Misc/abrash-black-book
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James Gregory 2013-12-30 13:57:02 +11:00
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<P>I know that&rsquo;s hard to believe, but the display adapter cycle-eater gives out just so many display memory accesses in a given time, and no more, no matter how fast the processor is. In fact, the faster the processor, the more the display adapter cycle-eater hurts the performance of instructions that access display memory. The display adapter cycle-eater is not only still present in 286/386 computers, it&rsquo;s worse than ever.
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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"><A NAME="Heading13"></A><FONT COLOR="#000077">New Instructions and Features: The 286</FONT></H4>
<H4 ALIGN="LEFT"><A NAME="Heading13"></A>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.
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<P>The 286 has a number of instructions designed for protected-mode operations. As I&rsquo;ve said, we&rsquo;re not going to discuss protected mode in this book; in any case, protected-mode instructions are generally used only by operating systems. (I should mention that the 286&rsquo;s protected mode brings with it the ability to address 16 MB of memory, a considerable improvement over the 8088&rsquo;s 1 MB. In real mode, however, programs are still limited to 1 MB of addressable memory on the 286. In either mode, each segment is still limited to 64K.)</P>
<P>There are also a handful of 286-specific real-mode instructions, and they can be quite useful. <B>BOUND</B> checks array bounds. <B>ENTER</B> and <B>LEAVE</B> support compact and speedy stack frame construction and removal, ideal for interfacing to high-level languages such as C and Pascal (although these instructions are actually relatively slow on the 386 and its successors, and should be used with caution when performance matters). <B>INS</B> and <B>OUTS</B> are new string instructions that support efficient data transfer between memory and I/O ports. Finally, <B>PUSHA</B> and <B>POPA</B> push and pop all eight general-purpose registers.</P>
<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"><A NAME="Heading14"></A><FONT COLOR="#000077">New Instructions and Features: The 386</FONT></H4>
<H4 ALIGN="LEFT"><A NAME="Heading14"></A>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>
<P>The 386 has many new instructions, as well as new registers, addressing modes and data sizes that have trickled down from protected mode. Let&rsquo;s take a quick look at these new real-mode features.</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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