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AT bus
Last modified: Friday, March 21, 2003 

The expansion bus on the IBM PC/AT and compatible computers. The bus is the collection of wires and electronic components that connect all device controllers and add-in cards. The controllers are the components that attach to peripheral devices. The bus, therefore, is the main highway for all data moving in and out of the computer.

The AT bus, which runs at 8 megahertz and has a 16-bit data path, is the de facto standard for PCs. Because all IBM PCs (until the high-end PS/2 models) had an AT bus, it has been possible for manufacturers to produce expansion boards that will work with any PC.

The AT bus is sometimes referred to as the ISA bus, which stands for Industry Standard Architecture. However, ISA also includes the XT bus, which is an 8-bit version of the AT bus.

As processors have become more powerful, and applications more demanding, the AT bus has turned out to be the chief bottleneck in PCs. In response, IBM introduced the Micro Channel Architecture (MCA) in 1987. However, MCA was not accepted by the computer industry because it was not backward compatible with the AT-bus, so IBM has been forced to drop it.

A more successful alternative to the AT bus is the Extended Industry Standard Architecture (EISA ), a high-speed 32-bit bus architecture developed by a group of IBM's competitors. Unlike MCA, EISA is backward compatible with the AT bus, so a computer equipped with an EISA bus can accept AT or EISA expansion boards.

While EISA has had limited success, its speeds are still insufficient for modern graphical applications. The most common solution to bypassing the AT-bus bottleneck is to include a local bus on the motherboard. A local bus communicates directly with the processor rather than using the standard computer bus. Currently, there are two competing standards for local buses: VESA local bus (VL-bus), promoted by the VESA standards group, and Peripheral Component Interconnect (PCI), designed and promoted by Intel.

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