Specifications

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Chapter 15 Video Hardware
√√ See “Video Adapter BIOS,” p. 523.
Other implementations of the VGA differ in their hardware but respond to the same BIOS calls and
functions. New features are added as a superset of the existing functions, and VGA remains compatible
with the graphics and text BIOS functions built into the PC systems from the beginning. The VGA can
run almost any software that originally was written for the CGA or EGA, unless it was written to
directly access the hardware registers of these cards.
A standard VGA card displays up to 256 colors onscreen, from a palette of 262,144 (256KB) colors;
when used in the 640×480 graphics or 720×400 text mode, 16 colors at a time can be displayed.
Because the VGA outputs an analog signal, you must have a monitor that accepts an analog input.
VGA displays originally came not only in color, but also in monochrome VGA models, which use color
summing. With color summing, 64 gray shades are displayed instead of colors. The summing routine is
initiated if the BIOS detects a monochrome display when the system boots. This routine uses an algo-
rithm that takes the desired color and rewrites the formula to involve all three color guns, producing
varying intensities of gray. Users who preferred a monochrome display, therefore, could execute color-
based applications.
Note
For a listing of the VGA display modes supported by the original IBM VGA card (and thus all subsequent VGA-type cards),
see “VGA Display Modes” in Chapter 15 of Upgrading and Repairing PCs, 12th Edition, available in electronic form on
the disc supplied with this book.
Even the least-expensive video adapters on the market today can work with modes well beyond the
VGA standard. VGA, at its 16-color, 640×480 graphics resolution, has come to be the baseline for PC
graphical display configurations. VGA is accepted as the least common denominator for all Windows
systems and must be supported by the video adapters in all systems running Windows. The installa-
tion programs of all Windows versions use these VGA settings as their default video configuration. In
addition to VGA, virtually all adapters support a range of higher screen resolutions and color depths,
depending on the capabilities of the hardware. If a Windows 9x/Me or Windows XP/2000 system must
be started in Safe Mode because of a startup problem, the system defaults to VGA in the 640×480, 16-
color mode. Windows 2000 and Windows XP also offer a VGA Mode startup that also uses this mode
(Windows XP uses 800×600 resolution) but doesn’t slow down the rest of the computer the way Safe
Mode (which replaces 32-bit drivers with BIOS services) does.
IBM introduced higher-resolution versions of VGA called XGA and XGA-2 in the early 1990s, but most
of the development of VGA standards has come from the third-party video card industry and its trade
group, the Video Electronic Standards Association (VESA).
Note
If you are interested in reading more about the XGA and XGA-2 display adapters, see “XGA and XGA-2” in Chapter 8 of
Upgrading and Repairing PCs, 10th Anniversary Edition, included on the disc with this book.
Super VGA
When IBM’s XGA and 8514/A video cards were introduced, competing manufacturers chose not to
attempt to clone these incremental improvements on their VGA products. Instead, they began produc-
ing lower-cost adapters that offered even higher resolutions. These video cards fall into a category
loosely known as Super VGA (SVGA).
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