Extended Industry Standard Architecture

Infobox Computer Hardware Bus
name = EISA
fullname = Enhanced Industry Standard Architecture

caption = Three EISA Slots.
invent-date = 1988
invent-name = Gang of Nine
super-name = PCI
super-date = 1993
width = 32
numdev = 1 per slot
speed = 8.33 MHz
style = p
hotplug = No
external = No
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 4.77MHz. The PC/AT, introduced in 1984, had three 8-bit slots and five 16-bit slots, all running at the system clock speed of 8MHz 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 16MHz. 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 an 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 sported 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 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 8.33MHz). Thus, even systems which didn't use the EISA bus gained the advantage of having the ISA standardized, which contributed to its longevity.

Technical data

Although the EISA bus had a slight performance disadvantage over MCA (bus speed of 8.33MHz, compared to 10 MHz), EISA contained almost all of the technological benefits that MCA boasted, including bus mastering, burst mode, software configurable resources, and 32-bit data/address buses. These brought EISA nearly to par with MCA from a performance standpoint, and EISA easily defeated MCA in industry support.

EISA replaced the tedious jumper configuration common with ISA cards with software-based configuration. Every EISA system shipped with an EISA configuration utility; this was usually a slightly customized version of the standard utilities written by the EISA chipset makers. The user would boot into this utility, either from floppy disk or on a dedicated hard drive partition. The utility software would detect all EISA cards in the system, and could configure any hardware resources (interrupts, memory ports, etc) on any EISA card (each EISA card would include a disk with information that described the available options on the card), or on the EISA system motherboard. The user could also enter information about ISA cards in the system, allowing the utility to automatically reconfigure EISA cards to avoid resource conflicts.

Similarly, Windows 95, with its Plug-and-Play capability, was not able to change the configuration of EISA cards, but it could detect the cards, read their configuration, and reconfigure Plug and Play hardware to avoid resource conflicts. Windows 95 would also automatically attempt to install appropriate drivers for detected EISA cards.

Industry Acceptance

EISA's success was far from guaranteed. Many manufacturers, including those in the "Gang of Nine", researched the possibility of using MCA. For example, Compaq actually produced prototype DeskPro systems using the bus. However, these were never put into production, and when it was clear that MCA had lost, Compaq allowed its MCA license to expire (the license actually cost relatively little; the primary costs associated with MCA, and at which the industry revolted, were royalties to be paid per system shipped). On the other hand, when it became clear to IBM that Micro Channel was dying, IBM actually licensed EISA for use in a few server systems. As a final jab at their competitor, Compaq (leader of the EISA consortium) didn't cash the first check sent by IBM for the EISA license. Instead, the check was framed and put on display in the company museum at Compaq's main campus in Houston, Texas.

See also

* Industry Standard Architecture (ISA)
* Micro Channel architecture (MCA)
* NuBus
* VESA Local Bus (VESA)
* Peripheral Component Interconnect (PCI)
* Accelerated Graphics Port (AGP)
* PCI Express (PCIe)
* List of device bandwidths
* PC/104
* CompactPCI
* PC card
* Low Pin Count (LPC)
* Universal Serial Bus

External links

* [http://www.techfest.com/hardware/bus/eisa.htm EISA bus technical summary]
* [http://www.mindshare.com/pdf/eisabook.pdf "EISA System Architecture" published by Mindshare (pdf)]
* [http://www.intel.com/design/archives/periphrl/index.htm Intel EISA Controllers]

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