Misc/abrash-black-book
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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"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<!DOCTYPE html>
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<td><a href="57-03.html">Previous</a></td>
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<a href="57-03.html">Previous</a>
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<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="58-01.html">Next</a></td>
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<p><br /></p>
<p>Within Listing 57.2, all the important optimization is in the loop that draws across each destination scan line, near the end of the listing. One optimization is elimination of the call to the set-pixel routine used to draw each pixel in Listing 57.1. Function calls are expensive operations, to be avoided when performance matters. Also, although Mode X (the undocumented 320x240 256-color VGA mode X-Sharp runs in) doesn&rsquo;t lend itself well to pixel-oriented operations like line drawing or texture mapping, the inner loop has been set up to minimize Mode X&rsquo;s overhead. A rotating plane mask is maintained in AL, with DX pointing to the Map Mask register; thus, only a rotate and an <b>OUT</b> are required to select the plane to which to write, cycling from plane 0 through plane 3 and wrapping back to 0. Better yet, because we know that we&rsquo;re simply stepping horizontally across the destination scan line, we can use a clever optimization to both step the destination and reduce the overhead of maintaining the mask. Two copies of the current plane mask are maintained, one in each nibble of AL. (The Map Mask register pays attention only to the lower nibble.) Then, when one copy rotates out of the lower nibble, the other copy rotates into the lower nibble and is ready to be used. This approach eliminates the need to test for the mask wrapping from plane 3 to plane 0, all the more so because a carry is generated when wrapping occurs, and that carry can be added to DI to advance the screen pointer. (Check out the next chapter, however, to see the best Map Mask optimization of all&mdash;setting it once and leaving it unchanged.)</p>
<p>In all, the overhead of drawing each pixel is reduced from a call to the set-pixel routine and full calculation of the screen address and plane mask to five instructions and no branches. This is an excellent example of converting full, from-scratch calculations to incremental processing, whereby only information that has changed since the last operation (the plane mask moving one pixel, for example) is recalculated.</p>
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<p>I&rsquo;m always interested in getting your feedback on and hearing about potential improvements to X-Sharp. Contact me through the publisher. There is no truth to the rumor that I can be reached under the alias &ldquo;sheep-shearer,&rdquo; at least not for another 9,999 sheep.</p>
<p><br /></p>
<center>
<table border="1">
<tr>
<td><a href="57-03.html">Previous</a></td>
<td>
<a href="57-03.html">Previous</a>
</td>
<td><a href="index.html">Table of Contents</a></td>
<td>
<a href="index.html">Table of Contents</a>
</td>
<td><a href="58-01.html">Next</a></td>
<td>
<a href="58-01.html">Next</a>
</td>
</tr>
</table>
</center>