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Atari coding BBS
| 5 instr mapper |
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Posted by: MKM/Lamers
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Sep,09.2016-21:40
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Hi,
I have a question about famous "5 instr texture mapper". It is described often on both atari and amiga sites... and I almost got it working, but have one doubt. If we have standart:
move.w d0,d1
move.b d2,d1
move.w (a0,d1.l*2),(a1)+
add.l d3,d0
addx.b d4,d2
with layout:
uu__VVvv (d0 - current value, d3 - step)
______UU (d2 - current value, d4 - step)
(I guess I don't need to describe more as it is common pattern)
I obviously works when UUuu step is positive. But for me it seems not to be working when UUuu step is negavite. All values will be computed correctly appart of the fact that Extend flag will be added to d2 when it should be substracted in negative case.
So, it seems for me I need separate code path with subx.b for negative case, but on the other hand fact that all "5 instr mapper" posts I found does not mention it, seems to get me wonder that I am missing something there. Am I?
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