Fix

1 files changed, 24 insertions, 32 deletions

diff --git a/Optimizing-assembly-code.md b/Optimizing-assembly-code.md
index c55d8fd..107dbd9 100644
--- a/Optimizing-assembly-code.md
+++ b/Optimizing-assembly-code.md
@@ -19,7 +19,7 @@ WikiTI's advice fully applies here:
   - [Set `a` to 0](#set-a-to-0)
   - [Invert the bits of `a`](#invert-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 carry](set-a-to-one-constant-or-another-depending-on-carry)
+  - [Set `a` to one constant or another depending on the carry flag](#set-a-to-one-constant-or-another-depending-on-the-carry-flag)
   - [Multiply `hl` by 2](#multiply-hl-by-2)
   - [Add `a` to a 16-bit register](#add-a-to-a-16-bit-register)
   - [Add `a` to an address](#add-a-to-an-address)
@@ -71,9 +71,9 @@ Don't use the optimized versions if you need to preserve flags. As such, `ld a,
 
 ```asm
 	ld a, [wIsTrainerBattle]
-	and a ; NZ if [wIsTrainerBattle] is nonzero
-	ld a, 0
-	jr nz, .trainer
+	and a   ; sets flag Z if [wIsTrainerBattle] is 0
+	ld a, 0 ; sets a to 0 without affecting Z
+	jr nz, .is_trainer_battle
 ```
 
 
@@ -112,15 +112,17 @@ But do:
 ```
 
 
-### Set `a` to one constant or another depending on carry
+### Set `a` to one constant or another depending on the carry flag
+
+(The example sets `a` to `ONE` if the carry flag is set (`c`), or `TWO` is the carry flag is not set (`nc`).)
 
 Don't do:
 
 ```asm
 	; 6 bytes, 6 or 7 cycles
-	ld a, CONST
+	ld a, ONE
 	jr c, .carry
-	ld a, 0
+	ld a, TWO
 .carry
 ```
 
@@ -128,48 +130,38 @@ And don't do:
 
 ```asm
 	; 6 bytes, 6 or 7 cycles
-	ld a, 0
+	ld a, TWO
 	jr nc, .no_carry
-	ld a, CONST
+	ld a, ONE
 .no_carry
 ```
 
 But do:
 
 ```asm
-	; 3 bytes, 3 cycles
-	sbc a
-	and CONST
+	; 5 bytes, 5 cycles
+	sbc a         ; if carry, then $ff, else 0
+	and ONE ^ TWO ; $ff becomes ONE ^ TWO, 0 stays 0
+	xor TWO       ; ONE ^ TWO becomes ONE, 0 becomes TWO
 ```
 
-Likewise, don't do:
+If `ONE` is 0 (i.e. set `a` to 0 if carry), then do:
 
 ```asm
-	; 6 bytes, 6 or 7 cycles
-	ld a, 0
-	jr c, .carry
-	ld a, CONST
-.carry
+	; 4 bytes, 4 cycles
+	ccf     ; invert carry flag
+	sbc a   ; if originally carry, then 0, else $ff
+	and TWO ; 0 stays 0, $ff becomes TWO
 ```
 
-And don't do:
+If `TWO` is 0 (i.e. set `a` to 0 if not carry), then do:
 
 ```asm
-	; 6 bytes, 6 or 7 cycles
-	ld a, CONST
-	jr nc, .no_carry
-	ld a, 0
-.no_carry
+	; 3 bytes, 3 cycles
+	sbc a   ; if carry, then $ff, else 0
+	and ONE ; $ff becomes ONE, 0 stays 0
 ```
 
-But do:
-
-```asm
-	; 4 bytes, 4 cycles
-	ccf
-	sbc a
-	xor CONST
-```
 
 
 ### Multiply `hl` by 2