Industry Standard Architecture

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Industry Standard Architecture (in practice almost always shortened to ISA) was a computer bus standard for IBM compatible computers.

Contents

History

The ISA bus was developed by a team lead by Mark Dean at IBM as part of the IBM PC project in 1981. It originated as an 8-bit system and was extended in 1983 for the XT system architecture. The newer 16-bit standard, the IBM AT bus, was introduced in 1984. In 1988, the Gang of Nine IBM PC compatible manufacturers put forth the 32-bit EISA standard and in the process retroactively renamed the AT bus to "ISA" to avoid infringing IBM's trademark on its PC/AT computer. IBM designed the 8-bit version as a buffered interface to the external bus of the Intel 8088 (16/8 bit) CPU used in the original IBM PC and PC/XT, and the 16-bit version as an upgrade for the external bus of the Intel 80286 CPU used in the IBM AT. Therefore, the ISA bus was synchronous with the CPU clock, until sophisticated buffering methods were developed and implemented by chipsets to interface ISA to much faster CPUs.

Designed to connect peripheral cards to the motherboard, ISA allows for bus mastering although only the first 16 MB of main memory are available for direct access. The 8-bit bus ran at 4.77 MHz (the clock speed of the IBM PC and IBM PC/XT's 8088 CPU), while the 16-bit bus operated at 6 or 8 MHz (because the 80286 CPUs in IBM PC/AT computers ran at 6 MHz in early models and 8 MHz in later models.) IBM RT/PC also used the 16-bit bus. It was also available on some non-IBM compatible machines such as the short-lived AT&T Hobbit and later PowerPC based BeBox.

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