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Diffstat (limited to 'Optimizing-assembly-code.md')
| -rw-r--r-- | Optimizing-assembly-code.md | 63 |
1 files changed, 62 insertions, 1 deletions
diff --git a/Optimizing-assembly-code.md b/Optimizing-assembly-code.md index 4e84545..c55d8fd 100644 --- a/Optimizing-assembly-code.md +++ b/Optimizing-assembly-code.md @@ -1,6 +1,6 @@ Sometimes the simplest way to write something in assembly code isn't the best. All of your resources are limited: CPU speed, ROM size, RAM space, register use. You can rewrite code to use those resources more efficiently (sometimes by trading one for another). -Most of these tricks come from either [Jeff's GB Assembly Code Tips v1.0](http://www.devrs.com/gb/files/asmtips.txt), or [WikiTI's Z80 Optimization page](http://wikiti.brandonw.net/index.php?title=Z80_Optimization). (Note that Z80 assembly is *not* the same as GBZ80; it has more registers and some different instructions.) +Most of these tricks come from [Jeff's GB Assembly Code Tips v1.0](http://www.devrs.com/gb/files/asmtips.txt), [WikiTI's Z80 Optimization page](http://wikiti.brandonw.net/index.php?title=Z80_Optimization), and [z80 Heaven's optimization tutorial](http://z80-heaven.wikidot.com/optimization). (Note that Z80 assembly is *not* the same as GBZ80; it has more registers and some different instructions.) WikiTI's advice fully applies here: @@ -19,6 +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) - [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) @@ -111,6 +112,66 @@ But do: ``` +### Set `a` to one constant or another depending on carry + +Don't do: + +```asm + ; 6 bytes, 6 or 7 cycles + ld a, CONST + jr c, .carry + ld a, 0 +.carry +``` + +And don't do: + +```asm + ; 6 bytes, 6 or 7 cycles + ld a, 0 + jr nc, .no_carry + ld a, CONST +.no_carry +``` + +But do: + +```asm + ; 3 bytes, 3 cycles + sbc a + and CONST +``` + +Likewise, don't do: + +```asm + ; 6 bytes, 6 or 7 cycles + ld a, 0 + jr c, .carry + ld a, CONST +.carry +``` + +And don't do: + +```asm + ; 6 bytes, 6 or 7 cycles + ld a, CONST + jr nc, .no_carry + ld a, 0 +.no_carry +``` + +But do: + +```asm + ; 4 bytes, 4 cycles + ccf + sbc a + xor CONST +``` + + ### Multiply `hl` by 2 Don't do: |