Various flag related issues and others

[gulrak]

I came across your emulator via Reddit and as it is one of the CHIP-8 implementations with more stars and thus potentially used for inspiration from newcomers, I thought I give you a heads-up on a few issues:

The 8xyn opcodes have various bugs related to VF handling. They have mainly to do with the order of setting Vx and VF. In case x is F, the flag result needs to be the result, not the result of the math operation. Also subtracting equal numbers needs no borrow, so 1 needs to be in VF for that case, but both subtraction variants only set VF only on >, not >=, giving wrong flag result on equal numbers.

Also the implementation is more of a mixture of SuperChip and CHIP-8, as the shift opcodes (shifting just VX instead of first setting VX to VY as classic and modern variants (like XO-CHIP) do it) and Fx55/Fx65 (that doesn't increment I) behave as SuperChip, not CHIP-8 but it doesn't have the SuperChip features. The inaccuracies come probably from the used technical reference that sadly has some issues. For a more correct documentation I would point for e.g. https://github.com/mattmikolay/chip-8

I recommend the tests from https://github.com/Timendus/chip8-test-suite to use at least the tests up to the flags test to verify the implementation.

I didn't open single issues for each one of the problems, as I can totally understand if you don't want to work on the old project after that time, but I opened this ticket anyway, in case you are interested.

[f0lg0]

Thanks for pointing these out. As you imagined, I don't know if I'll be able to work on this project again but I'm defintely leaving this issue open, as it's useful for developers referencing this project.
Thanks!