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julie_m · 9th Jul 2019, 3:10 pm

Ding, ding, we have a winner! This is 2 bytes shorter and 7 cycles faster:

Code:

.dotest
LDY offsets,X
INX
LDA (fxb),Y
LDY offsets,X
INX
CMP (mvb),Y
RTS

I'd still need to INX again sometime if I used offsets+1 in the second LDY -- and bear in mind there must be a conditional branch after the JSR that called this subroutine. The offsets are only 8 bits (actually, they're only 4 bits) but it's not worth unpacking 8 bytes into 16 for this. (The unpacking code would need 4 bytes for the LSR A if using the high nybble, two for the conditional branch around them and two more for the AND &F if using the low nybble -- and that's the saving wiped out before we've even done an LDA or a TAX!) The rest of the project makes very heavy use of bit-packed co-ordinates (12 bit signed X and Y values in 3 bytes); and in some places, uses robbed bits to specify a plot mode -- move, draw, triangle or close. But one byte saved per point plotted (and points saved through the use of "close") is worth it.

Splitting the table of offsets would be a slap in the face for anybody trying to reverse-engineer the code in future. (I do plan to publish the Source Code, but you never know .....) So I'm holding off that technique unless and until the code ever gets one byte the wrong side of running on a model B!

9th Jul 2019, 3:10 pm	#24
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Fun with 6502 Assembler Ding, ding, we have a winner! This is 2 bytes shorter and 7 cycles faster: Code: .dotest LDY offsets,X INX LDA (fxb),Y LDY offsets,X INX CMP (mvb),Y RTS I'd still need to INX again sometime if I used offsets+1 in the second LDY -- and bear in mind there must be a conditional branch after the JSR that called this subroutine. The offsets are only 8 bits (actually, they're only 4 bits) but it's not worth unpacking 8 bytes into 16 for this. (The unpacking code would need 4 bytes for the LSR A if using the high nybble, two for the conditional branch around them and two more for the AND &F if using the low nybble -- and that's the saving wiped out before we've even done an LDA or a TAX!) The rest of the project makes very heavy use of bit-packed co-ordinates (12 bit signed X and Y values in 3 bytes); and in some places, uses robbed bits to specify a plot mode -- move, draw, triangle or close. But one byte saved per point plotted (and points saved through the use of "close") is worth it. Splitting the table of offsets would be a slap in the face for anybody trying to reverse-engineer the code in future. (I do plan to publish the Source Code, but you never know .....) So I'm holding off that technique unless and until the code ever gets one byte the wrong side of running on a model B! __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.