Comparing to an existing, known-good emulator is probably the most convenient way. This can be printf debugging like you describe, or even trying to compare fra

Comparing to an existing, known-good emulator is probably the most convenient way. This can be printf debugging like you describe, or even trying to compare framebuffer contents etc. However I feel like that "spoils" a bit of the fun. A more adventurous approach is to test with test ROMs [1] - they are simpler to follow by hand and discover why they don't work than real games. Of course if some game ROM is relying on some more obscure hardware quirks this might not be enough. Just wanted to bring it to attention for anyone interested in writing and debugging their emulators. [1]: https://gbdev.gg8.se/wiki/articles/Test_ROMs https://gbdev.gg8.se/wiki/articles/Test_ROMs