Video Graphics Array
Video Graphics Array is a video display controller and accompanying de facto graphics standard, first introduced with the IBM PS/2 line of computers in 1987, which became ubiquitous in the IBM PC compatible industry within three years. The term can now refer to the computer display standard, the 15-pin D-subminiature VGA connector, or the resolution characteristic of the VGA hardware.
VGA was the last IBM graphics standard to which the majority of IBM PC compatible computer manufacturers conformed, making it the lowest common denominator that virtually all post-1990 PC graphics hardware can be expected to implement.
VGA was adapted into many extended forms by third parties, collectively known as Super VGA, then gave way to custom graphics processing units which, in addition to their proprietary interfaces and capabilities, continue to implement common VGA graphics modes and interfaces to the present day.
The VGA analog interface standard has been extended to support resolutions of up to for general usage, with specialized applications improving it further still.
Hardware design
The color palette random access memory and its corresponding digital-to-analog converter were integrated into one chip and the cathode-ray tube controller was integrated into a main VGA chip, which eliminated several other chips in previous graphics adapters, so VGA only additionally required external video RAM and timing crystals.This small part count allowed IBM to include VGA directly on the PS/2 motherboard, in contrast to prior IBM PC modelsPC, PC/XT, and PC ATwhich required a separate display adapter installed in a slot in order to connect a monitor. The term "array" rather than "adapter" in the name denoted that it was not a complete independent expansion device, but a single component that could be integrated into a system.
Unlike the graphics adapters that preceded it there was initially no discrete VGA card released by IBM. The first commercial implementation of VGA was a built-in component of the IBM PS/2, in which it was accompanied by 256 KiB of video RAM, and a new DE-15 connector replacing the DE-9 used by previous graphics adapters. IBM later released the standalone IBM PS/2 Display Adapter, which utilized the VGA but could be added to machines that did not have it built in.
On some machines and cables, pin 9 was missing. Pin 9's purpose is to power an EEPROM chip in the monitor which tells the graphics card the capabilities on the monitor. Systems or cables missing this are likely using an older version of VGA.
Capabilities
The VGA supports all graphics modes supported by the MDA, CGA and EGA cards, as well as multiple new modes.Standard graphics modes
- in 4 or 16 colors
- in 256 colors
- and in 16 colors or monochrome
- 640×480| in 16 colors or monochrome
The other modes defaulted to standard EGA or CGA compatible palettes and instructions, but still permitted remapping of the palette with VGA-specific commands.
graphics mode
The resolution was originally used by IBM in PGC graphics but did not see wide adoption until VGA was introduced. As the VGA began to be cloned in great quantities by manufacturers who added ever-increasing capabilities, its, 16-color mode became the de facto lowest common denominator of graphics cards. By the mid 1990s, a ×16 graphics mode using the VGA memory and register specifications was expected by operating systems such as Windows 95 and OS/2 Warp 3.0, which provided no support for lower resolutions or bit depths, or support for other memory or register layouts without additional drivers. Well into the 2000s, even after the VESA standard for graphics cards became commonplace, the "VGA" graphics mode remained a compatibility option for PC operating systems.Other graphics modes
Nonstandard display modes can be implemented, with horizontal resolutions of:- 512 to 800 pixels wide, in 16 colors
- 256 to 400 pixels wide, in 256 colors
- 200, or 350 to 410 lines at 70 Hz refresh rate, or
- 224 to 256, or 448 to 512 lines at 60 Hz refresh rate
- 512 to 600 lines at reduced vertical refresh rates, depending on individual monitor compatibility.
"Narrow" modes such as tend to preserve the same pixel ratio as in e.g. mode unless the monitor is adjusted to stretch the image out to fill the screen, as they are derived simply by masking down the wider mode instead of altering pixel or line timings, but can be useful for reducing memory requirements and pixel addressing calculations for arcade game conversions or console emulators.
The PC version of Pinball Fantasies has the option to use non-standard, "high res" modes, such as, allowing it to display a larger portion of the pinball table on screen. The game Scorched Earth uses a default resolution of, with many other nonstandard resolutions available.
