Misc/abrash-black-book
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Remove colour attributes from body and strip most of the font tags out

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James Gregory 2013-12-30 13:57:02 +11:00
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<P>Logical function 0, which writes the CPU data unmodified, is the standard mode of operation of the ALUs. In this mode, the CPU data is combined with the latched data by ignoring the latched data entirely. Expressed as a logical function, this could be considered CPU data ANDed with 1 (or ORed with 0). This is the mode to use whenever you want to place CPU data into display memory, replacing the previous contents entirely. It may occur to you that there is no need to latch display memory at all when the data unmodified function is selected. In the sample program, that is true, but if the bit mask is being used, the latches must be loaded even for the data unmodified function, as I&rsquo;ll discuss in the next chapter.
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<P>Logical functions 1 through 3 cause the CPU data to be ANDed, ORed, and XORed with the latched data, respectively. Of these, XOR is the most useful, since exclusive-ORing is a traditional way to perform animation. The uses of the AND and OR logical functions are less obvious. AND can be used to mask a blank area into display memory, or to mask off those portions of a drawing operation that don&rsquo;t overlap an existing display memory image. OR could conceivably be used to force an image into display memory over an existing image. To be honest, I haven&rsquo;t encountered any particularly valuable applications for AND and OR, but they&rsquo;re the sort of building-block features that could come in handy in just the right context, so keep them in mind.</P>
<H3><A NAME="Heading4"></A><FONT COLOR="#000077">Notes on the ALU/Latch Demo Program</FONT></H3>
<H3><A NAME="Heading4"></A>Notes on the ALU/Latch Demo Program</H3>
<P>VGA settings such as the logical function select should be restored to their default condition before the BIOS is called to output text or draw pixels. The VGA BIOS does not guarantee that it will set most VGA registers except on mode sets, and there are so many compatible BIOSes around that the code of the IBM BIOS is not a reliable guide. For instance, when the BIOS is called to draw text, it&rsquo;s likely that the result will be illegible if the Bit Mask register is not in its default state. Similarly, a mode set should generally be performed before exiting a program that tinkers with VGA settings.
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<P>Along the same lines, the sample program does not explicitly set the Map Mask register to ensure that all planes are enabled for writing. The mode set for mode 10H leaves all planes enabled, so I did not bother to program the Map Mask register, or any other register besides the Data Rotate register, for that matter. However, the profusion of compatible BIOSes means there is some small risk in relying on the BIOS to leave registers set properly. For the highly safety-conscious, the best course would be to program data control registers such as the Map Mask and Read Mask explicitly before relying on their contents.</P>
@ -75,7 +75,7 @@ MOV DX,(VALUE2 SHL 8) OR VALUE1
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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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