VGA was the last graphical standard introduced by IBM that the majority of PC clone manufacturers conformed to, making it today (as of 2010) the lowest common denominator that all PC graphics hardware can be expected to implement without device-specific driver software. 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VGA was officially superseded by IBM's Extended Graphics Array (XGA) standard, but in reality it was superseded by numerous slightly different extensions to VGA made by clone manufacturers that came to be known collectively as Super VGA.
Hardware
VGA compared to other standard resolutions.
VGA is referred to as an "array" instead of an "adapter" because it was implemented from the start as a single chip (an ASIC), replacing the Motorola 6845 and dozens of discrete logic chips that covered the full-length ISA boards of the MDA, CGA, and EGA. Its single-chip implementation also allowed the VGA 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. (Contrast this with all of the "family one" IBM PC desktop models—the PC [machine-type 5150], PC/XT [5160], and PC AT [5170]—which required a display adapter installed in a slot in order to connect a monitor.)
The VGA specifications are as follows:
- 256 KB Video RAM (The very first cards could be ordered with 64 KB or 128 KB of RAM at the cost of losing some video modes).
- 16-color and 256-color modes
- 262,144-value color palette (six bits each for red, green, and blue)
- Selectable 25.175 MHz or 28.322 MHz master clock
- Maximum of 800 horizontal pixels
- Maximum of 600 lines
- Refresh rates at up to 70 Hz
- Vertical blank 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 (18.7 mA – 13 mW)
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). Compatibility is almost full at BIOS level, but even at register level, a very high value of compatibility is reached. VGA is not compatible with the special IBM PCjr or HGC video modes.
