Video Electronics Standards Association


The association responsible for SVGA and the VL-Bus. Originally also used to describe the VLB.

VESA Local Bus (or VL Bus)

(V-L-B or V-L-bus)

An expansion bus standard proposed to replace ISA, EISA, and Micro Channel. It lost in competition with the PCI expansion bus.

Industry Standard Architecture


An expansion bus design which EISA and Micro Channel were supposed to replace but still exists in some forms today; particularly in older machines. ISA extended the original bus architecture to 16 bits and allowed for bus mastering (the first 16MB of main memory was made available for direct access).

Extended Industry Standard Architecture


An ISA expansion architecture designed by Compaq and a consortium of other vendors and announced in 1988. It was designed to compete with IBM's Micro Channel architecture. EISA introduced a 32-bit bus and allowed multiple CPU's to share the bus. Memory access was expanded to 4GB.

While widely available for a short time, better choices evolved (e.g., PCI) and have taken over both EISA and Micro Channel.

Micro Channel Architecture


A bus architecture developed by IBMto replace the original PCbus. Because of the lack of backward compatibility and high license fees, MCA never caught on very well in the market.

The first thing you notice about the MCA bus is its connector. The PC bus uses 62-pin connectors (0.1-inch spacing). MCA uses 90-pin, 22-pin, and 84-pin connectors (0.05-inch spacing). There are also auxiliary connectors designed for special purposes.

Card size is also some 40% smaller in the MCA specification. This, along with the connector specifications, made the cards difficult to construct; limiting the market.

But, like beta was technically superior to VHS but still lost in the market, MCA was technically superior to the PC bus. The bus supported multiple processors using a system of priorities. Also, eight DMA channels were supported. The PC bus used edge-triggered interrupts whereas MCA used level-triggered interrupts. The level-triggered interrupts made it easy for multiple devices to share interrupt lines. Additional memory address lines were also included in the MCA specification. MCA also supported a 16-bit I/O; increasing the nunber of devices that could be addressed.

MCA also introduced an effective plug-and-play system with its programmable option select feature. Jumpers and switches on cards were replaced with a programmable register that was configured automatically when the computer started. This was accomplished through a manufacturer-supplied Adapter Description File provided on disk with the adapter card and keyed to the card by a unique identification number.

There were other benefits to MCA but market factors made it only a limited success.

Peripheral Component Interface


A high-speed expansion architecture. PCI allows multiple devices fast access between external devices and the computer.

PCI is an Intel design released late in 1993 and is now supported by major PC manufacturers. PCI runs at 20-33MHz and carries either 32 bits using a 124-pin connector or 64 bits using a 188-pin connector.

While often called a bus, PCI is really more of a bridge or mezzanine which decouples the peripherals connected to PCI from the CPU. The slower peripherals are allowed to operate asynchronously so everything runs at closer to its optimum speed.