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z80_unpacker: optimisation: -1B and -1T in decode_bit = 173B

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Peter Helcmanovsky (Ped)
2022-09-18 23:41:51 +02:00
parent c8924456aa
commit 1fb29f3a1b
2 changed files with 3 additions and 3 deletions
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z80_unpacker/example/example.sna
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z80_unpacker/unpack.asm
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@@ -259,16 +259,16 @@ decode_bit:
; so 7*128 > 250 and thus edge case `ADD hl=(7*128+0),de=(-250)` => CF=1
.bit_is_0_2:
; *** adjust probs[context_index]
ld e,a ; preserve prob
rra ; + (bit<<4) ; part of -prob_offset, needs another -16
and $FC ; clear/keep correct bits to get desired (prob>>4) + extras, CF=0
rra
rra
rra ; A = (bit<<4) + (prob>>4), CF=(prob & 8)
adc a,-16 ; A = (bit<<4) - 16 + ((prob + 8)>>4) ; -prob_offset = (bit<<4) - 16
sub e ; A = (bit<<4) - 16 + ((prob + 8)>>4) - prob ; = ((prob + 8)>>4) - prob_offset - prob
neg ; A = prob_offset + prob - ((prob + 8)>>4)
ld e,a
pop bc
ld a,(bc) ; A = prob (cheaper + shorter to re-read again from memory)
sub e ; A = 16 - (bit<<4) + prob - ((prob + 8)>>4) ; = prob_offset + prob - ((prob + 8)>>4)
ld (bc),a ; probs[context_index] = prob_offset + prob - ((prob + 8) >> 4);
add a,d ; restore CF = bit (D = bit ? $FF : $00 && A > 0)
pop de