There was also some manual testing needed in order to get both the screen and the keypad to work. I wrote some simple programs to run on the chip, and corrected some misconceptions I have on CHIP-8 behavior (for instance, memory loads and stores include multiple registers, and sprite drawing wraps around). I was able to correct my Rust CHIP-8 emulator in the process; it’s funny how it was able to run many games despite getting these things completely wrong.
The CHIP-8 specification includes 16 one-byte registers (V0 to VF) and 20 two-byte words of stack. Initially I wanted them to be separate arrays, but it was really hard to coordinate access so that they get synthesized as RAM, so I decided to map them to memory instead (see the memory map described at the top of cpu.v).
I also mapped screen contents to system memory, so two different modules (CPU and screen) had to compete for memory access somehow. I decided to pause the CPU (i.e. not make it process the next instruction) whenever we want to read the screen contents.
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