Stephen Patrick Thompson

VGA—design choices for a new video subsystem

The VGA (Video Graphics Array) video subsystem is provided as standard on the system boards of the IBM Personal System/2® Models 50 and above. VGA was designed to meet the objectives set for these new systems and to support compatibility with older IBM offerings, while at the same time providing greater performance and increased function. The IBM Enhanced Graphics Adapter (EGA) was chosen as the compatibility base for VGA, since EGA had become the video standard for IBM-compatible computer systems. Six new modes of operation were designed to meet the needs of new business and consumer applications and to improve the ergonomics of the systems. Higher-performance video presents several design problems, including electromagnetic interference, physical design size, and cost. These design problems were contained by implementing the VGA function in a single-gate array and by using an analog display interface. The use of a video digital-to-analog converter (DAC) allows the VGA subsystem to show any color from a choice of 256K colors when a color display is used, or 64 gray shades when a monochrome display is used. The VGA subsystem was designed to provide a uniform interface for color and monochrome that allows a color mode to be selected when a monochrome display is used, or a monochrome mode to be used on a color display. A color-summing algorithm was designed and implemented in the BIOS (Basic Input/Output System) software that will allow colors to be shown as shades of gray on the monochrome display.