mirror of
https://github.com/michaelcmartin/Ophis.git
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295 lines
4.6 KiB
HTML
295 lines
4.6 KiB
HTML
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<!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 Solution</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="Advanced Memory Segments"
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HREF="c419.html"><LINK
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REL="PREVIOUS"
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TITLE="Advanced Memory Segments"
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HREF="c419.html"><LINK
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REL="NEXT"
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TITLE="Where to go from here"
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HREF="x449.html"></HEAD
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><BODY
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CLASS="SECTION"
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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="c419.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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>Advanced Memory Segments</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="x449.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="SECTION"
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><H1
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CLASS="SECTION"
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><A
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NAME="AEN430"
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>The Solution</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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>.data</TT
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> and <TT
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CLASS="LITERAL"
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>.text</TT
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>
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commands can take a label name after them—this names a new
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segment. We'll define a new segment
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called <TT
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CLASS="LITERAL"
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>zp</TT
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> (for <SPAN
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CLASS="QUOTE"
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>"zero page"</SPAN
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>) and
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have our zero-page variables be placed there. We can't actually
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use the default origin of $0000 here either, though, because the
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Commodore 64 reserves memory locations 0 and 1 to control its
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memory mappers:
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</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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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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>.data zp
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.org $0002</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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> Now, actually, the rest of the zero page is reserved too:
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locations $02-$7F are used by the BASIC interpreter, and
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locations $80-$FF are used by the KERNAL. We don't need the
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BASIC interpreter, though, so we can back up all of $02-$7F at
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the start of our program and restore it all when we're done:
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</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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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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>.scope
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; Cache BASIC's zero page at top of available RAM.
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ldx #$7E
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* lda $01, x
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sta $CF81, x
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dex
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bne -
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jsr _main
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; Restore BASIC's zero page and return control.
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ldx #$7E
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* lda $CF81, x
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sta $01, x
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dex
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bne -
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rts
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_main:
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; _main points at the start of the real program,
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; which is actually outside of this scope
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.scend</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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> The new, improved header file is <A
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HREF="x497.html"
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><I
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><I
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>c64-2.oph</I
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></I
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></A
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>.
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</P
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><P
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> Our <TT
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CLASS="LITERAL"
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>print'str</TT
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> routine is then rewritten to
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declare and use a zero-page variable, like so:
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</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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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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>; PRINTSTR routine. Accumulator stores the low byte of the address,
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; X register stores the high byte. Destroys the values of $10 and
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; $11.
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.scope
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.data zp
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.space _ptr 2
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.text
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printstr:
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sta _ptr
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stx _ptr+1
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ldy #$00
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_lp: lda (_ptr),y
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beq _done
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jsr chrout
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iny
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bne _lp
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_done: rts
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.scend</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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> Also, we ought to put in an extra check to make sure our
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zero-page allocations don't overflow, either:
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</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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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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>.data zp
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.checkpc $80</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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> That concludes our tour. The final source file
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is <A
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HREF="x501.html"
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><I
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><I
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>tutor7.oph</I
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></I
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></A
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>.
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</P
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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="c419.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="x449.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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>Advanced Memory Segments</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="c419.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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>Where to go from here</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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>
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