Misc/abrash-black-book
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James Gregory 2013-12-30 12:25:34 +11:00
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<P>Finally, it&#146;s possible in real mode to use the 386&#146;s new addressing modes, in which <I>any</I> 32-bit general-purpose register or pair of registers can be used to address memory. What&#146;s more, multiplication of memory-addressing registers by 2, 4, or 8 for look-ups in word, doubleword, or quadword tables can be built right into the memory addressing mode. (The 32-bit addressing modes are discussed further in later chapters.) In protected mode, these new addressing modes allow you to address a full 4 gigabytes per segment, but in real mode you&#146;re still limited to 64K, even with 32-bit registers and the new addressing modes, unless you play some unorthodox tricks with the segment registers.</P>
<TABLE WIDTH="100%"><TD WIDTH="5%" VALIGN="TOP" ALIGN="LEFT"><IMG SRC="images/11-06i.jpg"><TD WIDTH="95%" ALIGN="LEFT" VALIGN="TOP"><SMALL><I>Note well: Those tricks don&#146;t necessarily work with system software such as Windows, so I&#146;d recommend against using them. If you want 4-gigabyte segments, use a 32-bit environment such as Win32.</I></SMALL>
<TABLE WIDTH="100%"><TD WIDTH="5%" VALIGN="TOP" ALIGN="LEFT"><IMG SRC="images/i.jpg"><TD WIDTH="95%" ALIGN="LEFT" VALIGN="TOP"><SMALL><I>Note well: Those tricks don&#146;t necessarily work with system software such as Windows, so I&#146;d recommend against using them. If you want 4-gigabyte segments, use a 32-bit environment such as Win32.</I></SMALL>
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<H4 ALIGN="CENTER"><A NAME="Heading15"></A><FONT COLOR="#000077">Optimization Rules: The More Things Change...</FONT></H4>
<P>Let&#146;s see what we&#146;ve learned about 286/386 optimization. Mostly what we&#146;ve learned is that our familiar PC cycle-eaters still apply, although in somewhat different forms, and that the major optimization rules for the PC hold true on ATs and 386-based computers. You won&#146;t go wrong on any of these computers if you keep your instructions short, use the registers heavily and avoid memory, don&#146;t branch, and avoid accessing display memory like the plague.