VIDEO GRAPHICS ARRAY

Video Graphics Array (VGA) a computer display standard marketed in 1987 by IBM. VGA belongs to a family of earlier IBM video standards and largely remains backward compatible with them. VGA can be seen as an enhancement of and sucessor to the previous EGA graphics adapter.

As with most IBM hardware the VGA was extensively cloned by other manufacturers. While the VGA has been obsolete in original form for some time it was the last IBM standard that the majority of clone manufacturers decided to follow, making it even today the only standard graphics interface that be relied on to be present on the PC architecture. The VGA was technically superseded by IBM's XGA standard, in reality it was superseded by the numerous extensions to the VGA by clone manufactuers that came to be known as Super VGAs.

Even today, VGA remains a relavant graphics standard. It forms the "lowest common denominator" that all PC graphics cards need to support prior to a device-specific driver being loaded. On Windows machines, the Microsoft Windows splash screen appears while the machine is still operating in VGA mode, which is the reason that this screen always appears in reduced resolution and color depth than following screens.

The VGA specifications are as follows:

• 256KByte Video RAM
• 16 colour or 256 colour modes
• 262144 colour palette
• Selectable 25Mhz or 28Mhz master clock
• Maximum of 720 horizontal pixels
• Maximum of 480 lines
• Refresh rates at up to 70 Hz
• 4 video planes
• Hardware smooth scrolling support
• Some 'Raster Ops' support
• Barrel shifter
• Split screen support
• Soft fonts

The VGA supports both All Points Addressable graphics modes, and Alphanumeric Text modes. Standard graphics modes are:

• 640x480 in 16 colours
• 640x400 in 16 colours
• 320x200 in 16 colours
• 320x200 in 256 colours (Mode 13h)

An undocumented, but popular 256 colour mode called Mode X was used to make available programming techniques and graphics resolutions not possible in the standard Mode 13h. This was a trade off for extra complexity and performance loss in some types of graphics operations.

Standard alphanumeric text modes for the VGA are 80x25 and 40x25 text cells. Each cell may choose from one of 16 available colours for its foreground and 8 colours for the background. The character may also be made to blink, or at the expense of the blinking option, the background may be selected from 16 colours.

As well as the standard modes the VGA supports many of the modes of its predecessors the EGA, CGA, MDA and due to its configurable nature, un-documented modes.