VGA

The term Video Graphics Array (VGA) refers specifically to the display hardware first introduced with the IBM PS/2 line of computers in 1987 , but through its widespread adoption has also come to mean either an analog computer display standard, the 15-pin D-subminiature VGA connector, or the 640×480 resolution itself. While this resolution has been superseded in the personal computer market, it is becoming a popular resolution on mobile devices

VGA was the last graphical standard introduced by IBM that the majority of PC clone manufacturers conformed to, making it today (as of 2008) the lowest common denominator that all PC graphics hardware supports before a device-specific driver is loaded into the computer. 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.

VGA was officially superseded by IBM's XGA standard, but in reality it was superseded by numerous slightly different extensions to VGA made by clone manufacturers that came to be known collectively as "Super VGA".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 IBM PS/2 models were equipped with VGA on the motherboard.

The VGA specifications are as follows:

* 256 KB Video RAM
* 16-color and 256-color modes
* 262,144-value color palette (six bits each for red, green, and blue)
* Selectable 25.175 MHz [3] or 28.3 MHz master clock
* Maximum of 720 horizontal pixels
* Maximum of 480 lines
* Refresh rates at up to 70 Hz
* Vertical Blanking interrupt (Not all clone cards support this.)
* 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
* 0.7 V peak-to-peak
* 75 ohm double-terminated impedance (18.7mA - 13mW)

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).