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196 lines
3.2 KiB
HTML
196 lines
3.2 KiB
HTML
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<HTML
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>16-bit addition and subtraction</TITLE
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TITLE="Programming with Ophis"
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TITLE="The Second Step"
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HREF="c486.html"><LINK
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TITLE="Unsigned arithmetic"
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TITLE="16-bit comparisons"
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>The Second Step</TD
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><A
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NAME="AEN527"
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>16-bit addition and subtraction</A
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></H1
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><P
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> Time to use the carry bit for what it was meant to do. Adding two
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8 bit numbers can produce a 9-bit result. That 9th bit is stored
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in the carry flag. The <TT
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CLASS="LITERAL"
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>ADC</TT
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> command adds the
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carry value to its result, as well. Thus, carries work just as
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we'd expect them to. Suppose we're storing two 16-bit values, low
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byte first, in $C100-1 and $C102-3. To add them together and
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store them in $C104-5, this is very easy:
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</P
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WIDTH="100%"
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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> CLC
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LDA $C100
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ADC $C102
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STA $C104
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LDA $C101
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ADC $C103
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STA $C105</PRE
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></TD
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></TR
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></TABLE
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><P
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> Subtraction is identical, but you set the carry bit first
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with <TT
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CLASS="LITERAL"
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>SEC</TT
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> (because borrow is the complement of
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carry—think about how the unsigned compare works if this
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puzzles you) and, of course, using the <TT
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CLASS="LITERAL"
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>SBC</TT
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>
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instruction instead of <TT
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CLASS="LITERAL"
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>ADC</TT
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>.
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</P
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><P
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> The carry/borrow bit is set appropriately to let you continue,
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too. As long as you just keep working your way up to bytes of
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ever-higher significance, this generalizes to 24 (do it three
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times instead of two) or 32 (four, etc.) bit integers.
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</P
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> |