Standard text modes
VGA also implements several text modes:- , rendered with a pixel font, with an effective resolution of
- , with a font, with an effective resolution of
- or, with an font grid, with an effective resolution of or pixels.
One variant that is sometimes seen is or, using an or font and an effective pixel display, which trades use of the more flickery 60 Hz mode for an additional 5 or 10 lines of text and square character blocks.
Technical details
Unlike the cards that preceded it, which used binary TTL signals to interface with a monitor, the VGA introduced a video interface using pure analog RGB signals, with a range of 0.7 volts peak-to-peak max. In conjunction with a 18-bit RAMDAC, this produced a color gamut of 262,144 colors.The original VGA specifications follow:
- Selectable 25.175 MHz or 28.322 MHz master pixel clock
- Maximum of 640 horizontal pixels in graphics mode, and 720 pixels in text mode
- Maximum of 480 lines
- Refresh rates at 60 or 70 Hz
- Vertical blank interrupt
- Planar mode: up to 16 colors
- Packed-pixel mode: 256 colors
- Hardware smooth scrolling support
- No Blitter
- *Supports fast data transfers via "VGA latch" registers
- Barrel shifter
- Split screen support
Signal timings
All "derived" VGA timings can be varied by software that bypasses the VGA firmware interface and communicates directly with the VGA hardware, as many MS-DOS based games did. However, only the standard modes, or modes that at least use almost exactly the same H-sync and V-sync timings as one of the standard modes, can be expected to work with the original late-1980s and early-1990s VGA monitors. The use of other timings may in fact damage such monitors and thus was usually avoided by software publishers.
Third-party "multisync" CRT monitors were more flexible, and in combination with "super EGA", VGA, and later SVGA graphics cards using extended modes, could display a much wider range of resolutions and refresh rates at arbitrary sync frequencies and pixel clock rates.
For the most common VGA mode, the horizontal timings can be found in the HP Super VGA Display Installation Guide and in other places.
Typical uses of selected modes
@ 70 Hz is traditionally the video mode used for booting VGA-compatible x86 personal computers that show a graphical boot screen, while text-mode boot uses @ 70 Hz.This convention has been eroded in recent years, however, with POST and BIOS screens moving to higher resolutions, taking advantage of EDID data to match the resolution to a connected monitor.
@ 60 Hz is the default Windows graphics mode, up to Windows 2000. It remains an option in XP via the boot menu "low resolution video" option and per-application compatibility mode settings, despite newer versions of Windows now defaulting to and generally not allowing any resolution below to be set.
The need for such a low-quality, universally compatible fallback has diminished since the turn of the millennium, as.
at 70 Hz was the most common mode for early 1990s PC games, with pixel-doubling and line-doubling performed in hardware to present a at 70 Hz signal to the monitor.
The Windows 95/98/Me LOGO.SYS boot-up image was 320 × 400 resolution, displayed with pixel-doubling to present a at 70 Hz signal to the monitor. The 400-line signal was the same as the standard text mode, which meant that pressing to return to text mode didn't change the frequency of the video signal, and thus the monitor did not have to resynchronize.
Connector
The standard VGA monitor interface is a 15-pin D-subminiature connector in the "E" shell, variously referred to as "DE-15", "HD-15" and erroneously "DB-15".All VGA connectors carry analog RGBHV video signals. Modern connectors also include VESA DDC pins, for identifying attached display devices.
Because VGA uses low-voltage analog signals, signal degradation becomes a factor with low-quality or overly long cables. Solutions include shielded cables, cables that include a separate internal coaxial cable for each color signal, and "broken out" cables utilizing a separate coaxial cable with a BNC connector for each color signal.
BNC breakout cables typically use five connectors, one each for Red, Green, Blue, Horizontal Sync, and Vertical Sync, and do not include the other signal lines of the VGA interface. With BNC, the coaxial wires are fully shielded end-to-end and through the interconnect so that virtually no crosstalk and very little external interference can occur. The use of BNC RGB video cables predates VGA in other markets and industries.