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| -rw-r--r-- | Optimizing-assembly-code.md | 17 |
1 files changed, 17 insertions, 0 deletions
diff --git a/Optimizing-assembly-code.md b/Optimizing-assembly-code.md index 2c4b827..867cb07 100644 --- a/Optimizing-assembly-code.md +++ b/Optimizing-assembly-code.md @@ -201,6 +201,23 @@ If `FOO` equals `BAR - 2`, then do: sbc -BAR ; -1 becomes BAR - 2 aka FOO, 0 becomes BAR ``` +If `FOO` is 1 and `BAR` is 0 (i.e. set `a` to 1 if carry or 0 if not carry), then do: + +```asm + ; 3 bytes, 3 cycles + ccf ; invert carry flag + sbc a ; if originally carry, then 0, else $ff aka -1 + inc a ; 0 becomes 1, -1 becomes 0 +``` + +If `FOO` is 0 and `BAR` is 0 (i.e. set `a` to 0 if carry or 1 if not carry), then do: + +```asm + ; 2 bytes, 2 cycles + sbc a ; if carry, then $ff aka -1, else 0 + inc a ; -1 becomes 0, 0 becomes 1 +``` + ### Shift `a` right by 3 bits |