mirror of
https://github.com/michaelcmartin/Ophis.git
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185 lines
2.8 KiB
HTML
185 lines
2.8 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<HTML
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><HEAD
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><TITLE
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>The Second Step</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK
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REL="HOME"
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TITLE="Programming with Ophis"
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HREF="book1.html"><LINK
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REL="UP"
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TITLE="To HLL and Back"
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HREF="p481.html"><LINK
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REL="PREVIOUS"
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TITLE="To HLL and Back"
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HREF="p481.html"><LINK
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REL="NEXT"
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TITLE="The solution"
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HREF="x498.html"></HEAD
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><BODY
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CLASS="CHAPTER"
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BGCOLOR="#FFFFFF"
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TEXT="#000000"
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LINK="#0000FF"
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VLINK="#840084"
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ALINK="#0000FF"
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><DIV
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CLASS="NAVHEADER"
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><TABLE
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SUMMARY="Header navigation table"
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WIDTH="100%"
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BORDER="0"
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CELLPADDING="0"
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CELLSPACING="0"
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><TR
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><TH
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COLSPAN="3"
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ALIGN="center"
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>Programming with Ophis</TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="p481.html"
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ACCESSKEY="P"
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><<< Previous</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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></TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="x498.html"
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ACCESSKEY="N"
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>Next >>></A
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></TD
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></TR
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="CHAPTER"
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><H1
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><A
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NAME="HLL-1"
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></A
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>The Second Step</H1
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><P
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> This essay discusses how to do 16-or-more bit addition and
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subtraction on the 6502, and how to do unsigned comparisons
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properly, thus making 16-bit arithmetic less necessary.</P
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><DIV
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CLASS="SECTION"
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><H1
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CLASS="SECTION"
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><A
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NAME="AEN489"
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>The problem</A
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></H1
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><P
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> The <TT
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CLASS="LITERAL"
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>ADC</TT
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>, <TT
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CLASS="LITERAL"
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>SBC</TT
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>, <TT
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CLASS="LITERAL"
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>INX</TT
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>,
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and <TT
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CLASS="LITERAL"
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>INY</TT
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> instructions are the only real
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arithmetic instructions the 6502 chip has. In and of themselves,
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they aren't too useful for general applications: the accumulator
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can only hold 8 bits, and thus can't store any value over 255.
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Matters get even worse when we're branching based on
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values; <TT
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CLASS="LITERAL"
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>BMI</TT
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> and <TT
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CLASS="LITERAL"
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>BPL</TT
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> hinge on
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the seventh (sign) bit of the result, so we can't represent any
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value above 127.
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</P
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></DIV
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></DIV
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><DIV
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CLASS="NAVFOOTER"
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><HR
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
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SUMMARY="Footer navigation table"
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WIDTH="100%"
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BORDER="0"
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CELLPADDING="0"
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CELLSPACING="0"
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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><A
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HREF="p481.html"
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ACCESSKEY="P"
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><<< Previous</A
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></TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="book1.html"
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ACCESSKEY="H"
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>Home</A
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></TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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><A
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HREF="x498.html"
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ACCESSKEY="N"
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>Next >>></A
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></TD
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></TR
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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>To HLL and Back</TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="p481.html"
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ACCESSKEY="U"
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>Up</A
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></TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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>The solution</TD
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></TR
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></TABLE
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></DIV
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></BODY
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></HTML
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> |