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chatGPZ
Registered: Dec 2001
Posts: 11150 |
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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Frantic
Registered: Mar 2003
Posts: 1633 |
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: 11150 |
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: 16 |
So, let's publish that paper! |
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Bitbreaker
Registered: Oct 2002
Posts: 501 |
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: 1633 |
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: 501 |
Implement a turing complete stack machine with those two opcodes :-P |
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Cruzer
Registered: Dec 2001
Posts: 1048 |
Quoting BitbreakerImplement a turing complete stack machine with those two opcodes :-P Can't be done since they can't branch. |
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Perplex
Registered: Feb 2009
Posts: 254 |
Use TAS to store a branch opcode somewhere after the current program counter? :-) |
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Cruzer
Registered: Dec 2001
Posts: 1048 |
Ok, if you're allowed to cheat like that. :) |
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Mixer
Registered: Apr 2008
Posts: 422 |
Funny opcodes those LAS and TAS
TAS $fe00,y would let you insert any SP value on that page.
TAS $7e00,y would let you insert any SP value AND #$7f on that page, etc.
Sort of having an extra register.Except that SP gets replaced by A and X.
On the other hand perhaps this is a way to do 2 AND operations at the same time?
TAS ADDR,Y: ADDR=SP and ADDRHI+1, SP=A and X.
; perhaps when calculating and #$f8 and and #$07
ldy #$00
lda #$07
ldx ycoordinate,y
txs
tas $f700,y ; f700,y has Ycoord & F8 , SP has ycoord & 07
; perhaps use prepared stack, PLA here?
tas $fe00,y ; if this is correct, fe00,y now has ycoord & 07
;2+2+4+2+5+5
;$f700=y &f8, SP=Y &07
Same could be written without stackpointer like this.
ldy #$00
lax ycordinate,y
and #$f8
sta $f700,y
lda #$07
sax $fe00,y
;4+2+4+2+4+2
and with tables
ldy #$00
ldx yycoordinate,y
lda table1,x ; X and f8
sta $f700,y
lda table2,x ; X and 07
sta $fe00,y
;2+4+4+4+4+4 |
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