Specifications

Video Display Adapters
895
Obsolete Display Adapters
Although many types of display systems were at one time considered to be industry standards, few of
these are viable standards for today’s hardware and software.
Note
If you are interested in reading more about MDA, HGC, CGA, EGA, or MCGA display adapters, see Chapter 8 of
Upgrading and Repairing PCs, 10th Anniversary Edition, included on the disc with this book.
Current Display Adapters
When IBM introduced the PS/2 systems on April 2, 1987, it also introduced the VGA display. On that day,
in fact, IBM also introduced the lower-resolution MCGA and higher-resolution 8514 adapters. The MCGA
and 8514 adapters did not become popular standards like the VGA did, and both were discontinued.
All current display adapters that connect to the 15-pin VGA analog connector or the DVI analog/
digital connector are based on the VGA standard.
Digital Versus Analog Signals
Unlike earlier video standards, which are digital, VGA is an analog system. Why have displays gone
from digital to analog when most other electronic systems have gone digital? Compact disc players
(digital) have replaced most turntables (analog), mini DV camcorders are replacing 8MM and VHS-
based analog camcorders, and TiVo and UltimateTV digital video recorders are performing time-
shifting in place of analog VCRs for many users. With a digital television set, you can watch several
channels on a single screen by splitting the screen or placing a picture within another picture.
Most personal computer displays introduced before the PS/2 are digital. This type of display generates
different colors by firing the RGB electron beams in on-or-off mode, which allows for the display of up
to eight colors (2
3
). In the IBM displays and adapters, another signal doubles the number of color combi-
nations from 8 to 16 by displaying each color at one of two intensity levels. This digital display is easy to
manufacture and offers simplicity with consistent color combinations from system to system. The real
drawback of the older digital displays such as CGA and EGA is the limited number of possible colors.
In the PS/2 systems, IBM went to an analog display circuit. Analog displays work like the digital dis-
plays that use RGB electron beams to construct various colors, but each color in the analog display
system can be displayed at varying levels of intensity—64 levels, in the case of the VGA. This versatil-
ity provides 262,144 possible colors (64
3
), of which 256 could be simultaneously displayed. For realis-
tic computer graphics, color depth is often more important than high resolution because the human
eye perceives a picture that has more colors as being more realistic. IBM moved to analog graphics to
enhance the color capabilities of its systems.
Video Graphics Array
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 com-
mands and functions built into the system’s ROM-control software.
Chapter 15
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