Misc/abrash-black-book
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Remove colour attributes from body and strip most of the font tags out

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James Gregory 2013-12-30 13:57:02 +11:00
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@ -24,7 +24,7 @@
<!--CHAPTER=09//-->
<!--PAGES=182-185//-->
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<!--UNASSIGNED2//--></HEAD><BODY LINK=#0000FF ALINK=#000099 VLINK=#0000FF BGCOLOR=#FFFFFF>
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@ -128,7 +128,7 @@ main() {
}
</PRE>
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<H4 ALIGN="LEFT"><A NAME="Heading9"></A><FONT COLOR="#000077">Sweet Spot Revisited</FONT></H4>
<H4 ALIGN="LEFT"><A NAME="Heading9"></A>Sweet Spot Revisited</H4>
<P>Way back in Volume 1, Number 1 of <I>PC TECHNIQUES</I>, (April/May 1990) I wrote the very first of that magazine&rsquo;s HAX (#1), which extolled the virtues of placing your most commonly-used automatic (stack-based) variables within the stack&rsquo;s &ldquo;sweet spot,&rdquo; the area between +127 to -128 bytes away from BP, the stack frame pointer. The reason was that the 8088 can store addressing displacements that fall within that range in a single byte; larger displacements require a full word of storage, increasing code size by a byte per instruction, and thereby slowing down performance due to increased instruction fetching time.</P>
<P>This takes on new prominence in 386 native mode, where straying from the sweet spot costs not one, but two or three bytes. Where the 8088 had two possible displacement sizes, either byte or word, on the 386 there are three possible sizes: byte, word, or dword. In native mode (32-bit protected mode), however, a prefix byte is needed in order to use a word-sized displacement, so a variable located outside the sweet spot requires either two extra bytes (an extra displacement byte plus a prefix byte) or three extra bytes (a dword displacement rather than a byte displacement). Either way, instructions grow alarmingly.</P>
<P>Performance may or may not suffer from missing the sweet spot, depending on the processor, the memory architecture, and the code mix. On a 486, prefix bytes often cost a cycle; on a 386SX, increased code size often slows performance because instructions must be fetched through the half-pint 16-bit bus; on a 386, the effect depends on the instruction mix and whether there&rsquo;s a cache.</P>
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<hr width="90%" size="1" noshade>
<div align="center">
<font face="Verdana,sans-serif" size="1">Graphics Programming Black Book &copy; 2001 Michael Abrash</font>
Graphics Programming Black Book &copy; 2001 Michael Abrash
</div>
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