Misc/abrash-black-book
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James Gregory 2013-12-30 12:25:34 +11:00
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<P>The key to understanding Listing 26.1 is understanding the effect of ANDing the rotated CPU data with the contents of the Bit Mask register. The CPU data is the pattern for the character to be drawn, with bits equal to 1 indicating where character pixels are to appear. The Data Rotate register is set to rotate the CPU data to pixel-align it, since without rotation characters could only be drawn on byte boundaries.
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<TABLE WIDTH="100%"><TD WIDTH="5%" VALIGN="TOP"><IMG SRC="images/26-01i.jpg"><TD WIDTH="95%"><SMALL><I>As I pointed out in Chapter 25, the CPU is perfectly capable of rotating the data itself, and it&#146;s often the case that that&#146;s more efficient. The problem with using the Data Rotate register is that the <B>OUT</B> that sets that register is time-consuming, especially for proportional text, which requires a different rotation for each character. Also, if the code performs full-byte accesses to display memory&#151;that is, if it combines pieces of two adjacent characters into one byte&#151;whenever possible for efficiency, the CPU generally has to do extra work to prepare the data so the VGA&#146;s rotator can handle it.</I></SMALL>
<TABLE WIDTH="100%"><TD WIDTH="5%" VALIGN="TOP"><IMG SRC="images/i.jpg"><TD WIDTH="95%"><SMALL><I>As I pointed out in Chapter 25, the CPU is perfectly capable of rotating the data itself, and it&#146;s often the case that that&#146;s more efficient. The problem with using the Data Rotate register is that the <B>OUT</B> that sets that register is time-consuming, especially for proportional text, which requires a different rotation for each character. Also, if the code performs full-byte accesses to display memory&#151;that is, if it combines pieces of two adjacent characters into one byte&#151;whenever possible for efficiency, the CPU generally has to do extra work to prepare the data so the VGA&#146;s rotator can handle it.</I></SMALL>
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<P>At the same time that the Data Rotate register is set, the Bit Mask register is set to allow the CPU to modify only that portion of the display memory byte accessed that the pixel-aligned character falls in, so that other characters and/or graphics data won&#146;t be wiped out. The result of ANDing the rotated CPU data byte with the contents of the Bit Mask register is a bit mask that allows only the bits equal to 1 in the original character pattern (rotated and masked to provide pixel alignment) to be modified by the CPU; all other bits come straight from the latches. The latches should have previously been loaded from the target address, so the effect of the ultimate synthesized bit mask value is to allow the CPU to modify only those pixels in display memory that correspond to the 1 bits in that part of the pixel-aligned character that falls in the currently addressed byte. The color of the pixels set by the CPU is determined by the contents of the Set/Reset register.
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