Converter to turn games with illegal instructions into legal ones?

[jpivarski]

I wanted to try playing Little Sisyphus, but in OpenEmu's NES emulator (FCEU), it flickers so much that it's hard to see. On the chat page, I see that I'm not the only one, though I don't know if other users are using the same emulator as I am.

I just noticed that you're the author of both Little Sisyphus and NESFab! I'm jpivarski on both itch.io and GitHub. (And for some reason, I'm not allowed to edit my comment on itch.io or add another...? My email is verified...)

Is the Little Sisyphus source code open? If so, recompiling it with make ISA=legal would be a good first project for me in NESFab.

Before I knew you were the developer of both the programming language and the game, I was going to ask about whether you think it's possible to create a tool that converts a closed-source binary with illegal instructions into another binary without illegal instructions. For instance, if you have a table in which every illegal instruction X on register x can be replaced with a sequence A, B, C on registers a1, a2, b1, b2, ... (possibly moving other data out of the way and back), then I could contribute a standalone script that applies those changes. How complicated do you think that would be?

[pubby]

Heya.

Sisyphus is closed-source for now. It's likely to get recompiled with legal opcodes in the future though. Probably soon, even. So keep your eyes out.

I was going to ask about whether you think it's possible to create a tool that converts a closed-source binary with illegal instructions into another binary without illegal instructions

It's actually pretty difficult. Although one can replace an illegal instruction with several legal instructions, the code size is no longer the same. This means all code addresses break. It also means you may need a bin-packing algorithm to reassemble the ROM.

One option that may exist (or not - I don't know) is adding illegal opcode support to OpenEmu. It's usually not too hard, and since the game works in FCEUX it might be as simple as porting one file over