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zasm: optimize handleRST a little bit

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This commit is contained in:
Virgil Dupras 2019-11-12 19:45:56 -05:00
parent 4de2ce3ceb
commit 0e9173a89a
1 changed files with 8 additions and 10 deletions
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18
apps/zasm/instr.asm
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@ -559,16 +559,14 @@ handleLDrr:
handleRST:
ld a, (INS_CURARG1+1)
; verify that A is either 0x08, 0x10, 0x18, 0x20, 0x28, 0x30 or 0x38.
; Good news: they're all multiples of 8.
; to verify that we're within range, we to 8-bit rotation. If any of
; the first 3 bytes are set (thus not a multiple of 8), the cp 8
; later will yield NC because those bits will now be upwards.
rrca \ rrca \ rrca
cp 8
jr nc, .error
; good, we have a proper arg. Now let's get those 3 bits in position
rlca \ rlca \ rlca
or 0b11000111
; Good news: the relevant bits (bits 5:3) are already in place. We only
; have to verify that they're surrounded by zeroes.
ld c, 0b11000111
and c
jr nz, .error
; We're in range. good.
ld a, (INS_CURARG1+1)
or c
ld (INS_UPCODE), a
ld c, 1
ret