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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<P><BR></P>
<P>It&rsquo;s actually not hard to figure out which instructions go through which pipes; just back up until you find an instruction that can&rsquo;t pair or can only go through the U-pipe, and work forward from there, given the knowledge that that instruction executes in the U-pipe. The easiest thing to look for is branches. All branch target instructions execute in the U-pipe, as do all instructions after conditional branches that fall through. Instructions with prefix bytes are generally good U-pipe markers, although they&rsquo;re expensive instructions that should be avoided whenever possible, and have at least one aberration with regard to pipe usage, as discussed below. Shifts, rotates, <B>ADC, SBB</B>, and all other instructions not listed in Table 20.1 in the last chapter are likewise U-pipe markers.</P>
<H3><A NAME="Heading7"></A><FONT COLOR="#000077">Pentium Optimization in Action</FONT></H3>
<H3><A NAME="Heading7"></A>Pentium Optimization in Action</H3>
<P>Now, let&rsquo;s take a look at one of the simplest, tightest pieces of code imaginable, and see what our new Pentium perspective reveals. Listing 21.1 shows a loop implementing the TCP/IP checksum, a 16-bit checksum that wraps carries around to the low bit so that the result is endian-independent. This makes it easy to perform checksums on blocks of data regardless of the endian characteristics of the machines on which those blocks are generated and received. (Thanks to fellow performance enthusiast Terje Mathisen for suggesting this checksum as fertile ground for Pentium optimization, in the ibm.pc/fast.code forum on Bix.) The loop in Listing 21.1 consists of exactly five instructions; it&rsquo;s hard to imagine that there&rsquo;s a lot of performance to be wrung from this snippet, right?
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<P><B>LISTING 21.1 L21-1.ASM</B></P>
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<font face="Verdana,sans-serif" size="1">Graphics Programming Black Book &copy; 2001 Michael Abrash</font>
Graphics Programming Black Book &copy; 2001 Michael Abrash
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