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Monte Carlos
Registered: Jun 2004
Posts: 351 |
Event id #2417 : First CSDb "Unintended OpCode coding challenge"
So here it is. The First CSDb "Unintended OpCode coding challenge" starts over.
First CSDb "Unintended OpCode Coding Challenge"
Please give some feedback about your interest in this compo.
For those who have been part of the discussion
http://csdb.dk/forums/?roomid=12&topicid=112819#112927
please let me know if you are in better agreement with the reworked rules than before.
However, there will not be a rule change anymore. |
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Rastah Bar
Registered: Oct 2012
Posts: 336 |
Quote: Nice! A optimization that comes to my mind at a chort glance:
eor #$60
beq start ; shift a zero bit in
sec ; shift a one bit in
jmp start+1
;isn't that the same as:
eor #$60
cmp #$01 ;sets carry on $20 $40 $60 and clears carry on $00
bne start+1 ;branch always
The lda #$60 + eor #$60 gives me the feeling that it can be optimized too, twice the same value :-D Also it would help to get a negated carry, hmm
Yes, thanks for the optimization!
I'm studying Groepaz's document now to see if another UOC can be sneaked in advantageously :-) |
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Bitbreaker
Registered: Oct 2002
Posts: 501 |
Next try, and hope i don't spoil the fun, also: untested and just sketched in an editor :-D
lda #seed1
sta zp1
lda #seed2
start:
clc
ldx #$60
rol zp1
rol
sbx #$00 ;x = a & $60
beq start ;x = 0?
sbx #$60 ;$60-$60=0, $20 - $60=$c0 $40-$60=$e0
cpx #$01 ;clear carry on x = 0, else set
bne start+1
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Rastah Bar
Registered: Oct 2012
Posts: 336 |
Nice. The code after start: is 1 cycle faster, isn't it? But it does use the x-register as well.
For practical use of the MLS the code should probably be modified such that you can go through one update of it (as a subroutine?) and use the output elsewhere.
I don't want to give up too easily on RLA ;-) |
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Bitbreaker
Registered: Oct 2002
Posts: 501 |
lda #seed1
sta zp1
lda #seed2
clc
start:
ldx #$60
rol zp1
rol
sbx #$e0 ;x = a & $60 - $e0 -> clc
bpl start
cpx #$40
;$20-$40 -> clc $40-$40 -> sec $60-$40 -> sec
bne start
Still untested, so i might be very wrong. Of course this method would not work well as a call, but i guess it was not the focus? :-D |
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Rastah Bar
Registered: Oct 2012
Posts: 336 |
I am not sure I understand this version. X&A may contain $00, $20, $40, or $60, flags in SBX are set like CMP, so the BPL start never happens??
Focus? There wasn't any focus ... I was just looking for a nice application for opcodes like SLO, SRE, RLA, RRA :-D |
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Bitbreaker
Registered: Oct 2002
Posts: 501 |
Umm, i guess it should be bmi if i get it right, but flags are set perfectly well by sbx, so no problem.
The values i have noted are the input and resulting values. We need to focus on $00, $20, $40, $60 (00,01,10,11) by subtracting $e0 we manage to receive a negative result only upon $60 as input value, so we can branch out beforehand here as we would end up with a wrong carry in the following code. What remains are $00 - $e0, $20 - $e0 and $40 - $e0 as further results. so for 00 we have $20, for case 01 we have $40 and for case 10 we have $60 as result in X. If we now do a cpx we end up to have the carry cleared for case 00 and set for case 01 and case 10, right? After that we branch always. However i just notice that we should better do a cpx #$3f to fullfill the branch always :-D
lda #seed1
sta zp1
lda #seed2
clc
start:
ldx #$60
rol zp1
rol
sbx #$e0 ;x = a & $60 - $e0 -> clc
bmi start
cpx #$3f
;$20-$40 -> clc $40-$40 -> sec $60-$40 -> sec
bne start
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Rastah Bar
Registered: Oct 2012
Posts: 336 |
That is even more efficient. Cool!
From this and other examples such as the ones posted by Monte Carlos I get the impression that SBX is a pretty useful UOC. |
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Oswald
Registered: Apr 2002
Posts: 5034 |
anything to emulate cpx ,y or cpy ,x ? |
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algorithm
Registered: May 2002
Posts: 702 |
@Oswald, maybe something like the below?
stx loc+1
loc cpy #$00
with code in zero page, would use 5 cycles.
No single illegal/legal opcode for the above :-( |
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Oswald
Registered: Apr 2002
Posts: 5034 |
I mean cpx abs,y or vice versa, they seem so obvious to have still not implemented.
damn you chuck ;) |
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