Misc/abrash-black-book
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James Gregory 2013-12-30 12:25:34 +11:00
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@ -39,7 +39,7 @@
<H3><A NAME="Heading5"></A><FONT COLOR="#000077">Overscan</FONT></H3>
<P>While we&#146;re at it, I&#146;m going to touch on overscan. Overscan is the color of the border of the display, the rectangular area around the edge of the monitor that&#146;s outside the region displaying active video data but inside the blanking area. The overscan (or border) color can be programmed to any of the 64 possible colors by either setting Attribute Controller register 11H directly or calling video function 10H, subfunction 1.
</P>
<TABLE WIDTH="100%"><TD WIDTH="5%" VALIGN="TOP"><IMG SRC="images/29-03i.jpg"><TD WIDTH="95%"><SMALL><I>On ECD-compatible monitors, however, there&#146;s too little scan time to display a proper border when the EGA is in 350-scan-line mode, so overscan should always be 0 (black) unless you&#146;re in 200-scanmode. Note, though, that a VGA can easily display a border on a VGA-compatible monitor, and VGAs are in fact programmed at mode set for an 8-pixel-wide border in all modes; all you need do is set the overscan color on any VGA to see the border.</I></SMALL>
<TABLE WIDTH="100%"><TD WIDTH="5%" VALIGN="TOP"><IMG SRC="images/i.jpg"><TD WIDTH="95%"><SMALL><I>On ECD-compatible monitors, however, there&#146;s too little scan time to display a proper border when the EGA is in 350-scan-line mode, so overscan should always be 0 (black) unless you&#146;re in 200-scanmode. Note, though, that a VGA can easily display a border on a VGA-compatible monitor, and VGAs are in fact programmed at mode set for an 8-pixel-wide border in all modes; all you need do is set the overscan color on any VGA to see the border.</I></SMALL>
</TABLE>
<H3><A NAME="Heading6"></A><FONT COLOR="#000077">A Bonus Blanker</FONT></H3>
<P>An interesting bonus: The Attribute Controller provides a very convenient way to blank the screen, in the form of the aforementioned bit 5 of the Attribute Controller Index register (at address 3C0H after the Input Status 1 register&#151;3DAH in color, 3BAH in monochrome&#151;has been read and on every other write to 3C0H thereafter). Whenever bit 5 of the AC Index register is 0, video data is cut off, effectively blanking the screen. Setting bit 5 of the AC Index back to 1 restores video data immediately. Listing 29.4 illustrates this simple but effective form of screen blanking.