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Video Graphics Array (VGA) is an analog computer display standard first marketed in 1987 by IBM. While it has been obsolete for some time except in the pocket pc market where it is becoming the new standard, it was the last graphical standard that the majority of manufacturers decided to follow, making it the lowest common denominator that all PC graphics hardware supports prior to a device-specific driver being loaded. For example, 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.
The term VGA is often used to refer to a resolution of 640×480, regardless of the hardware that produces the picture. It may also refer to the 15-pin D-subminiature VGA connector which is still widely used to carry analog video signals of all resolutions.
VGA was officially superseded by IBM's XGA standard, but in reality it was superseded by numerous extensions to VGA made by clone manufacturers that came to be known as "Super VGA".
Technical details
VGA is referred to as an "array" instead of an "adapter" because it was implemented from the start as a single chip, replacing the Motorola 6845 and dozens of discrete logic chips covering a full-length ISA board that the MDA, CGA, and EGA used. This also allowed it to be placed directly on a PC's motherboard with a minimum of difficulty (it only required video memory, timing crystals and an external RAMDAC), and the first PS/2 models were equipped with VGA on the motherboard.
The VGA specifications are as follows:
256 KiB Video RAM;
16-color and 256-color modes;
262144-value color palette (six bits each for red, green, and blue);
Selectable 25 MHz or 28 MHz master clock;
Maximum of 720 horizontal pixels;
Maximum of 480 lines;
Refresh rates at up to 70 Hz;
Planar mode: up to 16 colors (4 bit planes);
Packed-pixel mode: 256 colors (Mode 13h);
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
640×480 in 16 colors;
640×350 in 16 colors;
320×200 in 16 colors;
320×200 in 256 colors (Mode 13h);
As well as the standard modes, VGA can be configured to emulate many of the modes of its predecessors (EGA, CGA, and MDA).
Programming tricks
An undocumented but popular technique nicknamed Mode X (first coined by Michael Abrash) was used to make available programming techniques and graphics resolutions not possible in the standard Mode 13h. This was done by "unchaining" the 256 KiB VGA memory into four separate "planes", which would make all of VGA's 256 KiB of RAM available in 256-color modes. There was a trade-off for extra complexity and performance loss in some types of graphics operations, but this was mitigated by other operations becoming faster in certain situations:
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