Industry Standard Architecture (in practice almost always shortened to ISA) is a computer bus standard for IBM compatible computers.
ISA originated as an 8-bit system in the IBM PC in 1981, and was extended in 1983 as the XT bus architecture. The newer 16-bit standard was introduced in 1984. Designed to connect peripheral cards to the motherboard, the protocol also allows for bus mastering although only the first 16 MiB of main memory is available for direct access. The 8-bit bus ran at 4.77 MHz, while the 16-bit bus operated at 8 MHz. In reference to the XT bus, it is sometimes referred to as the AT bus architecture. It was also available on some non-IBM compatible machines such as the short-lived AT&T Hobbit and later PowerPC based BeBox.
In 1987, IBM moved to replace the ISA bus with their proprietary Micro Channel Architecture (MCA) in an effort to regain control of the PC architecture, and the PC market. The system was far more advanced than ISA, and computer manufacturers responded with the Extended Industry Standard Architecture (EISA) and later, the VESA Local Bus (VLB). In fact, VLB used some parts originally intended for MCA due to the fact that component manufacturers already had the ability to manufacture it. Both were compatible expansions of the ISA standard.
