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| -rw-r--r-- | Optimizing-assembly-code.md | 24 |
1 files changed, 24 insertions, 0 deletions
diff --git a/Optimizing-assembly-code.md b/Optimizing-assembly-code.md index cfeb403..17dbc88 100644 --- a/Optimizing-assembly-code.md +++ b/Optimizing-assembly-code.md @@ -241,6 +241,30 @@ Or if you really want to optimize for size over speed, then do: ld a, b ``` +Or if you really want to optimize for speed over size, then do: + +```asm + ; 6 bytes, 12 cycles + ; (4 bytes, 5 cycles if you don't need the push hl/pop hl) + push hl + ld h, HIGH(ReversedBitTable) + ld l, a + ld a, [hl] + pop hl +``` + +```asm + ; 256 bytes; placed in ROM0 or the same ROMX section as the bit reversal +SECTION "ReversedBitTable", ROM0, ALIGN[8] +ReversedBitTable:: +x = 0 +rept 256 + ; http://graphics.stanford.edu/~seander/bithacks.html#ReverseByteWith32Bits + db LOW(((((x * $802) & $22110) | ((x * $8020) & $88440)) * $10101) >> 16) +x = x + 1 +endr +``` + ### Set `a` to some constant minus `a` |