CHIP-8 was a virtual machine for microcomputers

CHIP-8 was first used for the COSMAC VIP microcomputer and later introduced in the popular HP-48 calculator. A revision named SCHIP was developed later, with extra instructions.

The CHIP-8 VM is one of the most recommended systems to implement to get started with emulators development.

• It is simple enough to be done over a few days.
• The architecture is extensively documented, and there are tutorials.
• Test ROMs do exist.

Architecture

• 4KB of memory, with 1-byte words.
• 16 general-purpose registers of 1 byte each, called V0..VF.
  • The last of these registers, VF is used for the carry flag.
• 1 16-bit register named I that stores memory addresses.
• 1 special 1-byte register named ST that is a timer for sound.
• 1 special 1-byte register named DT that is a generic countdown timer.
• A stack with capacity for 16 slots. This is used to handle calls to subroutines.

Memory addresses range from 0x000 to 0xFFF, but the first range is reserved for the interpreter itself. CHIP-8 ROMS load from address 0x200.

In addition to the CPU, there's also a 64×32 monochrome display and a synthesizer/buzzer that emits a tone when the ST register is non-zero.

Opcodes

There are 35 opcodes in the standard CHIP-8 set, with extra instruction for SCHIP.

All opcodes are always 2-byte long in big endian.

See a table with all the opcodes and their variations/bugs accross versions of the interpreters.

Resources

• Cowgod's CHIP-8 technical reference
• Guide to making a CHIP-8 emulator
• Octo assembly language and tools for CHIP-8
• Github repository with Test ROMs
• My own implementation of an emulator for CHIP-8 in Rust + WebAssembly