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Extended Graphics Array

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XGA logo used internally within IBM, designed by Paul Rand[1]

The Extended Graphics Array (XGA) is an IBM display standard introduced in 1990.[2] Later it became the most common appellation of the 1024 × 768 pixels display resolution[3], but the official definition is broader than that.[4] 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.

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.[5]

History

It was initially built into the new PS/2 Model 90 and 90 XP, and was also available as an upgrade for existing PS/2 systems, as the “IBM PS/2 XGA Display Adapter/A”.

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:[6]

  • 640 × 480 pixels in 8 bit/px (256 color) palette-indexed mode.
  • 640 × 480 pixels in direct 16 bits-per-pixel (65,536 color) RGB hi-color (required 1 MB video memory option)
  • 1024 × 768 pixels with a 16-color (4 bit/px) palette, using a low frequency interlaced refresh rate
  • 1024 × 768 pixels with a 256-color (8 bit/px) palette, using a low frequency interlaced refresh rate (required 1 MB VRAM[7])

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 range of fixed-frequency monitors, neither adapter offered support for 800 × 600 SVGA modes.

Software support included drivers for OS/2 1.2 and 1.3, Windows 2.1 and 3.0, and popular software packages like AutoCAD.[6]

XGA-2, released in 1992, 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 bit/px) and interlaced, non-interlaced and flicker-free refresh rates even before the release of the XGA-2.

See also

References:

  1. ^ "XGA Logo". Paul Rand Foundation. Retrieved September 25, 2021.
  2. ^ IBM US (October 30, 1990). "IBM PS/2 XGA DISPLAY ADAPTER/A - IBM Announcement Letter Number 190-182". ardent-tool.com.
  3. ^ "XGA Full Form". GeeksforGeeks. 2020-05-13. Retrieved 2022-12-12.
  4. ^ Team, AnimationXpress (2006-07-19). "XGA (Extended Graphics array) -". Retrieved 2022-12-12.
  5. ^ IBM 3270 Workstation Program User's Guide and Reference (PDF). International Business Machines Corporation. 1987. p. GL-17. Retrieved 2014-01-23.
  6. ^ a b Necasek, Michal. "The XGA Graphics Chip". The OS/2 Museum. Retrieved 2013-08-01.
  7. ^ Necasek, Michal. "The 8514/A Graphics Accelerators". The OS/2 Museum. Retrieved 2013-08-01.
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