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0 pointswhizzter1y ago0 comments

Yes and no, with the 8bits and probably 16bits like the C64, NES, etc you really want cycle accuracy and emulating something like the PS2 really well would probably need a fair bit of accuracy.

HOWEVER.. writing an recompiler (not feasible for 8bits due to the amount of self-mod code) you could probably insert pattern checks to detect writes to DMA-starting locations,etc (thinking back to our PS2 games) and transform it to safe sequences without accurate timing for most games that really only would be happy with some extra cycles.

0 comments

OnlyMortal1y ago

Good point about 8bit self modifying code.

The UK C64 disk for “Skate or Die” had copy protection that did exactly that, as an example.

tom_1y ago

It's pretty much best practice on the 6502. Loading code from disk or tape is basically the sane thing too.

OnlyMortal1y ago

With Skate or Die, it’d pull a byte off the genuine disk and use it to decrypt (xor) a loop immediately after the current PC.

I think it went through about five iterations.

My crack was to step it with the original disk to decrypt the encrypted part and modify the code to jump over the decryption. It was pretty sophisticated for the day.

To another extreme, Ocean disk games had a “IsGenuineDisk” style function. It returned 1 or 0 in the accumulator. I could crack it in under a minute with a disk sector editor.

Oddly, I ended up doing Mac copy protection years later.

j / k navigate · click thread line to collapse

0 pointswhizzter1y ago0 comments

Yes and no, with the 8bits and probably 16bits like the C64, NES, etc you really want cycle accuracy and emulating something like the PS2 really well would probably need a fair bit of accuracy.

0 comments

OnlyMortal1y ago

Good point about 8bit self modifying code.

The UK C64 disk for “Skate or Die” had copy protection that did exactly that, as an example.

tom_1y ago

It's pretty much best practice on the 6502. Loading code from disk or tape is basically the sane thing too.

OnlyMortal1y ago

With Skate or Die, it’d pull a byte off the genuine disk and use it to decrypt (xor) a loop immediately after the current PC.

I think it went through about five iterations.

My crack was to step it with the original disk to decrypt the encrypted part and modify the code to jump over the decryption. It was pretty sophisticated for the day.

To another extreme, Ocean disk games had a “IsGenuineDisk” style function. It returned 1 or 0 in the accumulator. I could crack it in under a minute with a disk sector editor.

Oddly, I ended up doing Mac copy protection years later.

j / k navigate · click thread line to collapse