The Extended Industry Standard Architecture (in practice almost always shortened to EISA and frequently pronounced "eee-suh") is a bus standard for IBM compatible computers. It was announced in late 1988 by PC clone vendors (the "Gang of Nine") as a counter to IBM's use of its proprietary MicroChannel Architecture (MCA) in its PS/2 series.
EISA extends the AT bus, which the Gang of Nine retroactively renamed to the ISA bus to avoid infringing IBM's trademark on its PC/AT computer, to 32 bits and allows more than one CPU to share the bus. The bus mastering support is also enhanced to provide access to 4 GB of memory. Unlike MCA, EISA can accept older XT and ISA boards — the lines and slots for EISA are a superset of ISA.
EISA was much favoured by manufacturers due to the proprietary nature of MCA, and even IBM produced some machines supporting it. It was somewhat expensive to implement (though not as much as MCA), so it never became particularly popular in desktop PCs. However, it was reasonably successful in the server market, as it was better suited to bandwidth-intensive tasks (such as disk access and networking). Most EISA cards produced were either SCSI or network cards. EISA was also available on some non-IBM compatible machines such as the AlphaServer, HP 9000-D, SGI Indigo2 and MIPS Magnum.
By the time there was a strong market need for a bus of these speeds and capabilities, the VESA Local Bus and later PCI filled this niche and EISA vanished into obscurity.
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 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, there was still no standards body to "police" the industry.
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