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292 lines
5.4 KiB
HTML
292 lines
5.4 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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>Pointers and Indirection</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="p618.html"><LINK
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REL="PREVIOUS"
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TITLE="A modest example: Insertion sort on linked lists"
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HREF="x856.html"><LINK
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REL="NEXT"
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TITLE="Pointer arithmetic"
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HREF="x919.html"></HEAD
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CLASS="CHAPTER"
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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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><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="HLL3"
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></A
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>Pointers and Indirection</H1
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><P
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> The basics of pointers versus cursors (or, at the 6502 assembler
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level, the indirect indexed addressing mode versus the absolute
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indexed ones) were covered in <A
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HREF="c680.html"
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>the Chapter called <I
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>Structured Programming</I
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></A
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> This essay seeks
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to explain the uses of the indirect modes, and how to implement
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pointer operations with them. It does <I
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CLASS="EMPHASIS"
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>not</I
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> seek to explain
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why you'd want to use pointers for something to begin with; for a
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tutorial on proper pointer usage, consult any decent C textbook.</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="AEN890"
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>The absolute basics</A
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></H1
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><P
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> A pointer is a variable holding the address of a memory location.
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Memory locations take 16 bits to represent on the 6502: thus, we
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need two bytes to hold it. Any decent assembler will have ways of
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taking the high and low bytes of an address; use these to acquire
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the raw values you need. The 6502 chip does not have any
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simple <SPAN
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CLASS="QUOTE"
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>"pure"</SPAN
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> indirect modes (except
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for <TT
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CLASS="LITERAL"
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>JMP</TT
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>, which is a matter for a later essay);
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all are indexed, and they're indexed different ways depending on
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which index register you use.
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</P
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><DIV
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CLASS="SECTION"
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><H2
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CLASS="SECTION"
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><A
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NAME="AEN895"
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>The simplest example</A
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></H2
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><P
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> When doing a simple, direct dereference (that is, something
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equivalent to the C code <TT
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CLASS="LITERAL"
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>c=*b;</TT
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>) the code
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looks like this:
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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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> ldy #0
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lda (b), y
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sta c</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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> Even with this simple example, there are several important
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things to notice.
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</P
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><P
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></P
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><UL
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><LI
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><P
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> The variable <TT
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CLASS="LITERAL"
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>b</TT
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> <I
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CLASS="EMPHASIS"
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>must be on the
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zero page</I
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>, and furthermore, it <I
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CLASS="EMPHASIS"
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>cannot
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be $FF.</I
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> All your pointer values need to be
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either stored on the zero page to begin with or copied
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there before use.
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</P
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></LI
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><LI
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><P
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> The <TT
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CLASS="LITERAL"
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>y</TT
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> in the <TT
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CLASS="LITERAL"
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>lda</TT
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>
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statement must be y. It cannot be x (that's a different
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form of indirection), and it cannot be a constant. If
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you're doing a lot of indirection, be sure to keep your Y
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register free to handle the indexing on the
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pointers.
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</P
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></LI
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><LI
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><P
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> The <TT
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CLASS="LITERAL"
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>b</TT
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> variable is used alone. Statements
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like <TT
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CLASS="LITERAL"
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>lda (b+2), y</TT
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> are syntactically valid
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and sometimes even correct: it dereferences the value next
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to <TT
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CLASS="LITERAL"
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>b</TT
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> after adding y to the value therein.
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However, it is almost guaranteed that what you *really*
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wanted to do was compute <TT
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CLASS="LITERAL"
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>*(b+2)</TT
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> (that is,
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take the address of b, add 2 to <I
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CLASS="EMPHASIS"
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>that</I
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>,
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and dereference that value); see the next section for how to
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do this properly.
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</P
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></LI
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></UL
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><P
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> In nearly all cases, it is the Y-register's version (Indirect
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Indexed) that you want to use when you're dealing with pointers.
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Even though either version could be used for this example, we
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use the Y register to establish this habit.
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</P
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></DIV
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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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><TR
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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="x856.html"
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><<< Previous</A
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>Next >>></A
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></TD
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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 modest example: Insertion sort on linked lists</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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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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>Pointer arithmetic</TD
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></TR
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> |