Extended Graphics Array

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 90 MHz, enabling a 75 Hz refresh rate at . The resolution was added with 16 bit high color support. The DAC was increased to 8 bits per channel, and the accelerated functions were enabled at 16 bit 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.

References

References

1. "XGA Logo". Paul Rand Foundation.
2. Jon Peddie. (8 April 2019). "Famous Graphics Chips: IBM's XGA. The End of an Era". IEEE.org.
3. (2003). "The PC Graphics Handbook". CRC Press.