VESA Local Bus

The VESA Local Bus is a local bus defined by the Video Electronics Standards Association, mostly used in personal computers based on the Intel 80486 CPU. VESA Local Bus worked alongside the ISA bus; it acted as a high-speed conduit for memory-mapped I/O and DMA, while the ISA bus handled interrupts and port-mapped I/O.

The VESA Local Bus was was designed as a stop-gap solution to the problem of ISA's limited bandwidth, and had several flaws that limited its useful life substantially:

• 80486 dependence. The VESA Local Bus relied heavily on the 80486's memory bus design. When the Pentium processor started to gain mass acceptance, circa 1995, there were major differences in its bus design, and the VESA bus was not easily adaptable. This also made moving the bus to non-Intel architectures nearly impossible.

• Limited number of slots available. Most PCs that used VESA Local Bus had only one or two slots available, as opposed to 5 or 6 ISA slots. This was because, as a direct branch of the 80486 memory bus, the VESA Local Bus didn't have the electrical ability to drive more than 1 or 2 cards at a time.

• Reliability problems. The same electrical problems that limited the VESA Local Bus to 2 slots also limited its reliability. Glitches between cards were common, especially on low-end motherboards, and when important devices such as hard disk controllers were attached to the bus, there was the all-too-common possibility of massive data corruption.

By 1996, the Pentium (driven by Intel's Triton chipset and Peripheral Component Interconnect (PCI) architecture) had all but eliminated the 80486 market, and VESA Local Bus with it; most of the last 80486 motherboards made have PCI slots in addition to (or completely replacing) the VESA Local Bus slots.

See also:

• Extended Industry Standard Architecture
• Industry Standard Architecture
• Micro Channel Architecture
• Peripheral Component Interconnect
• XT bus architecture

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Article based based on VESA Local Bus at FOLDOC, used with permission.