Lookup table

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`