The BIOS is typically placed in a ROM chip that comes with the computer (it is often called a ROM BIOS). This ensures that the BIOS will always be available and will not be damaged by disk failures. It also makes it possible for a computer to boot itself. Because RAM is faster than ROM, though, many computer manufacturers design systems so that the BIOS is copied from ROM to RAM each time the computer is booted. This is known as shadowing.

Many modern PCs have a flash BIOS, which means that the BIOS has been recorded on a flash memory chip, which can be updated if necessary.

The PC BIOS is fairly standardized, so all PCs are similar at this level (although there are different BIOS versions). Additional DOS functions are usually added through software modules. This means you can upgrade to a newer version of DOS without changing the BIOS.

PC BIOSes that can handle Plug-and-Play (PnP) devices are known as PnP BIOSes, or PnP-aware BIOSes. These BIOSes are always implemented with flash memory rather than ROM.

EIDE is sometimes referred to as Fast ATA or Fast IDE, which is essentially the same standard, developed and promoted by Seagate Technologies. It is also sometimes called ATA-2.

There are four EIDE modes defined. The most common is Mode 4, which supports transfer rates of 16.6 MBps. There is also a new mode, called ATA-3 or Ultra ATA, that supports transfer rates of 33 MBps.

The EISA bus was designed by nine IBM competitors (sometimes called the Gang of Nine): AST Research, Compaq Computer, Epson, Hewlett-Packard, NEC, Olivetti, Tandy, WYSE, and Zenith Data Systems. They designed the architecture to compete with IBM's own high-speed bus architecture called the Micro Channel architecture (MCA).

The principal difference between EISA and MCA is that EISA is backward compatible with the ISA bus (also called the AT bus), while MCA is not. This means that computers with an EISA bus can use new EISA expansion cards as well as old AT expansion cards. Computers with an MCA bus can use only MCA expansion cards.

EISA and MCA are not compatible with each other. This means that the type of bus in your computer determines which expansion cards you can install.

Neither EISA nor MCA has been very successful. Instead, a new technology called local bus (PCI) is being used in combination with the old ISA bus.

FPM RAM is being replaced by newer types of memory, such as SDRAM.

Although it really refers to a general technology, most people use the term to refer the ATA specification, which uses this technology.

In 1993, Intel and Microsoft introduced a new version of the ISA specification called Plug and Play ISA. Plug and Play ISA enables the operating system to configure expansion boards automatically so that users do not need to fiddle with DIP switches and jumpers.

PCI is a 64-bit bus, though it is usually implemented as a 32-bit bus. It can run at clock speeds of 33 or 66 MHz. At 32 bits and 33 MHz, it yields a throughput rate of 133 MBps.

Although it was developed by Intel, PCI is not tied to any particular family of microprocessors.

In 1997, Intel announced that it would license the Rambus technology for use on its future motherboards, thus making it the likely de facto standard for memory architectures. However, a consortium of computer vendors is working on an alternative memory architecture called SyncLink DRAM (SLDRAM).

RDRAM is already being used in place of VRAM in some graphics accelerator boards, but it is not expected to be used for the main memory of PCs until 1998 or 1999. Intel and Rambus are also working a new version of RDRAM, called nDRAM, that will support data transfer speeds at up to 1,600 MHz.

SCSI interfaces provide for faster data transmission rates (up to 80 megabytes per second) than standard serial and parallel ports. In addition, you can attach many devices to a single SCSI port, so that SCSI is really an I/O bus rather than simply an interface.

The following varieties of SCSI are currently implemented:

Wide Ultra2 SCSI: Uses a 16-bit bus and supports data rates of 80 MBps.

Today's fastest Pentium systems use CPU buses running at 100 MHz, so SDRAM can keep up with them, though barely. Future PCs, however, are expected to have CPU buses running at 200 MHz or faster. SDRAM is not expected to support these high speeds which is why new memory technologies, such as RDRAM and SLDRAM, are being developed.

The bus from a SIMM to the actual memory chips is 32 bits wide. A newer technology, called dual in-line memory module (DIMM), provides a 64-bit bus. For modern Pentium microprocessors that have a 64-bit bus, you must use either DIMMs or pairs of SIMMs.

A special type of VRAM, called Windows RAM (WRAM), yields even better performance than conventional VRAM.

WRAM is similar to VRAM, but achieves even faster performance at less cost because it supports addressing of large blocks (windows) of video memory.