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Diffstat (limited to 'Optimizing-assembly-code.md')
| -rw-r--r-- | Optimizing-assembly-code.md | 36 |
1 files changed, 28 insertions, 8 deletions
diff --git a/Optimizing-assembly-code.md b/Optimizing-assembly-code.md index a4a1347..9b28f12 100644 --- a/Optimizing-assembly-code.md +++ b/Optimizing-assembly-code.md @@ -46,6 +46,7 @@ WikiTI's advice fully applies here: - [Compare `a` to 1](#compare-a-to-1) - [Compare `a` to 255](#compare-a-to-255) - [Compare `a` to 0 after masking it](#compare-a-to-0-after-masking-it) + - [Test whether `a` is negative (compare `a` to $80)](#test-whether-a-is-negative-compare-a-to-80) - [Subroutines (functions)](#subroutines-functions) - [Tail call optimization](#tail-call-optimization) - [Call `hl`](#call-hl) @@ -133,37 +134,37 @@ But do: Don't do: ```asm - rl a ; 2 bytes, 2 cycles + rl a ; 2 bytes, 2 cycles; updates Z and C flags ``` ```asm - rlc a ; 2 bytes, 2 cycles + rlc a ; 2 bytes, 2 cycles; updates Z and C flags ``` ```asm - rr a ; 2 bytes, 2 cycles + rr a ; 2 bytes, 2 cycles; updates Z and C flags ``` ```asm - rrc a ; 2 bytes, 2 cycles + rrc a ; 2 bytes, 2 cycles; updates Z and C flags ``` But do: ```asm - rla ; 1 byte, 1 cycle + rla ; 1 byte, 1 cycle; updates C flag ``` ```asm - rlca ; 1 byte, 1 cycle + rlca ; 1 byte, 1 cycle; updates C flag ``` ```asm - rra ; 1 byte, 1 cycle + rra ; 1 byte, 1 cycle; updates C flag ``` ```asm - rrca ; 1 byte, 1 cycle + rrca ; 1 byte, 1 cycle; updates C flag ``` The exception is if you need to set the zero flag when the operation results in 0 for `a`; the two-byte operations can set `z`, the one-byte operations cannot. @@ -912,6 +913,25 @@ But do: ``` +### Test whether `a` is negative (compare `a` to $80) + +If you don't need to preserve the value in `a`, then don't do: + +``` + ; 4 bytes, 4/5 cycles + cp $80 ; high bit means negative + jr nc, .negative +``` + +Instead, do: + +``` + ; 3 bytes, 3/4 cycles + rlca ; high bit means negative + jr c, .negative +``` + + ## Subroutines (functions) |