use of the lesser-documented 80-column Low Resolution Graphics mode which is not without its difficulties. Even screen columns are on Page 2 of screen memory, while odd columns are on Page 1. Switching between memory pages is accomplished by writing to the correct "soft switch".

Meanwhile, even rows are written to the "low-nibble" (the 4 least significant bits) of the corresponding address and odd rows written to the "high-nibble". All of this means that the same memory address may correspond to one of four possible pixels. Special care must be taken to ensure the desired pixel is turned on.

Dependencies

brew install criterion sdl2 cc65 c2t

Build

Non Apple //e build:

make sdl
./bin/sdl/chip8 Path/to/a/valid/CHIP8/ROM/file.ch8

Apple //e build:

# Compiles code into an executable 6502 file
make apple

# Generates a .wav file for analog audio transfer to a real-world //e
make wav

# Generates a .dsk file for use with Virtual ][ or other //e emulators
make disk

TODO:

• Get keyboard input working on SDL build
• Get audio audio output working on the SDL build
• Rewrite //e implementation's draw screen routine in assembly (current C code is slower than desired)
• Incorporate load_rom.rb into the Makefile build processes
• Make instruction and machine extern values