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| -rw-r--r-- | Optimizing-assembly-code.md | 56 |
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 |