Hardware Reference
In-Depth Information
programs to talk to the BIOS-based driver rather than directly to the hardware. This allowed
programs to call a consistent set of commands and functions that would work on different hardware,
as long as a compatible VGA BIOS interface was present. The original 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 displays, which are analog
by nature. When a display is connected via VGA, the digital signals 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 12.4
; the pinouts are shown in
Table 12.9
.
Figure 12.4. The standard 15-pin analog VGA connector.
Table 12.9. 15-Pin Analog VGA Connector Pinout
The mating VGA cable connector that plugs into this connector normally has pin 9 missing. This was
designed such that the mating hole in the connector on the video card could be plugged, but it is
usually open (and merely unused) instead. The connector is keyed by virtue of the D-shape shell and
pin alignment, so it is difficult to plug in backward even without the key pin. Pin 5 is used only for
testing purposes, and pin 15 is rarely used; they are often missing as well. To identify the type of
monitor connected to the system, some early VGA cards used the presence or absence of the monitor
ID pins in various combinations.
