hl = a * 16

1 files changed, 102 insertions, 0 deletions

diff --git a/Optimizing-assembly-code.md b/Optimizing-assembly-code.md
index 8346020..d1dd25f 100644
--- a/Optimizing-assembly-code.md
+++ b/Optimizing-assembly-code.md
@@ -34,6 +34,7 @@ WikiTI's advice fully applies here:
   - [Add `a` to a 16-bit register](#add-a-to-a-16-bit-register)
   - [Subtract an 8-bit constant from a 16-bit register](#subtract-an-8-bit-constant-from-a-16-bit-register)
   - [Add `a` to an address](#add-a-to-an-address)
+  - [Set a 16-bit register to `a` \* 16](#set-a-16-bit-register-to-a--16)
   - [Increment or decrement a 16-bit register](#increment-or-decrement-a-16-bit-register)
   - [Load from an address to `hl`](#load-from-an-address-to-hl)
   - [Exchange two 16-bit registers](#exchange-two-16-bit-registers)
@@ -631,6 +632,107 @@ But do:
 ```
 
 
+### Set a 16-bit register to `a` \* 16
+
+(The example uses `hl`, but `bc` or `de` would also work.)
+
+You can do:
+
+```
+; 7 bytes, 11 cycles
+	ld l, a
+	ld h, 0
+	add hl, hl
+	add hl, hl
+	add hl, hl
+	add hl, hl
+```
+
+```
+; 7 bytes, 11 cycles
+	ld l, a
+	ld h, 0
+rept 4
+	add hl, hl
+endr
+```
+
+But if `a` is definitely small enough, and its value can be changed, then do:
+
+```
+; 7 bytes, 10 cycles; sets a = a * 2; requires a < $80
+	add a
+	ld l, a
+	ld h, 0
+	add hl, hl
+	add hl, hl
+	add hl, hl
+```
+
+```
+; 7 bytes, 9 cycles; sets a = a * 4; requires a < $40
+	add a
+	add a
+	ld l, a
+	ld h, 0
+	add hl, hl
+	add hl, hl
+```
+
+```
+; 7 bytes, 8 cycles; sets a = a * 8; requires a < $20
+	add a
+	add a
+	add a
+	ld l, a
+	ld h, 0
+	add hl, hl
+```
+
+```
+; 7 bytes, 7 cycles; sets a = a * 16; requires a < $10
+	add a
+	add a
+	add a
+	add a
+	ld l, a
+	ld h, 0
+```
+
+```
+; 7 bytes, 7 cycles; sets a = a * 16; requires a < $10
+rept 4
+	add a
+endr
+	ld l, a
+	ld h, 0
+```
+
+Or if the value of `a` can be changed and you want to optimize for speed over size, do:
+
+```
+; 8 bytes, 8 cycles; sets a = l
+	swap a
+	ld l, a
+	and $f
+	ld h, a
+	xor l
+	ld l, a
+```
+
+Or do:
+
+```
+; 8 bytes, 8 cycles; sets a = h
+	swap a
+	ld h, a
+	and $f0
+	ld l, a
+	xor h
+	ld h, a
+```
+
+
 ### Increment or decrement a 16-bit register
 
 When possible, avoid doing: