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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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<center>
<table border="1">
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<td><a href="11-04.html">Previous</a></td>
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<a href="11-04.html">Previous</a>
</td>
<td><a href="index.html">Table of Contents</a></td>
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<a href="index.html">Table of Contents</a>
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<td><a href="11-06.html">Next</a></td>
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<p><br /></p>
<p>Under ideal conditions, a 286 can access memory much, much faster than an 8088. A 10 MHz 286 is capable of accessing a word of system memory every 0.20 ms with <b>REP MOVSW</b>, dwarfing the 1 byte every 1.31 &micro;s that the 8088 in a PC can manage. However, access to display memory is anything but ideal for a 286. For one thing, most display adapters are 8-bit devices, although newer adapters are 16-bit in nature. One consequence of that is that only 1 byte can be read or written per access to display memory; word-sized accesses to 8-bit devices are automatically split into 2 separate byte-sized accesses by the AT&rsquo;s bus. Another consequence is that accesses are simply slower; the AT&rsquo;s bus inserts additional wait states on accesses to 8-bit devices since it must assume that such devices were designed for PCs and may not run reliably at AT speeds.</p>
<p>However, the 8-bit size of most display adapters is but one of the two factors that reduce the speed with which the 286 can access display memory. Far more cycles are eaten by the inherent memory-access limitations of display adapters&mdash;that is, the limited number of display memory accesses that display adapters make available to the 286. Look at it this way: If <b>REP MOVSW</b> on a PC can use more than half of all available accesses to display memory, then how much faster can code running on a 286 or 386 possibly run when accessing display memory?</p>
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<p>What can we do about this new, more virulent form of the display adapter cycle-eater? The workaround is the same as it was on the PC: Access display memory as little as you possibly can.</p>
<h4 align="left" id="Heading13">New Instructions and Features: The 286</h4>
<h4 id="Heading13">New Instructions and Features: The 286</h4>
<p>The 286 and 386 offer a number of new instructions. The 286 has a relatively small number of instructions that the 8088 lacks, while the 386 has those instructions and quite a few more, along with new addressing modes and data sizes. We&rsquo;ll discuss the 286 and the 386 separately in this regard.</p>
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<p>A couple of old instructions gain new features on the 286. For one, the 286 version of <b>PUSH</b> is capable of pushing a constant on the stack. For another, the 286 allows all shifts and rotates to be performed for not just 1 bit or the number of bits specified by CL, but for <i>any</i> constant number of bits.</p>
<h4 align="left" id="Heading14">New Instructions and Features: The 386</h4>
<h4 id="Heading14">New Instructions and Features: The 386</h4>
<p>The 386 is somewhat more complex than the 286 regarding new features. Once again, we won&rsquo;t discuss protected mode, which on the 386 comes with the ability to address up to 4 gigabytes per segment and 64 terabytes in all. In real mode (and in virtual-86 mode, which allows the 386 to multitask MS-DOS applications, and which is identical to real mode so far as MS-DOS programs are concerned), programs running on the 386 are still limited to 1 MB of addressable memory and 64K per segment.</p>
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<p>The 386 also comes with a slew of new real-mode instructions beyond those supported by the 8088 and 286. These instructions can scan data on a bit-by-bit basis, set the Carry flag to the value of a specified bit, sign-extend or zero-extend data as it&rsquo;s moved, set a register or memory variable to 1 or 0 on the basis of any of the conditions that can be tested with conditional jumps, and more. (Again, beware: Many of these complex 386-specific instructions are slower than equivalent sequences of simple instructions on the 486 and especially on the Pentium.) What&rsquo;s more, both old and new instructions support 32-bit operations on the 386. For example, it&rsquo;s relatively simple to copy data in chunks of 4 bytes on a 386, even in real mode, by using the <b>MOVSD</b> (&ldquo;move string double&rdquo;) instruction, or to negate a 32-bit value with <b>NEG eax</b>.</p>
<p><br /></p>
<center>
<table border="1">
<tr>
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<a href="index.html">Table of Contents</a>
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<a href="11-06.html">Next</a>
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