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Concept
Extended Industry Standard Architecture
Summary
The Extended Industry Standard Architecture (in practice almost always shortened to EISA and frequently pronounced "eee-suh") is a bus standard for IBM PC compatible computers. It was announced in September 1988 by a consortium of PC clone vendors (the Gang of Nine) as an alternative to IBM's proprietary Micro Channel architecture (MCA) in its PS/2 series.
In comparison with 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, EISA is extended 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 ISA cards — 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 DEC AlphaServer, HP 9000 D-class, SGI Indigo2 and MIPS Magnum.
By the time there was a strong market need for a bus of these speeds and capabilities for desktop computers, 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 physically interfered with the extended portion of the slot) and continue to run in 8-bit mode.
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