Extended Graphics Array

A resolution of 1024 × 768 display.

The Extended Graphics Array (XGA) is an IBM display standard introduced in 1990. Later it became the most common appellation of the 1024 × 768 pixels display resolution, but the official definition is broader than that. It was not a new and improved replacement for Super VGA, but rather became one particular subset of the broad range of capabilities covered under the "Super VGA" umbrella.

The initial version of XGA (and its predecessor, the IBM 8514/A) expanded upon IBM's older VGA by adding support for four new screen modes (three, for the 8514/A), including one new resolution:

  • 640 × 480 pixels in direct 16 bits-per-pixel (65,536 colors) RGB hi-color (XGA only, with 1 MB video memory option) and 8 bpp (256 colors) palette-indexed mode.
  • 1024 × 768 pixels with a 16- or 256-color (4 or 8 bpp) palette, using a low frequency interlaced refresh rate (again, the higher 8 bpp mode required 1 MB VRAM).

Like the 8514, XGA offered fixed-function hardware acceleration to offload processing of 2D drawing tasks. Both adapters allowed offloading of line-draw, bitmap-copy (BitBlt), and color-fill operations from the host CPU. XGA's acceleration was faster than 8514's, and more comprehensive, supporting more drawing primitives, the VGA-res hi-color mode, versatile "brush" and "mask" modes, system memory addressing functions, and a single simple hardware sprite typically used to provide a low CPU load mouse pointer. It was also capable of wholly independent function, as it incorporated support for all existing VGA functions and modes – the 8514 itself was a simpler add-on adapter that required a separate VGA to be present. As they were designed for use with IBM's own range of fixed-frequency monitors, neither adapter offered support for 800 × 600 SVGA modes.

XGA-2 added a 24-bit DAC, but this was used only to extend the available master palette in 256-color mode, e.g. to allow true 256-greyscale output instead of the 64 grey levels previously available; there was still no direct True Color mode despite the adapter featuring enough default onboard VRAM (1 MB) to support it. Other improvements included the provision of the previously missing 800 × 600 resolution (using an SVGA or multisync monitor) in up to 65,536 colors, faster screen refresh rates in all modes (including non-interlace, flicker-free output for 1024 × 768), and improved accelerator performance and versatility.

IBM licensed the XGA technology and architecture to certain third-party hardware developers, and its characteristic modes (although not necessarily the accelerator functions, nor the MCA data-bus interface) were aped by many others. These accelerators typically did not suffer from the same limitations on available resolutions and refresh rate, and featured other now-standard modes like 800 × 600 (and 1280 × 1024) at various color depths (up to 24 bpp Truecolor) and interlaced, non-interlaced and flicker-free refresh rates even before the release of the XGA-2.

All standard XGA modes have a 4:3 aspect ratio with square pixels, although this does not hold for certain standard VGA and third-party extended modes (640 × 400, 1280 × 1024).

XGA should not be confused with EVGA (Extended Video Graphics Array), a contemporaneous VESA standard that also has 1024 × 768 pixels. It should also not be confused with the Expanded Graphics Adapter, a peripheral for the IBM 3270 PC which can also be referred to as XGA.

Key Terms

refresh rate

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Sources & Credits

Last modified on May 29 2020
Content adapted from Wikipedia
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