Quirks

[ivan-pi]

Over the years, CHIP-8 emulators have accumulated different "quirks" or differences in the way certain instructions are supposed to behave.

More information is available at:

• https://chip-8.github.io/extensions/
• https://chip8.gulrak.net/
• https://github.com/Timendus/chip8-test-suite#quirks-test (and thread on Reddit: https://www.reddit.com/r/EmuDev/comments/viri5r/i_wrote_a_chip8_test_suite/)
• http://johnearnest.github.io/Octo/ (check the options under Toolbox -> Compatibility)

We could introduce a bitmask or individual flags for the quirks which are checked at runtime in each emulator cycle, or we could implement different sub-interpreters (e.g. classic CHIP8, S-CHIP, XO-CHIP, etc.). The performance penalty should be negligible thanks to CPU branch predictors.

Sketch (taken from here):

type :: quirks
    logical :: VfReset
    logical :: Memory
    logical :: DisplayBlank
    logical :: Clipping
    logical :: Shifting
    logical :: Jumping
end type

[ivan-pi]

A related project to document the quirks of different games out in the wild is found here: https://github.com/chip-8/chip-8-database