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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: Aiming the 486</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: Aiming the 486</title>
<meta name="chapter" content="13" />
<meta name="pages" content="256-258" />
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</head>
<body>
@ -37,12 +30,12 @@
<p><br /></p>
<h3><a id="Heading7"></a>32-Bit Addressing Modes</h3>
<h3 id="Heading7">32-Bit Addressing Modes</h3>
<p>The 386 and 486 both support 32-bit addressing modes, in which any register may serve as the base memory addressing register, and almost any register may serve as the potentially scaled index register. For example,</p><!-- CODE SNIP //-->
<p>The 386 and 486 both support 32-bit addressing modes, in which any register may serve as the base memory addressing register, and almost any register may serve as the potentially scaled index register. For example,</p>
<pre>
mov al,BaseTable[ecx+edx*4]
</pre><!-- END CODE SNIP //-->
</pre>
<p>uses a perfectly valid 32-bit address, with the byte accessed being the one at the offset in DS pointed to by the sum of EDX times 4 plus the offset of <b>BaseTable</b> plus ECX. This is a very powerful memory addressing scheme, far superior to 8088-style 16-bit addressing, but it&rsquo;s not without its quirks and costs, so let&rsquo;s take a quick look at 32-bit addressing. (By the way, 32-bit addressing is not limited to protected mode; 32-bit instructions may be used in real mode, although each instruction that uses 32-bit addressing must have an address-size prefix byte, and the presence of a prefix byte costs a cycle on a 486.)</p>
@ -60,16 +53,16 @@ mov al,BaseTable[ecx+edx*4]
</tr>
</table>
<p>However, because 32-bit addressing supports many more addressing combinations than 16-bit addressing, the Mod-R/M byte can&rsquo;t describe all the combinations. Therefore, whenever an index register (as described above) is involved, a second byte, the SIB byte, follows the Mod-R/M byte to provide additional address information. Consequently, whenever you use a scaled memory addressing register or use the sum of two registers to point to memory, you automatically add 1 cycle and 1 byte to that instruction. This is not to say that you shouldn&rsquo;t use index registers when they&rsquo;re needed, but if you find yourself using them inside key loops, you should see if it&rsquo;s possible to move the index calculation outside the loop as, for example, in a loop like this:</p><!-- CODE SNIP //-->
<p>However, because 32-bit addressing supports many more addressing combinations than 16-bit addressing, the Mod-R/M byte can&rsquo;t describe all the combinations. Therefore, whenever an index register (as described above) is involved, a second byte, the SIB byte, follows the Mod-R/M byte to provide additional address information. Consequently, whenever you use a scaled memory addressing register or use the sum of two registers to point to memory, you automatically add 1 cycle and 1 byte to that instruction. This is not to say that you shouldn&rsquo;t use index registers when they&rsquo;re needed, but if you find yourself using them inside key loops, you should see if it&rsquo;s possible to move the index calculation outside the loop as, for example, in a loop like this:</p>
<pre>
LoopTop:
add ax,DataTable[ebx*2]
inc ebx
dec cx
jnz LoopTop
</pre><!-- END CODE SNIP //-->
</pre>
<p>You could change this to the following for greater performance:</p><!-- CODE SNIP //-->
<p>You could change this to the following for greater performance:</p>
<pre>
add ebx,ebx ;ebx*2
LoopTop:
@ -78,7 +71,7 @@ LoopTop:
dec cx
jnz LoopTop
shr ebx,1 ;ebx*2/2
</pre><!-- END CODE SNIP //-->
</pre>
<p>I&rsquo;ll end this chapter with two more quirks of 32-bit addressing. First, as with 16-bit addressing, addressing that uses EBP as a base register both accesses the SS segment by default and always has a displacement of at least 1 byte. This reflects the common use of EBP to address a stack frame, but is worth keeping in mind if you should happen to use EBP to address non-stack memory.</p>
@ -101,10 +94,6 @@ LoopTop:
<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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