Add or subtract the carry flag from a register besides `a`

1 files changed, 84 insertions, 2 deletions

diff --git a/Optimizing-assembly-code.md b/Optimizing-assembly-code.md
index 893ade2..27c9433 100644
--- a/Optimizing-assembly-code.md
+++ b/Optimizing-assembly-code.md
@@ -28,7 +28,8 @@ WikiTI's advice fully applies here:
   - [Increment or decrement `a` when the carry flag is set](#increment-or-decrement-a-when-the-carry-flag-is-set)
   - [Divide `a` by 8 (shift `a` right 3 bits)](#divide-a-by-8-shift-a-right-3-bits)
   - [Divide `a` by 16 (shift `a` right 4 bits)](#divide-a-by-16-shift-a-right-4-bits)
-  - [Set `a` to some value plus carry](#set-a-to-some-value-plus-carry)
+  - [Set `a` to some value plus or minus carry](#set-a-to-some-value-plus-or-minus-carry)
+  - [Add or subtract the carry flag from a register besides `a`](#add-or-subtract-the-carry-flag-from-a-register-besides-a)
   - [Load from HRAM to `a` or from `a` to HRAM](#load-from-hram-to-a-or-from-a-to-hram)
 - [16-bit registers](#16-bit-registers)
   - [Multiply `hl` by 2](#multiply-hl-by-2)
@@ -616,7 +617,7 @@ But do:
 ```
 
 
-### Set `a` to some value plus carry
+### Set `a` to some value plus or minus carry
 
 (The example uses `b` and `c`, but any registers besides `a` would also work, including `[hl]`.)
 
@@ -629,6 +630,13 @@ Don't do:
 	adc 0
 ```
 
+```asm
+	; 4 bytes, 4 cycles
+	ld b, a
+	ld a, c
+	sbc 0
+```
+
 And don't do:
 
 ```asm
@@ -638,6 +646,13 @@ And don't do:
 	adc c
 ```
 
+```asm
+	; 4 bytes, 4 cycles
+	ld b, a
+	ld a, 0
+	sbc c
+```
+
 But do:
 
 ```asm
@@ -647,6 +662,13 @@ But do:
 	sub b
 ```
 
+```asm
+	; 3 bytes, 3 cycles
+	ld b, a
+	sbc c
+	add b
+```
+
 Also, don't do:
 
 ```asm
@@ -656,6 +678,13 @@ Also, don't do:
 	adc 0
 ```
 
+```asm
+	; 5 bytes, 5 cycles
+	ld b, a
+	ld a, N
+	sbc 0
+```
+
 And don't do:
 
 ```asm
@@ -665,6 +694,13 @@ And don't do:
 	adc N
 ```
 
+```asm
+	; 5 bytes, 5 cycles
+	ld b, a
+	ld a, 0
+	sbc N
+```
+
 But do:
 
 ```asm
@@ -674,6 +710,52 @@ But do:
 	sub b
 ```
 
+```asm
+	; 4 bytes, 4 cycles
+	ld b, a
+	sbc N
+	add b
+```
+
+(If the original value of `a` was not backed up in `b`, this optimization would not apply.)
+
+
+### Add or subtract the carry flag from a register besides `a`
+
+(The example uses `b`, but any of `c`, `d`, `e`, `h`, or `l` would also work.)
+
+Don't do:
+
+```asm
+	; 4 bytes, 4 cycles
+	ld a, b
+	adc 0
+	ld b, a
+```
+
+```asm
+	; 4 bytes, 4 cycles
+	ld a, b
+	sbc 0
+	ld b, a
+```
+
+But do:
+
+```asm
+	; 3 bytes, 3 or 4 cycles
+	jr nc, .no_carry
+	inc b
+.no_carry
+```
+
+```asm
+	; 3 bytes, 3 or 4 cycles
+	jr nc, .no_carry
+	dec b
+.no_carry
+```
+
 
 ### Load from HRAM to `a` or from `a` to HRAM