Category:Z80 Assembly: Difference between revisions
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Suppose you have a byte at some memory location and you want to know if it's odd or even.
<syntaxhighlight lang=z80>
If you don't need the actual value, the first choice would be:▼
JR nz,odd</lang>▼
<lang z80>LD A,(HL) ; 1 byte, 7 cycles▼
BIT 0,A ; 2 bytes, 8 cycles
JR nz,odd
</syntaxhighlight>
<syntaxhighlight lang=z80>
BIT 0,(HL) ; 2 bytes, 12 cycles
</syntaxhighlight>
But there are also a few other ways to do it, which are faster and/or take fewer bytes to execute. <code>BIT 0,A</code> takes 2 bytes to encode. So does <code>AND 1</code> but it's a little faster. It does destroy the accumulator but if you don't care about that, it's better to use <code>AND 1</code>.▼
<syntaxhighlight lang=z80>
▲But there are also a few other ways to do it, which are faster and/or take fewer bytes to execute. <code>BIT 0,A</code> takes 2 bytes to encode. So does <code>AND 1</code> but it's a little faster. It does destroy the accumulator but if you don't care about that, it's better to use AND 1.
AND 1 ; 2 bytes, 7 cycles
JR nz,odd
</syntaxhighlight>
Or is it? There's an even faster way than this that takes only ONE byte to encode:
<syntaxhighlight lang=z80>
▲<lang z80>LD A,(HL) ; 1 byte, 7 cycles
RRCA ; 1 byte, 4 cycles
JR c,odd
</syntaxhighlight>
If you don't care about destroying the accumulator (i.e. you only need the result of the test and not the number being tested) then <code>RRCA</code> outperforms <code>AND 1</code> in every way. <code>AND 1</code> in this case still has its uses if you need bit 0 to reflect the result of the test. If you don't, use <code>RRCA</code> instead.
===Bit Shifting===
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