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James Gregory 2013-12-30 15:11:04 +11:00
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@ -10,16 +10,9 @@
<meta name="vspubdate" content="07/01/97" />
<meta name="vscategory" content="Web and Software Development: Game Development,Web and Software Development: Graphics and Multimedia Development" />
<title>Michael Abrash's Graphics Programming Black Book Special Edition: Zenning and the Flexible Mind</title><!-- HEADER -->
<!-- Empty Reference Subhead -->
<!--ISBN=1576101746//-->
<!--TITLE=Michael Abrash's Graphics Programming Black Book Special Edition//-->
<!--AUTHOR=Michael Abrash//-->
<!--PUBLISHER=The Coriolis Group, Inc.//-->
<title>Michael Abrash's Graphics Programming Black Book Special Edition: Zenning and the Flexible Mind</title>
<meta name="chapter" content="22" />
<meta name="pages" content="419-420" />
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<body>
@ -37,7 +30,7 @@
<p><br /></p>
<p><b>LISTING 22.5 L22-5.ASM</b></p><!-- CODE //-->
<p><b>LISTING 22.5 L22-5.ASM</b></p>
<pre>
ClearS proc near
push bp ;save caller&rsquo;s BP
@ -56,13 +49,13 @@ Bye:
pop bp ;restore caller&rsquo;s BP
ret EndMrk-RetAddr-2 ;return, clearing the parms from the stack
ClearS endp
</pre><!-- END CODE //-->
</pre>
<p>(We could get rid of yet another instruction by having the calling code pack both the attribute and the fill value into the same word, but that&rsquo;s not part of the specification for this particular routine.)</p>
<p>Another nifty instruction-rearrangement trick saves 6 more bytes. <b>ClearS</b> checks to see whether the far pointer is null (zero) at the start of the routine...then loads and uses that same far pointer later on. Let&rsquo;s get that pointer into registers and keep it there; that way we can check to see whether it&rsquo;s null with a single comparison, and can use it later without having to reload it from memory. This technique is shown in Listing 22.6.</p>
<p><b>LISTING 22.6 L22-6.ASM</b></p><!-- CODE //-->
<p><b>LISTING 22.6 L22-6.ASM</b></p>
<pre>
ClearS proc near
push bp ;save caller&rsquo;s BP
@ -80,7 +73,7 @@ Bye:
pop bp ;restore caller&rsquo;s BP
ret EndMrk-RetAddr-2 ;return, clearing the parms from the stack
ClearS endp
</pre><!-- END CODE //-->
</pre>
<p>Well. Now we&rsquo;re down to 28 bytes, having reduced the size of this subroutine by nearly 50 percent. Only 13 instructions remain. Realistically, how much smaller can we make this code?</p>
@ -96,7 +89,7 @@ ClearS endp
<p>With that problem dealt with, Listing 22.7 shows the Zenned version of <b>ClearS</b>.</p>
<p><b>LISTING 22.7 L22-7.ASM</b></p><!-- CODE //-->
<p><b>LISTING 22.7 L22-7.ASM</b></p>
<pre>
ClearS procnear
pop dx ;get the return address
@ -114,7 +107,7 @@ ClearS procnear
Bye:
jmp dx ;return to the calling code
ClearS endp
</pre><!-- END CODE //-->
</pre>
<p>At long last, we&rsquo;re down to the bare metal. This version of <b>ClearS</b> is just 19 bytes long. That&rsquo;s just 37 percent as long as the original version, <i>without any change whatsoever in the functionality that <b>ClearS</b> makes available to the calling code</i>. The code is bound to run a bit faster too, given that there are far fewer instruction bytes and fewer memory accesses.</p>
@ -137,10 +130,6 @@ ClearS endp
<div align="center">
Graphics Programming Black Book &copy; 2001 Michael Abrash
</div><!-- all of the reference materials (books) have the footer and subfoot reveresed -->
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