Video Hardware, Part 2 - Video Graphics Array
(Page 9 of 10 )
PS/2 systems incorporated the primary display adapter circuitry onto the motherboard, and both IBM and third-party companies introduced separate VGA cards to enable other types of systems to enjoy the advantages of VGA.
Although the IBM MicroChannel (MCA) computers, such as the PS/2 Model 50 and above, introduced VGA, it's impossible today to find a brand-new replacement for VGA that fits into the obsolete MCA-bus systems. However, a few surplus and used third-party cards might be available if you look hard enough.
The VGA BIOS is the control software residing in the system ROM for controlling VGA circuits. With the BIOS, software can initiate commands and functions without having to manipulate the VGA directly. Programs become somewhat hardware independent and can call a consistent set of commands and functions built into the system's ROM-control software.
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 640x480 graphics or 720x400 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 algorithm 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, 640x480 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 installation 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 640x480, 16-color mode. Windows 2000 and Windows XP also offer a VGA Mode startup that also uses this mode (Windows XP uses 800x600 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.
 | This chapter is from Upgrading and Repairing PCs, 16th edition,by Scott Mueller. (Que Books, 2004, ISBN: 0789731738). Check it out at your favorite bookstore today. Buy this book now! |
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