Chip-8 Emulator

A CHIP-8 emulator written in C++ using SDL for graphics and audio, and ImGUI for the debugging window. Initially I wanted to go for an NES emulator but after learning about the emulation space in general it seemed that a good first step would be a CHIP-8 emulator. It’s been tested on a bunch of ROMs from here and I’ve yet to find any more issues with it. One interesting point is that due to the way sprites are drawn in CHIP-8, moving objects appear to flicker in the demo above. Sprites are drawn by XORing sprite data with the current screen state, and so to move an object you first XOR it at its current position, increment the X or Y position, and then XOR it again at the new position. I think this appeared more smooth on a CRT for which the ISA was designed, but on a modern display it is quite noticeable.

Probably the most difficult thing about building an emulator is getting the timing right, even in a simple system like CHIP-8. I ended up tying the instruction rate to the framerate, and relying on SDL to compute said framerate (the clock rate on the COSMAC VIP was ~1.76MHz but amounted to an instruction execution rate of about 500 inst/sec). This also allows for easier debugging than using real-time clocks, since the debugger can simply stop between frames rather than have to control clocks / tick counts. This is probably fine for a simple emulator like this that should run at a stable rate on most machines but there are more involved techniques for more complex systems, especially where there are multiple units (e.g. the PPU and CPU of the NES) that run at different rates relative to eachother.

I also made a CHIP-8 assembler & disassembler for troubleshooting purposes, and a (semi-functional) CLI version during the early stages of development to avoid having to debug graphics issues before the core of the emulator was done.