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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 |
To my shame I must confess that I haven't found any use for LAS or TAS either.
You'd think TAS would be useful for clearing all of the registers and stack pointer during initialization but LXA #$00/TAY/TXS is a byte shorter, plus you probably want reset S to $FF anyway.
As for SLO/SRE I may have mentioned this before but below is the host side of a two-bit IRQ transfer running in 64 cycles per byte, including loop overhead. A asynchronous IRQ protocol works by toggling ATN, then waiting for the maximum delay period when the drive can be relied on to have the results back before sampling them and toggling ATN for the next bit pair.
By using the SLO/SRE RMW instructions we can both sample DATA/CLK and toggle ATN within two cycles inside the same instruction, instead of four as you would get with separate EOR/STA. ldy #%00000100
;Assert ATN
loop: arr #%11111010 ;cdab00--
arr #%11111010 ;dcdab00-
ror ;cdcdab00
sre $dd00-4,y ;cefdaB--
;Release ATN
sty $dd00
alr #%11111100 ;0cefdaB0
sta merge+1
slo $dd00 ;hcef1--0
;Assert ATN
and #%10000000 ;h0000000
merge: adc #%00000000 ;hcefdaBg
sta sector,x
sre $dd00-4,y ;0ab001--
;Release ATN
inx
sty $dd00
beq break
and #%01100000 ;0ab00000
slo $dd00 ;dab01---
bne loop ;(BRA) |
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lft
Registered: Jul 2007
Posts: 369 |
@doynax: Although it makes some assumptions about which vic bank is active, right? |
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Copyfault
Registered: Dec 2001
Posts: 478 |
Hey Groepaz,
Ninjas 6-Sprites-over-FLI-routine makes heavy use of SLO and RRA (maybe also RLA, not 100% sure), but I guess this example is already covered in the pdf ?!?
I've never used LAE/SHS ($bb/$9b) up to now. As you are looking for "real world examples" I guess that you completely found out what this "unstability" (which is usually connected to both opcodes) really means, right?
Looking forward to reading this paper;)
Bye-
CF |
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chatGPZ
Registered: Dec 2001
Posts: 11386 |
ninja contributed that routine, yes :) and yes, the instabilities are covered :) |
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doynax
Account closed
Registered: Oct 2004
Posts: 212 |
Quoting lftAlthough it makes some assumptions about which vic bank is active, right? Yes, only banks 0 and 3 work and IRQ-safe switching eats 10 cycles over $dd02 fiddling.
Plus the 14-cycle response is pushing it, famously failing on the 1571DCR in 1 MHz mode. |
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chatGPZ
Registered: Dec 2001
Posts: 11386 |
forgot: yes, only "real world" examples are useful, i HAVE used these opcodes in test programs (look in VICE repository) |
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chatGPZ
Registered: Dec 2001
Posts: 11386 |
who would have though =) |
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Cruzer
Registered: Dec 2001
Posts: 1048 |
The only thing I have used TAS for is for being impressed by how much stuff the CPU can do in 5 cycles. Too bad it's useless stuff. |
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Martin Piper
Registered: Nov 2007
Posts: 722 |
Maybe it's time for an optimising assembler that suggests replacement code by scanning for known instruction sequences. |
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