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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><BR></P>
<H2><A NAME="Heading1"></A><FONT COLOR="#000077">Chapter 28<BR>Reading VGA Memory
</FONT></H2>
<H3><A NAME="Heading2"></A><FONT COLOR="#000077">Read Modes 0 and 1, and the Color Don&rsquo;t Care Register</FONT></H3>
<H2><A NAME="Heading1"></A>Chapter 28<BR>Reading VGA Memory</H2>
<H3><A NAME="Heading2"></A>Read Modes 0 and 1, and the Color Don&rsquo;t Care Register</H3>
<P>Well, it&rsquo;s taken five chapters, but we&rsquo;ve finally covered the data write path and all four write modes of the VGA. Now it&rsquo;s time to tackle the VGA&rsquo;s two read modes. While the read modes aren&rsquo;t as complex as the write modes, they&rsquo;re nothing to sneeze at. In particular, read mode 1 (also known as color compare mode) is rather unusual and not at all intuitive.
</P>
<P>You may well ask, isn&rsquo;t <I>anything</I> about programming the VGA straightforward? Well...no. But then, clearing up the mysteries of VGA programming is what this part of the book is all about, so let&rsquo;s get started.</P>
<H3><A NAME="Heading3"></A><FONT COLOR="#000077">Read Mode 0</FONT></H3>
<H3><A NAME="Heading3"></A>Read Mode 0</H3>
<P>Read mode 0 is actually relatively uncomplicated, given that you understand the four-plane nature of the VGA. (If you don&rsquo;t understand the four-plane nature of the VGA, I strongly urge you to read Chapters 23-27 before continuing with this chapter.) Read mode 0, the read mode counterpart of write mode 0, lets you read from one (and only one) plane of VGA memory at any one time.
</P>
<P>Read mode 0 is selected by setting bit 3 of the Graphics Mode register (Graphics Controller register 5) to 0. When read mode 0 is active, the plane that supplies the data when the CPU reads VGA memory is the plane selected by bits 1 and 0 of the Read Map register (Graphics Controller register 4). When the Read Map register is set to 0, CPU reads come from plane 0 (the plane that normally contains blue pixel data). When the Read Map register is set to 1, CPU reads come from plane 1; when the Read Map register is 2, CPU reads come from plane 2; and when the Read Map register is 3, CPU reads come from plane 3.</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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