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James Gregory 2013-12-30 13:57:02 +11:00
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<H2><A NAME="Heading1"></A><FONT COLOR="#000077">Chapter 26<BR>VGA Write Mode 3
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<H3><A NAME="Heading2"></A><FONT COLOR="#000077">The Write Mode That Grows on You</FONT></H3>
<H2><A NAME="Heading1"></A>Chapter 26<BR>VGA Write Mode 3</H2>
<H3><A NAME="Heading2"></A>The Write Mode That Grows on You</H3>
<P>Over the last three chapters, we&rsquo;ve covered the VGA&rsquo;s write path from stem to stern&mdash;with one exception. Thus far, we&rsquo;ve only looked at how writes work in write mode 0, the straightforward, workhorse mode in which each byte that the CPU writes to display memory fans out across the four planes. (Actually, we also took a quick look at write mode 1, in which the latches are always copied unmodified, but since exactly the same result can be achieved by setting the Bit Mask register to 0 in write mode 0, write mode 1 is of little real significance.)
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<P>Write mode 0 is a very useful mode, but some of VGA&rsquo;s most interesting capabilities involve the two write modes that we have yet to examine: write mode 1, and, especially, write mode 3. We&rsquo;ll get to write mode 1 in the next chapter, but right now I want to focus on write mode 3, which can be confusing at first, but turns out to be quite a bit more powerful than one might initially think.</P>
<H3><A NAME="Heading3"></A><FONT COLOR="#000077">A Mode Born in Strangeness</FONT></H3>
<H3><A NAME="Heading3"></A>A Mode Born in Strangeness</H3>
<P>Write mode 3 is strange indeed, and its use is not immediately obvious. The first time I encountered write mode 3, I understood immediately how it functioned, but could think of very few useful applications for it. As time passed, and as I came to understand the atrocious performance characteristics of <B>OUT</B> instructions, and the importance of text and pattern drawing as well, write mode 3 grew considerably in my estimation. In fact, my esteem for this mode ultimately reached the point where in the last major chunk of 16-color graphics code I wrote, write mode 3 was used more than write mode 0 overall, excluding simple pixel copying. So write mode 3 is well worth using, but to use it you must first understand it. Here&rsquo;s how it works.</P>
<P>In write mode 3, set/reset is automatically enabled for all four planes (the Enable Set/Reset register is ignored). The CPU data byte is rotated and then ANDed with the contents of the Bit Mask register, and the result of this operation is used as the contents of the Bit Mask register alone would normally be used. (If this is Greek to you, have a look back at Chapters 23 through 25. There&rsquo;s no way to understand write mode 3 without understanding the rest of the VGA&rsquo;s write data path first.)</P>
<P>That&rsquo;s what write mode 3 does&mdash;but what is it <I>for?</I> It turns out that write mode 3 is excellent for a surprisingly large number of purposes, because it makes it possible to avoid the bane of VGA performance, <B>OUT</B>s. Some uses for write mode 3 include lines, circles, and solid and two-color pattern fills. Most importantly, write mode 3 is ideal for transparent text; that is, it makes it possible to draw text in 16-color graphics mode quickly without wiping out the background in the process. (As we&rsquo;ll see at the end of this chapter, write mode 3 is potentially terrific for opaque text&mdash;text drawn with the character box filled in with a solid color&mdash;as well.)</P>
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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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