Video Hardware - Upgrading and Repairing PCs - page 780

VGA
IBM introduced the VGA interface and display standard on April 2, 1987, along with a
family of systems it called PS/2. VGA originally included the display adapter, the monit-
or, and the connection between them. Since that time, the display adapters and monitors
haveevolved,buttheVGA15-pinanalogconnectionwentontobecomethemostpopular
video interface in history and is still used today in PC video adapters and displays.
VGA is an analog design. Analog uses a separate signal for each CRT color gun, but each
signalcanbesentatvaryinglevelsofintensity—64levels,inthecaseoftheoriginalVGA
standard. This provides 262,144 possible colors (64 3 ), of which 256 could be simultan-
eously displayed in the original design. For realistic computer graphics, color depth is of-
ten more important than high resolution because the human eye perceives a picture that
has more colors as being more realistic.
VGAwasdesignedtobeaddressedthroughtheVGABIOSinterface,asoftwareinterface
that forced programs to talk to the BIOS-based driver rather than directly to the hardware.
Thisallowedprogramstocallaconsistentsetofcommandsandfunctionsthatwouldwork
on different hardware, as long as a compatible VGA BIOS interface was present. The ori-
ginal VGA cards had the BIOS on the video card directly, in the form of a ROM chip
containing from 16KB to 32KB worth of code. Modern video cards and laptop graphics
processors still have this 32KB onboard BIOS. Typically, the only time the ROM-based
drivers are used is during boot, when running legacy DOS-based applications or games,
or when you run Windows in Safe Mode.
VGA also describes a 15-pin analog interface connection that can support a variety of
modes. The connection is analog because VGA was primarily designed to drive CRT dis-
plays, which are analog by nature. When a display is connected via VGA, the digital sig-
nals inside the PC are converted to analog signals by the DAC chip in the display adapter
and are then sent to the display via the analog VGA connection. The VGA connector is
shown in Figure 12.3 ; the pinouts are shown in Table 12.9 .
Figure 12.3 The standard 15-pin analog VGA connector.
Table 12.9 15-Pin Analog VGA Connector Pinout