IBM Video Graphics Array
The IBM Video Graphics Array (VGA) and its associated circuitry
consists of a video buffer, a video digital-to-analog converter
(DAC), and test circuitry. Then 256Kb video memory is mapped as
four planes of 64Kb by 8 bits (maps 0 through 3). The video DAC
drives the analog output to the display connector. The test
circuitry is used to test for the type of display attached,
color or monochrome.
The video subsystem supports all video modes available on the
IBM Monochrome Display Adapter, IBM Color/Graphics Adapter and
Enhanced Graphics Adapter. When a monochrome display is
attached, the colors for the color modes appear as shades of
gray.
The new modes available are:
640 x 480 16- and 2-color graphics
720 x 400 16-color and monochrome alphanumeric
350 x 400 16-color alphanumeric
320 x 200 256-color graphics
In the 200-scan-line modes, the data for each scan line is
scanned twice. The double scanning allows the 200-scan-line
image to be displayed as 400 scan lines.
The video subsystem serves as the interface between the system
microprocessor and video memory. When the system microprocessor
writes to or reads from video memory, all data passes through
the video subsystem.
The video subsystem controls the access to video memory form
the system and the cathode-ray tube (CRT) controller.
Therefore, programs do not need to wait for horizontal retrace
to update the display buffer in order to preserve screen
appearance. The system performs better when accessing the
display buffer during nonactive display times because there is
less interference from the CRT controller.
The video subsystem also controls the system addresses assigned
to video memory. Up to three different starting addresses can
be programmed for compatibility with previous video adapters.
In the graphics modes, the mode determine the way video
information is formatted into memory, and the way memory is
organized.
In alphanumeric modes, the system writes the ASCII character
code and attribute data to video memory maps 0 and 1,
respectively. Memory map 2 contains the character font loaded
by BIOS during an alphanumeric mode set. The font is used by
the character generator to create the character image on the
display.
Three fonts are contained in ROM. Two of these have dot
patterns that are the same as previous IBM display adapters.
The third font is a new 8-by-16 character font. Up to eight
256-character fonts can be loaded into video memory map 2 at
one time; two fonts can be active at any one time, allowing a
512-character font.
The video subsystem formats the information in video memory and
sends the output to the video DAC. For color displays, the
video DAC sends three analog color signals (red, green and
blue) to the display connector. For monochrome displays, BIOS
translates the color information in the DAC, and the DAC drives
the summed signal into the green output.
The auxiliary video connector allows video data to be passed
between the video subsystem and an adapter plugged into the
channel connector. The video subsystem can be disabled through
the POS registers. When it is disabled, the video subsystem
will not respond to video memory or I/O reads or writes, and
the video from the adapter can directly drive the video DAC.
Note: Compatibility with other hardware is best achieved by
using the BIOS interface or operating system interface
whenever possible.