Video Graphics Array

Video Graphics Array

Video Graphics Array (VGA) is a computer display standard first marketed in 1987 by IBM as MCGA. 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 successor to the previous EGA and CGA graphics adapters.

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. VGA was technically superseded by IBM's XGA standard, but in reality it was superseded by the numerous extensions to the VGA by clone manufactuers that came to be known as Super VGA.

VGA remains a relevant 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.

Table of contents

1 Technical details
2 Programming tricks
3 Text modes
4 Hardware details

Technical details

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)

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

For embedded devices, now there exist QVGA (320x240) and QQVGA (160x120) and 1/8 VGA (240x160).

(XVGA) is a resolution of 1024 by 768 pixels of 256 colours. IBM call this mode "8514".

Programming tricks

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.

Text modes

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.

VGA adapters usually support both a monochrome and a color text mode, even though the monochrome mode is almost never used. Black and white text on nearly all modern VGA adapters is drawn by using gray colored text on a black background in color mode.

Hardware details

The video memory for monochrome mode is mapped into real memory at 0xb0000-0xb7fff, and the video memory for color mode is mapped at 0xb8000-0xbffff. In color mode, each screen character is actually represented by two bytes. The lower, or character byte is the actual character for the current character set, and the higher, or attribute byte is bitfield used to select various video attributes such as color, blinking, character set, and so forth.