Misc/abrash-black-book
Watch
1
0
Fork
You've already forked abrash-black-book
 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

Remove colour attributes from body and strip most of the font tags out

Browse source
This commit is contained in:
James Gregory 2013-12-30 13:57:02 +11:00
commit c1f88ddb41
362 changed files with 1632 additions and 1706 deletions
Download patch file Download diff file Expand all files Collapse all files

8
25-03.html
View file

@ -24,7 +24,7 @@
<!--CHAPTER=25//-->
<!--PAGES=470-472//-->
<!--UNASSIGNED1//-->
<!--UNASSIGNED2//--></HEAD><BODY LINK=#0000FF ALINK=#000099 VLINK=#0000FF BGCOLOR=#FFFFFF>
<!--UNASSIGNED2//--></HEAD><body>
<CENTER>
<TABLE BORDER>
@ -42,12 +42,12 @@
<P>The example code in Listing 25.1 is designed to illustrate the use of the Data Rotate and Bit Mask registers, and is not as fast or as complete as it might be. The case where text <I>is</I> byte-aligned could be detected and performed much faster, without the use of the Bit Mask or Data Rotate registers and with only one display memory access per font byte (to write the font byte), rather than four (to read display memory and write the font byte to each of the two bytes the character spans). Likewise, non-aligned text drawing could be streamlined to one display memory access per byte by having the CPU rotate and combine the font data directly, rather than setting up the VGA&rsquo;s hardware to do it. (Listing 25.1 was designed to illustrate VGA data rotation and bit masking rather than the fastest way to draw text. We&rsquo;ll see faster text-drawing code soon.) One excellent rule of thumb is to minimize display memory accesses of all types, especially reads, which tend to be considerably slower than writes. Also, in Listing 25.1 it would be faster to use a table lookup to calculate the bit masks for the two halves of each character rather than the shifts used in the example.</P>
<P>For another (and more complex) example of drawing bit-mapped text on the VGA, see John Cockerham&rsquo;s article, &ldquo;Pixel Alignment of EGA Fonts,&rdquo; <I>PC Tech Journal</I>, January, 1987. Parenthetically, I&rsquo;d like to pass along John&rsquo;s comment about the VGA: &ldquo;When programming the VGA, <I>everything</I> is complex.&rdquo;</P>
<P>He&rsquo;s got a point there.</P>
<H3><A NAME="Heading5"></A><FONT COLOR="#000077">The VGA&rsquo;s Set/Reset Circuitry</FONT></H3>
<H3><A NAME="Heading5"></A>The VGA&rsquo;s Set/Reset Circuitry</H3>
<P>At last we come to the final aspect of data flow through the GC on write mode 0 writes: the set/reset circuitry. Figure 25.3 shows data flow on a write mode 0 write. The only difference between this figure and Figure 25.1 is that on its way to each plane potentially the rotated CPU data passes through the set/reset circuitry, which may or may not replace the CPU data with set/reset data. Briefly put, the set/reset circuitry enables the programmer to elect to independently replace the CPU data for each plane with either 00 or 0FFH.
</P>
<P>What is the use of such a feature? Well, the standard way to control color is to set the Map Mask register to enable writes to only those planes that need to be set to produce the desired color. For example, the Map Mask register would be set to 09H to draw in high-intensity blue; here, bits 0 and 3 are set to 1, so only the blue plane (plane 0) and the intensity plane (plane 3) are written to.</P>
<P><A NAME="Fig3"><!-- </A><A HREF="javascript:displayWindow('images/25-03.jpg',409,242 )"> --><IMG SRC="images/25-03.jpg"><BR><!-- </A>
<BR><A HREF="javascript:displayWindow('images/25-03.jpg',409,242)"> --><FONT COLOR="#000077"><B>Figure 25.3</B></FONT></A>&nbsp;&nbsp;<I>Data flow during a write mode 0 write operation.</I>
<BR><A HREF="javascript:displayWindow('images/25-03.jpg',409,242)"> --><B>Figure 25.3</B></A>&nbsp;&nbsp;<I>Data flow during a write mode 0 write operation.</I>
</P>
<P>Remember, though, that planes that are disabled by the Map Mask register are not written to or modified in any way. This means that the above approach works only if the memory being written to is zeroed; if, however, the memory already contains non-zero data, that data will remain in the planes disabled by the Map Mask, and the end result will be that some planes contain the data just written and other planes contain old data. In short, color control using the Map Mask does not force all planes to contain the desired color. In particular, it is not possible to force some planes to zero and other planes to one in a single write with the Map Mask register.
</P>
@ -64,7 +64,7 @@
<hr width="90%" size="1" noshade>
<div align="center">
<font face="Verdana,sans-serif" size="1">Graphics Programming Black Book &copy; 2001 Michael Abrash</font>
Graphics Programming Black Book &copy; 2001 Michael Abrash
</div>
<!-- all of the reference materials (books) have the footer and subfoot reveresed -->
<!-- reference_subfoot = footer -->