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chatGPZ
Registered: Dec 2001
Posts: 11386 |
useless opcodes riddle - wtf is up with LAS and TAS?
Due to my emulator related activities in the last year(s) i digged more into the so called "illegal" opcodes, and the result is a nice (hopefully) PDF that i'll publish soonish (when some of you lazy bastards are done proofreading =D) - which includes complete state of the art reference to all of these, plus a bunch of real world examples on how to use these (sometimes very weird behaving) instructions... (in large part provided by bitbreaker, thumbs up!) Its about time for a comprehensive document on this topic that is suitable for normal people =)
however, two opcodes seem to be completely useless and so far i cant think of what to use them for in real world code:
- LAS abs,y ($BB) (A,X,SP = addr & SP)
- TAS abs,y ($9B) (SP = A & X ; addr = SP & addrhi+1)
if any of you have used these before, or for whatever reason have an idea on what to do with them - let me know please, this is your chance to earn some karma upgrade points :o)
(and as a sidenote, some not so obvious *short* snippets that are using SLO, RLA, RRA or ISC are welcome too)
let's bust some more myths! |
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doynax
Account closed
Registered: Oct 2004
Posts: 212 |
Quoting Martin PiperMaybe it's time for an optimising assembler that suggests replacement code by scanning for known instruction sequences. I'm not convinced such a feature would be terribly useful unless the assembler had a fuller view of the semantics of the program, to separate side-effects from desired results.
Perhaps an emulator could be modified to report feedback on dead-stores/flags and the like. Plus with demo code coverage is largely a non-issue.
A super-optimizer would also be pretty neat, though the amount of complex immediates and tables in optimized 6502 code may put a damper on things. |
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lft
Registered: Jul 2007
Posts: 369 |
Quoting Martin PiperMaybe it's time for an optimising assembler that suggests replacement code by scanning for known instruction sequences.
Depends on the code. Often, the amount of cycles matters, and occasionally the amount of bytes.
But an optimising compiler for a high-level language should make use of illegal opcodes when it makes sense. |
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chatGPZ
Registered: Dec 2001
Posts: 11386 |
i have played with both, super optimizer and pattern based optimizer some years ago... it didnt make a lot of sense for hand written asm code (ie, it almost never found something to optimize). for code generated by cc65 on the other hand it was somewhat useful =) |
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Peacemaker
Registered: Sep 2004
Posts: 275 |
i call for an bruteforce ill. opcode tool that creates billions of combinations of routines and its results ;) |
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Frantic
Registered: Mar 2003
Posts: 1648 |
Although I've used quite a few of the illegals I can only confirm what others have said about failing to find a sensible use for LAS and TAS. :) |
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chatGPZ
Registered: Dec 2001
Posts: 11386 |
one thing i could think of... a clever combination of LAS $0100,y and TAS $0100,y with some other illegal(s) involving a shift operation to build a very fast pseudo random sequence generator. perhaps :) |
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metalux
Registered: Aug 2011
Posts: 17 |
So, let's publish that paper! |
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Bitbreaker
Registered: Oct 2002
Posts: 508 |
Quote: one thing i could think of... a clever combination of LAS $0100,y and TAS $0100,y with some other illegal(s) involving a shift operation to build a very fast pseudo random sequence generator. perhaps :)
one can at least do an and operation on an index with TAS (SP that is) and read the table via PLA then. But SBX #$xx + LDA table,x would do just as well then. Interesting is the feedback possibility in that TAS command, as the resulting SP is can again fed back into the next call/lookup. Just as with LAX ($xx,x). Still i could not think of any realworld example, either for LAS/TAS as well as for the mentioned lax variant. I finally successfully used SAX $xx,y yesterday however. |
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Frantic
Registered: Mar 2003
Posts: 1648 |
Would be a nice coding compo. Come up with the best uses for those two opcodes. (I wouldn't spend time on that myself though, but I'd like to see the result of such a compo. :D ) |
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Bitbreaker
Registered: Oct 2002
Posts: 508 |
Implement a turing complete stack machine with those two opcodes :-P |
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