PGA, PS/2 and XGA
Professional Graphics Adapter (PGA)
The PGA display adapter was introduced in 1984 alongside the EGA adapter. With a selling price of over $4,000, the PGA was aimed directly at the high-end business market. The PGA was the first processor based graphics card, containing its own Intel 8088 processor which controlled all video related tasks.
The entire PGA package contained a special PGA display monitor, and the graphics card set, which when fully expanded, took up three system expansion slots. While the set cost a great deal of money, its ability was impressive, offering full 60 fps of 3 dimensional rotation and clipping. This system was geared for engineering and scientific uses, but was ousted in favor of the VGA system, which gained support due to heavy OEM presence in the market.
PS/2 display systems
In 1987, IBM threw a few new, but related video adapters on the market: the 8514/A, the multicolor graphics Array (MCGA) and the all-familiar Video Graphics Array (VGA) that we still use today. The 8514/A and the MCGA offered slightly better ability than VGA, but were for use only on PS/2 model MCA (Micro Channel Architecture) bus systems. The 8514/A could operate with a standard VGA monitor, but it could only operate in its highest resolutions and color depth when paired with an 8514 display. The MCGA adapter was a lower cost version of the 8514/A. However, at this time, IBM encountered heavy resistance in the industry towards adoption of its new MCA bus. This happened because IBM insisted on charging a licensing royalty for third party companies to develop and build systems and components for the MCA system.
While the industry scoffed at IBM, Compaq and a small cadre attempted to push a new bus, called the EISA, which never took off. Shortly thereafter, the VL-Bus (Vesa Local Bus) was introduced, mainly seeing use in 486 class computers. Intel then introduced the PCI bus with the release of the second generation Pentium Classic.
Extended Graphics Array (XGA and XGA-2)
It wasn't too long before IBM dropped support for the 8514/A, MCGA, along with the VGA standard to introduce their new XGA display, which debuted in late 1990. XGA was grown out of a combination of the 8514/A and VGA technology, using the same 15 pin HD-15 connector. As is expected of new technologies, it was not compatible with the older VGA standard, or VGA display monitors. XGA was used only on IBM model PS/2 computer systems, and required an XGA monitor to operate. You could physically attach an XGA monitor to a VGA graphics card, or vice versa, but the display would be skewered, and not work correctly.
The primary difference between XGA and VGA was that XGA was a co-processor based adapter, while VGA was an accelerator-based adapter. XGA was overall more capable than VGA, but the two were incompatible with each other. Since contained it's own processor, the XGA was capable of operating completely independent of the systems' CPU, not requiring any external direction in order to do its job. The XGA-2 essentially just increased the resolution, color depth and refresh rates supported by the XGA adapter. XGA was capable of up to 1024X768 resolution, and had a 65,536 (16-bit) color palette, but only reached 16 bit color at 640x480. At 1024X768 resolution it was restricted to 256 colors. A minimum of 1MB of local video RAM was required to operated the XGA in both of these modes.