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| -rw-r--r-- | Optimizing-assembly-code.md | 38 |
1 files changed, 15 insertions, 23 deletions
diff --git a/Optimizing-assembly-code.md b/Optimizing-assembly-code.md index 5d80acf..9933b84 100644 --- a/Optimizing-assembly-code.md +++ b/Optimizing-assembly-code.md @@ -35,7 +35,6 @@ 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) - [Set a 16-bit register to `a` plus a 16-bit constant](#set-a-16-bit-register-to-a-plus-a-16-bit-constant) - - [Set a 16-bit register to `a` plus a nonzero 8-bit constant](#set-a-16-bit-register-to-a-plus-a-nonzero-8-bit-constant) - [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) @@ -679,21 +678,21 @@ But do: Don't do: ```asm - ld a, [hFoo] ; 3 bytes, 4 cycles + ld a, [hFoobar] ; 3 bytes, 4 cycles ``` ```asm - ld [hFoo], a ; 3 bytes, 4 cycles + ld [hFoobar], a ; 3 bytes, 4 cycles ``` But do: ```asm - ldh a, [hFoo] ; 2 bytes, 3 cycles + ldh a, [hFoobar] ; 2 bytes, 3 cycles ``` ```asm - ldh [hFoo], a ; 2 bytes, 3 cycles + ldh [hFoobar], a ; 2 bytes, 3 cycles ``` @@ -784,7 +783,7 @@ Don't do: ```asm ; 8 bytes, 8 cycles ld a, l - sub FOO + sub FOOBAR ld l, a ld a, h sbc 0 @@ -796,7 +795,7 @@ But do: ```asm ; 7 bytes, 7 or 8 cycles ld a, l - sub FOO + sub FOOBAR ld l, a jr nc, .no_carry dec h @@ -807,7 +806,7 @@ Or if you can spare another 16-bit register, do: ```asm ; 4 bytes, 5 cycles - ld de, -FOO + ld de, -FOOBAR add hl, de ``` @@ -822,7 +821,7 @@ Don't do: ; 7 bytes, 8 cycles; uses another 16-bit register ld e, a ld d, 0 - ld hl, Address + ld hl, FooBar add hl, de ``` @@ -830,7 +829,7 @@ And don't do: ```asm ; 8 bytes, 8 cycles - ld hl, Address + ld hl, FooBar add l ld l, a adc h @@ -842,8 +841,8 @@ And don't do: ```asm ; 8 bytes, 7 or 8 cycles - ld h, HIGH(Address) - add LOW(Address) + ld h, HIGH(FooBar) + add LOW(FooBar) ld l, a jr nc, .no_carry inc h @@ -854,31 +853,24 @@ But do: ```asm ; 7 bytes, 7 cycles - add LOW(Address) + add LOW(FooBar) ld l, a - adc HIGH(Address) + adc HIGH(FooBar) sub l ld h, a ``` - -### Set a 16-bit register to `a` plus a nonzero 8-bit constant - -(The example uses `hl`, but `bc` or `de` would also work.) - -Don't do any of [the above](#set-a-16-bit-register-to-a-plus-a-16-bit-constant) methods for a 16-bit constant. Instead, do: +And if the constant is 8-bit and nonzero (i.e. 0 < `FooBar` < 256), then do: ```asm ; 6 bytes, 6 cycles - sub -FOO + sub LOW(-FooBar) ld l, a sbc a inc a ld h, a ``` -This will work for any constant `FOO` from 1 to 255, but not 0. - ### Set a 16-bit register to `a` \* 16 |