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James Gregory 2013-12-30 12:25:34 +11:00
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<H3><A NAME="Heading4"></A><FONT COLOR="#000077">Copying Pixel Blocks within Display Memory</FONT></H3>
<P>Another fine use for the latches is copying pixels from one place in display memory to another. Whenever both the source and the destination share the same nibble alignment (that is, their start addresses modulo four are the same), it is not only possible but quite easy to use the latches to copy four pixels at a time. Listing 48.3 shows a routine that copies via the latches. (When the source and destination do not share the same nibble alignment, the latches cannot be used because the source and destination planes for any given pixel differ. In that case, you can set the Read Map register to select a source plane and the Map Mask register to select the corresponding destination plane. Then, copy all pixels in that plane, repeating for all four planes.)
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<TABLE WIDTH="100%"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="5%"><IMG SRC="images/48-01i.jpg"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="95%"><SMALL><I>Although copying through the latches is, in general, a speedy technique, especially on slower VGAs, it&#146;s not always a win. Reading video memory tends to be quite a bit slower than writing, and on a fast VLB or PCI adapter, it can be faster to copy from main memory to display memory than it is to copy from display memory to display memory via the latches.</I></SMALL>
<TABLE WIDTH="100%"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="5%"><IMG SRC="images/i.jpg"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="95%"><SMALL><I>Although copying through the latches is, in general, a speedy technique, especially on slower VGAs, it&#146;s not always a win. Reading video memory tends to be quite a bit slower than writing, and on a fast VLB or PCI adapter, it can be faster to copy from main memory to display memory than it is to copy from display memory to display memory via the latches.</I></SMALL>
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