48 pixel sprite positioning and optimisations

[Thomas Jentzsch]

The worse thing is that SBX is such a useful opcode, and is stable on all regular Atari consoles. It's just the Flashback2 that got it wrong.

I would always ignore that. If modern hardware has flaws then I couldn't care less.

[Joe Musashi]

Out of curiosity, how common was the use of illegal opcodes in classic games? I grepped for LAX in a couple of sources, but did not find anything.

[Thomas Jentzsch]

Out of curiosity, how common was the use of illegal opcodes in classic games? I grepped for LAX in a couple of sources, but did not find anything.

I don't think they got used at all. Sometimes even some legal opcodes seemed unknown.

 

E.g. Starmaster uses CLC, ROR instead of LSR.

Edited March 21, 2013 by Thomas Jentzsch

[Omegamatrix]

There is one case I know of, but I don't have time to look it up. I believe Airlock uses SAX of some form. It was a mistake IIRC, a bad branch was involved between and opcode and operand. It was a few years ago, but I seem to remember it looked like an error, as there was no point to the instruction, and if the branch was adjusted by 1 place it would make total sense.

[Mr SQL]

I don't think they got used at all. Sometimes even some legal opcodes seemed unknown.

 

E.g. Starmaster uses CLC, ROR instead of LSR.

Tom,

perhaps the programmer needed to pad cycles? Those opcodes are less frequently used but I would expect the Activision programmers to to be familiar.

[Thomas Jentzsch]

Tom,

perhaps the programmer needed to pad cycles? Those opcodes are less frequently used but I would expect the Activision programmers to to be familiar.

I found it hard to believe too, but it is very obvious. LSR is getting used, but probably not 100% understood. Else using CLC, ROR makes no sense. Just check the disassembly.