hl|bc|de -= N

1 files changed, 40 insertions, 0 deletions

diff --git a/Optimizing-assembly-code.md b/Optimizing-assembly-code.md
index 54af5ba..7d7afd9 100644
--- a/Optimizing-assembly-code.md
+++ b/Optimizing-assembly-code.md
@@ -28,6 +28,7 @@ WikiTI's advice fully applies here:
 - [16-bit registers](#16-bit-registers)
   - [Multiply `hl` by 2](#multiply-hl-by-2)
   - [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)
   - [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)
@@ -490,6 +491,45 @@ Or if you can spare another 16-bit register and want to optimize for size over s
 	add hl, de
 ```
 
+
+### Subtract an 8-bit constant from a 16-bit register
+
+(The example uses `hl`, but `bc` or `de` would also work.)
+
+Don't do:
+
+```asm
+	; 8 bytes, 8 cycles
+	ld a, l
+	sub FOO
+	ld l, a
+	ld a, h
+	sbc 0
+	ld h, a
+```
+
+But do:
+
+```asm
+	; 7 bytes, 7 or 8 cycles
+	ld a, l
+	sub FOO
+	ld l, a
+	jr nc, .no_carry
+	dec h
+
+.no_carry:
+```
+
+Or if you can spare another 16-bit register, do:
+
+```asm
+	; 4 bytes, 5 cycles
+	ld de, -FOO
+	add hl, de
+```
+
+
 ### Add `a` to an address
 
 (The example uses `hl`, but `bc` or `de` would also work.)