http://www.retroprogramming.com/2014/01/fast-z80-bit-reversal.html

1 files changed, 51 insertions, 0 deletions

diff --git a/Optimizing-assembly-code.md b/Optimizing-assembly-code.md
index bc5c107..b3453b4 100644
--- a/Optimizing-assembly-code.md
+++ b/Optimizing-assembly-code.md
@@ -22,6 +22,7 @@ WikiTI's advice fully applies here:
   - [Increment or decrement `a`](#increment-or-decrement-a)
   - [Invert the bits of `a`](#invert-the-bits-of-a)
   - [Rotate the bits of `a`](#rotate-the-bits-of-a)
+  - [Reverse the bits of `a`](#reverse-the-bits-of-a)
   - [Set `a` to some constant minus `a`](#set-a-to-some-constant-minus-a)
   - [Set `a` to one constant or another depending on the carry flag](#set-a-to-one-constant-or-another-depending-on-the-carry-flag)
   - [Increment or decrement `a` when the carry flag is set](#increment-or-decrement-a-when-the-carry-flag-is-set)
@@ -172,6 +173,56 @@ But do:
 The exception is if you need to set the zero flag when the operation results in 0 for `a`; the two-byte operations can set `z`, the one-byte operations cannot.
 
 
+### Reverse the bits of `a`
+
+(This optimization is from [Retro Programming](http://www.retroprogramming.com/2014/01/fast-z80-bit-reversal.html)).
+
+Don't do:
+
+```asm
+; 10 bytes, 59 cycles
+	ld bc, 8  ; lb bc, 0, 8
+.loop
+	rra
+	rl b
+	dec c
+	jr nz, .loop
+	ld a, b
+```
+
+And don't do:
+
+```asm
+; 27 bytes, 27 cycles
+	ld b, 0
+rept 8
+	rra
+	rl b
+endr
+	ld a, b
+```
+
+But do:
+
+```asm
+; 17 bytes, 17 cycles
+	ld b, a
+	rlca
+	rlca
+	xor b
+	and $aa
+	xor b
+	ld b, a
+	rlca
+	rlca
+	rlca
+	rrc b
+	xor b
+	and $66
+	xor b
+```
+
+
 ### Set `a` to some constant minus `a`
 
 Don't do: