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Retidy, removing align=left from headings and empty paragraphs

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James Gregory 2013-12-30 15:37:13 +11:00
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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<p><br /></p>
<h2 id="Heading1">Chapter 11<br />
Pushing the 286 and 386</h2>
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<p>This chapter provides an interesting look at the evolution of the x86 architecture, to a greater degree than you might expect, for the x86 family came into full maturity with the 386; the 486 and the Pentium are really nothing more than faster 386s, with very little in the way of new functionality. In contrast, the 286 added a number of instructions, respectable performance, and protected mode to the 8088&rsquo;s capabilities, and the 386 added more instructions and a whole new set of addressing modes, and brought the x86 family into the 32-bit world that represents the future (and, increasingly, the present) of personal computing. This chapter also provides insight into the effects on optimization of the variations in processors and memory architectures that are common in the PC world. So, although the 286 and 386 no longer represent the mainstream of computing, this chapter is a useful mix of history lesson, x86 overview, and details on two workhorse processors that are still in wide use.</p>
<h4 align="left" id="Heading3">Family Matters</h4>
<h4 id="Heading3">Family Matters</h4>
<p>While the x86 family is a large one, only a few members of the family&mdash;which includes the 8088, 8086, 80188, 80186, 286, 386SX, 386DX, numerous permutations of the 486, and now the Pentium&mdash;really matter.</p>
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<p>This leaves us with just two processors: the 286 and the 386. Each was <i>the</i> PC standard in its day. The 286 is no longer used in new systems, but there are millions of 286-based systems still in daily use. The 386 is still being used in new systems, although it&rsquo;s on the downhill leg of its lifespan, and it is in even wider use than the 286. The future clearly belongs to the 486 and Pentium, but the 286 and 386 are still very much a part of the present-day landscape.</p>
<h4 align="left" id="Heading4">Crossing the Gulf to the 286 and the 386</h4>
<h4 id="Heading4">Crossing the Gulf to the 286 and the 386</h4>
<p>Apart from vastly improved performance, the biggest difference between the 8088 and the 286 and 386 (as well as the later Intel CPUs) is that the 286 introduced protected mode, and the 386 greatly expanded the capabilities of protected mode. We&rsquo;re only going to talk about real-mode operation of the 286 and 386 in this book, however. Protected mode offers a whole new memory management scheme, one that isn&rsquo;t supported by the 8088. Only code specifically written for protected mode can run in that mode; it&rsquo;s an alien and hostile environment for MS-DOS programs.</p>
@ -75,22 +78,26 @@ mov dx,word ptr [LongVar+2]
<p>In short, taken as a whole, protected mode programming is a different kettle of fish altogether from what I&rsquo;ve been describing in this book. There&rsquo;s certainly a knack to optimizing specifically for protected mode under a given operating system...but it&rsquo;s not what we&rsquo;ve been learning, and now is not the time to pursue it further. In general, though, the optimization strategies discussed in this book still hold true in protected mode; it&rsquo;s just issues specific to protected mode or a particular operating system that we won&rsquo;t discuss.</p>
<h4 align="left" id="Heading5">In the Lair of the Cycle-Eaters, Part II</h4>
<h4 id="Heading5">In the Lair of the Cycle-Eaters, Part II</h4>
<p>Under the programming interface, the 286 and 386 differ considerably from the 8088. Nonetheless, with one exception and one addition, the cycle-eaters remain much the same on computers built around the 286 and 386. Next, we&rsquo;ll review each of the familiar cycle-eaters I covered in Chapter 4 as they apply to the 286 and 386, and we&rsquo;ll look at the new member of the gang, the data alignment cycle-eater.</p>
<p>The one cycle-eater that vanishes on the 286 and 386 is the 8-bit bus cycle-eater. The 286 is a 16-bit processor both internally and externally, and the 386 is a 32-bit processor both internally and externally, so the Execution Unit/Bus Interface Unit size mismatch that plagues the 8088 is eliminated. Consequently, there&rsquo;s no longer any need to use byte-sized memory variables in preference to word-sized variables, at least so long as word-sized variables start at even addresses, as we&rsquo;ll see shortly. On the other hand, access to byte-sized variables still isn&rsquo;t any <i>slower</i> than access to word-sized variables, so you can use whichever size suits a given task best.</p>
<p><br /></p>
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