Bus Types

Many types of buses have been introduced since the creation of the personal computer. Some, such as Industry Standard Architecture (ISA), have had long histories. Others, such as IBM's MicroChannel, were never widely adopted for one reason or another. The most widely used bus today is PCI (described earlier in the section, 'The Next Generations: The Pentium Family'). Not long ago, PCI was seen as adding better performance to emerging high-speed computers. Now, even the once-sought-after PCI is considered sluggish. Accelerated Graphics Port (AGP) is one bus technology that may hold some solutions.

IBM PC

The original IBM PC supported 8-bit expansion cards that ran at the same speed as its Intel 8088 processor, 4.77MHz.

IBM PC-AT, or Industry Standard Architecture (ISA)

The IBM PC-AT introduced two major enhancements: The data path was increased (by use of a second connector) to 16 bits, and the speed of the expansion cards, usually fixed at 8.33MHz, was made independent of the processor speed.

IBM MicroChannel Architecture (MCA)

IBM's third version of a motherboard expansion bus increased the width of the bus (to 32 bits) and increased the speed. However, unlike with the two original bus designs, IBM didn't freely allow all the other hardware vendors to build cards that were compatible with the MCA specifications.

Enhanced Industry Standard Architecture (EISA)

In response to IBM's proprietary MCA bus, the other major hardware vendors (led primarily by Compaq) developed this enhanced bus design.

Video Electronics Standards Association (VESA) Local Bus (VL-Bus)

The VESA Local Bus is not a replacement for the other bus types, but is instead usually used as an auxiliary bus. The primary devices that support the VL-Bus are, as might be expected by its name, video cards. However, some high-performance disk controllers were released that use this standard. Using VL-Bus technology, especially over the long term, has limitations. Major limitations of the VL-Bus include a restriction in the number of VL-Bus devices, a maximum 32-bit data path (preventing expansion to the new Intel Pentium 64-bit systems), and a clock-speed limit of only 33MHz.

megabytes per second (MBps)

A measurement of the transfer speed of a device in terms of millions of bytes per second.

Peripheral Component Interconnect (PCI)

The PCI architecture is a 32-bit wide local bus design running at 33MHz. As a local bus design, PCI devices have direct access to the CPU local bus. The PCI local bus is connected to the CPU local bus and system memory bus via a PCI-Host bridge. This is a caching device providing the interface between the CPU, memory, and PCI local bus. The cache enables the CPU to hand off executions to the PCI bus to finish freeing valuable CPU resources. The CPU can continue to fetch information from the caching bridge while the cache controller provides an expansion device with access to system memory.

More than one communication on more than one bus can occur at the same time. This concurrent bus operation could not happen with previous architectures (such as VESA). Additionally, PCI expansion devices are fully independent of the CPU local bus; there is no CPU dependency at all. This design enables the CPU to be upgraded without requiring new designs for devices on the CPU or expansion buses.

Accelerated Graphics Port (AGP)

AGP was developed as a replacement for PCI. AGP uses the Intel two-chip 440LX AGP set. This set of chips sits directly on the motherboard and provides similar functionality to PCI. The new chips are responsible for handling the transfer of data between memory, the processor, and the ISA cards all at the same time. Transfer of data to and from PCI cards still occurs at 132 megabytes per second (MBps) at 33MHz. The significant change from PCI is in the speed of transfers to RAM and to the accelerated graphics port. Both have transfer speeds of 528MBps each. This fourfold performance increase provides a significant boost, speeding data along to high-speed CPUs and RAM.




MCSA. MCSE 2003 JumpStart. Computer and Network Basics
MCSA/MCSE 2003 JumpStart
ISBN: 078214277X
EAN: 2147483647
Year: 2003
Pages: 203
Authors: Lisa Donald

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