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Category:Z80 Assembly: Difference between revisions
→Bit Shifting
Line 85:
SLA A
SLA A
SLA A ;8 bytes, 32 cycles total
AND %00001111
Line 91:
RLCA
RLCA
RLCA ;6 bytes, 23 cycles total</lang>
Not only is the second method shorter, it's also faster. The accumulator-specific bit rotates take 1 byte and 4 clock cycles each. They are different, however, because unlike the two-byte versions, these <i>do not affect the zero flag.</i> This isn't a big deal, however, as more often than not if you're rotating the accumulator you're not expecting to get zero as the output anyway
Correction: The AND <i>does</i> affect the Z flag. So the end result will be the same, right?
If you want to know zero or not, you could also use this:
<lang z80>RLCA
RLCA
RLCA
RLCA
AND %11110000 ;6 bytes, 23 cycles total</lang>
For 16-bit bit shifting, use A instead of the other half of the register pair for faster results (unless you're checking for equality to zero, or you need the accumulator for something else.)
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