Extended Industry Standard Architecture - History

History

The original IBM PC included five 8-bit slots, running at the system clock speed of 4.77 MHz. The PC/AT, introduced in 1984, had three 8-bit slots and five 16-bit slots, all running at the system clock speed of 6 MHz in the earlier models and 8 MHz in the last version of the computer. The 16-bit slots were a superset of the 8-bit configuration, so most 8-bit cards were able to plug into a 16-bit slot (some cards used a "skirt" design that physically interfered with the extended portion of the slot) and continue to run in 8-bit mode. One of the key reasons for the success of the IBM PC (and the PC clones that followed it) was the active ecosystem of third-party expansion cards available for the machines. IBM was restricted from patenting the bus, and widely published the bus specifications.

As the PC-clone industry continued to build momentum in the mid- to late-1980s, several problems with the bus began to be apparent. First, because the "AT slot" (as it was known at the time) was not managed by any central standards group, there was nothing to prevent a manufacturer from "pushing" the standard. One of the most common issues was that as PC clones became more common, PC manufacturers began ratcheting up the processor speed to maintain a competitive advantage. Unfortunately, because the ISA bus was originally locked to the processor clock, this meant that some 286 machines had ISA buses that ran at 10, 12, or even 16 MHz. In fact, the first system to clock the ISA bus at 8 MHz was the turbo 8088 clones that clocked the processors at 8 MHz. This caused many issues with incompatibility, where a true IBM-compatible third-party card (designed for an 8 MHz or 4.77 MHz bus) might not work in a higher speed system (or even worse, would work unreliably). Most PC makers eventually decoupled the slot clock from the system clock, but there was still no standards body to "police" the industry.

The AT bus architecture was so well entrenched that no single clone manufacturer had the leverage to create a standardized alternative, and there was no compelling reason for them to cooperate on a new standard. Because of this, when the first 386-based system (the Compaq Deskpro 386) hit the market in 1986, it still supported 16-bit slots. Other 386 PCs followed suit, and the AT (later ISA) bus remained a part of most systems even into the late 1990s. Some of the 386 systems had proprietary 32-bit extensions to the ISA bus.

Meanwhile, IBM began to worry that it was losing control of the industry it had created. In 1987, IBM released the PS/2 line of computers, which included the MCA bus. MCA included numerous enhancements over the 16-bit AT bus, including bus mastering, burst mode, software configurable resources, and 32-bit capabilities. However, in an effort to reassert its dominant role, IBM patented the bus, and placed stringent licensing and royalty policies on its use. A few manufacturers did produce licensed MCA machines (most notably NCR), but overall the industry balked at IBM's restrictions.

In response, a group of PC manufacturers (the "Gang of Nine"), led by Compaq, created a new bus, which was named the Extended (or Enhanced) Industry Standard Architecture, or "EISA". (The Industry Standard Architecture, or "ISA", name replaced the "AT" name commonly used for the 16-bit bus.) This provided virtually all of the technical advantages of MCA, while remaining compatible with existing 8-bit and 16-bit cards, and (most enticing to system and card makers) minimal licensing cost.

The first EISA computers to hit the market were the Compaq Deskpro 486 and the SystemPro. The SystemPro, being one of the first PC-style systems designed as a network server, was built from the ground up to take full advantage of the EISA bus. It included such features as multiprocessing, hardware RAID, and bus-mastering network cards.

Ironically, one of the benefits to come out of the EISA standard was a final codification of the standard to which ISA slots and cards should be held (in particular, clock speed was fixed at an industry standard of 8.33 MHz). Thus, even systems which didn't use the EISA bus gained the advantage of having the ISA standardized, which contributed to its longevity.