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196 lines
3.8 KiB
HTML
196 lines
3.8 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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>What about Indexed Indirect?</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="Pointers and Indirection"
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HREF="c748.html"><LINK
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REL="PREVIOUS"
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TITLE="Pointer arithmetic"
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HREF="x782.html"><LINK
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REL="NEXT"
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TITLE="Comparison with the other indexed forms"
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HREF="x805.html"></HEAD
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BGCOLOR="#FFFFFF"
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COLSPAN="3"
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>Programming with Ophis</TH
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><TR
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><TD
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WIDTH="10%"
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Pointers and Indirection</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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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="AEN798"
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>What about Indexed Indirect?</A
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></H1
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><P
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> This essay has concerned itself almost exclusively with the
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Indirect Indexed—or (Indirect), Y—mode. What about Indexed
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Indirect—(Indirect, X)? This is a <I
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CLASS="EMPHASIS"
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>much</I
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>
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less useful mode than the Y register's version. While the Y
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register indirection lets you implement pointers and arrays in
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full generality, the X register is useful for pretty much only one
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application: lookup tables for single byte values.
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</P
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><P
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> Even coming up with a motivating example for this is difficult,
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but here goes. Suppose you have multiple, widely disparate
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sections of memory that you're watching for signals. The
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following routine takes a resource index in the accumulator and
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returns the status byte for the corresponding resource.
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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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>; This data is sitting on the zero page somewhere
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resource_status_table: .word resource0_status, resource1_status,
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.word resource2_status, resource3_status,
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; etc. etc. etc.
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; This is the actual program code
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.text
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getstatus:
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clc ; Multiply argument by 2 before putting it in X, so that it
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asl ; produces a value that's properly word-indexed
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tax
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lda (resource_status_table, x)
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rts</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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> Why having a routine such as this is better than just having the
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calling routine access resourceN_status itself as an absolute
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memory load is left as an exercise for the reader. That aside,
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this code fragment does serve as a reminder that when indexing an
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array of anything other than bytes, you must multiply your index
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by the size of the objects you want to index. C does this
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automatically—assembler does not. Stay sharp.
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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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><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="x782.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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HREF="book1.html"
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>Home</A
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VALIGN="top"
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HREF="x805.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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>Pointer arithmetic</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="c748.html"
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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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>Comparison with the other indexed forms</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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> |