Wikipedia / Extended Graphics Array
Wikipedia / Extended Graphics Array
The eXtended Graphics Array (usually called XGA) is a graphics card manufactured by IBM and introduced for the IBM PS/2 line of personal computers in 1990 as a successor to the 8514/A. It supports, among other modes, a display resolution of pixels with 256 colors at 43.5Hz (interlaced), or at 60Hz (non-interlaced) with up to 65,536 colors. The XGA-2 added an 65,536 color mode and 60Hz non-interlaced.
The XGA was introduced at $1095 with 512K VRAM and additional $350 for the 512KB memory expansion (equivalent to $ and $, respectively, in ). As with the 8514/A, XGA required a Micro Channel architecture bus at a time when ISA systems were standard, however due to more extensive documentation and licensing ISA clones of XGA were made. XGA was integrated into the motherboard of the PS/2 Model 95 XP 486.
An improved version called XGA-2 was introduced in 1992 at $360, worth $ in dollars.
XGA gives its name to the resolution , as IBM's VGA gave its name to , despite the IBM 8514/A and PGC cards respectively supporting those resolutions prior to the eponyms.
Features
The 8514 had used a standardised API called the "Adapter Interface" or AI. This interface is also used by XGA, IBM Image Adapter/A, and clones of the 8514/A and XGA such as the ATI Technologies Mach 32 and IIT AGX. The interface allows computer software to offload common 2D-drawing operations (line-draw, color-fill, and block copies via a blitter) onto the hardware. This frees the host CPU for other tasks, and greatly improves the speed of redrawing a graphics visual (such as a pie-chart or CAD-illustration). Hardware-level documentation of the XGA was also made, which had not been available for the 8514/A.
XGA introduced a 64x64 hardware sprite which was typically used for the mouse pointer.
Differences from 8514/A
Register-compatible with VGA
Adds a 132 column text mode and high color in
Requires a minimum of 80386 host CPU
Provides a 3-dimensional drawing space called a "bitmap" which may reside anywhere in system memory
Adds a sprite for a hardware cursor
The Adapter Interface driver is moved to a .SYS file instead of TSR program
Provisions made for multitasking environment
XGA can act as bus master and access system memory directly
Hardware level documentation has been provided by IBM
XGA-2
XGA-2 added support for non-interlaced and made 1MB VRAM standard. It had a programmable PLL circuit and pixel clocks up to 90MHz, enabling a 75Hz refresh rate at . The resolution was added with 16bit high color support. The DAC was increased to 8 bits per channel, and the accelerated functions were enabled at 16bit color depth. Faster VRAM also improved performance.
Output capabilities
The XGA offered:
:
graphics mode with 256 colors at once (8-bit) out of 262,144 (18-bit RGB palette);
graphics with 65,536 colors at once (16-bit "high color");
text mode with 80×34 characters
:
graphics with 256 colors out of 262,144;
text with 85×38 or 146×51 characters
XGA-2 introduced:
graphics with 256 colors out of 16.7M (24-bit palette);
graphics with 65,536 colors at once;
graphics with 256 colors out of 16.7M
Later clone boards offered additional resolutions:
graphics with 16.7M accessible colors at once (if it were possible with pixels) (24-bit "true color");
graphics with 16.7M colors at once;
graphics with 65,536 and 16.7M colors at once
Clones
Unlike with the 8514/A, IBM fully documented the hardware interface to XGA. Further, IBM licensed the XGA design to SGS-Thomson (inmos) and Intel. The IIT AGX014 was largely compatible with the XGA-2 and offered some enhancements.
The VESA Group introduced a common standardized way to access features like hardware cursors, Bit Block transfers (Bit Blt), off screen sprites, hardware panning, drawing and other functions with VBE/accelerator functions (VBE/AF) in August 1996. This, along with standardised device drivers for operating systems such as Microsoft Windows, eliminated the need for a hardware standard for graphics.
See also
References
Further reading