(This document is part of the PC-Clone Unix Hardware Buyer's Guide. The Guide is maintained by Eric S. Raymond ; please email comments and corrections to him.)

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Bus Wars

This is yet another area in which progress has simplified your choices a lot. There used to be no fewer than four competing bus standards out there (ISA, EISA, VESA/VLB, PCI, and PCMCIA). Now there are effectively just two -- PCI for desktop/tower machines and PCMCIA for laptops.

Bus Types

PCI is Intel's fast 64-bit bus for the Pentium. Many PCI boards are actually PCI/ISA that supports both standards, so you can use less expensive ISA peripherals and controllers. Beware, though; dual-bus boards lose about 10% of their performance relative to single-bus PCI boards.

In the laptop market everything is PCMCIA. PCMCIA peripherals are about the size of credit cards (85x54mm) and vary in thickness between 5 and 10mm. They have the interesting feature that they can be hot-swapped (unplugged out and plugged in) while the computer is on. However, they are seldom seen in desktop machines. They require a special daemon to handle swapping; free versions are available under Linux.

Plug And Play

Many PCI cards have a feature called ``Plug and Play''. These cards negotiate with the operating system at boot time for things like IRQs and DMA channels -- they have no jumpers. Beware of these! Linux doesn't yet have full support for Plug and Play, though there are support utilities available. (Of the Microsoft OSs, only Windows 95 supports Plug and Play fully -- DOS can't handle it at all and Windows 3.1 requires manual intervention).

Historical Note

There used to be two ISA buses, the original 8-bit IBM PC and a 16-bit compatible extension sometimes called "AT bus". The term ISA didn't come into use until well into the lifetime of the latter. Here's a more complete list:

ISA: The original IBM PC bus architecture. The 8-bit version is completely extinct. The 16-bit AT version is still alive but has been declared obsolete by Intel in the PC99. specification.
MCA: Micro-Channel Architecture. A ``standard'' that IBM attempted to promulgate, esp. in the PS/2 series of machines. While they tried to claim it was faster/more efficient, it really was only marginally better than ISA. Its real advantage--to IBM--was that it was a closed architecture; they didn't publish the details of implementation as they had on virtually everything for the XT/AT. It failed horribly; customers didn't want to walk back into that trap.
EISA: Extended ISA. Required motherboard setup and manufacturer-provided descriptions that got loaded into flash ROM. Manually. By the user. Superseded by VLB years ago and now extinct.
VLB: VESA Local Bus. Usually seen on video cards providing high-speed graphics data transfer, it was also being touted for other cards. VLB slots accepted ISA and EISA cards and a further extension called the VESA local-bus specification. Supplanted by PCI.
PCI: Peripheral Component Interconnect. The winner of the bus wars.

Eric S. Raymond