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PCI=Peripheral Component Interconnect
A bus can best be defined as an avenue for transferring data between the central processing unit (CPU) and peripherals. The first IBM PCs, introduced in 1981, employed a bus known as ISA. These early models ran CGA graphics and DOS applications limited to several hundred thousand kilobytes. But the standard ISA bus --;with its 16-bit data path, 8MHz clock speed, and maximum data transfer rate of 5MB per second --has a hard time keeping up with the new generation of video, LAN, storage and Windows applications that require greater bandwidth. Enter PCI (Peripheral Component Interconnect), the newest standard interface bus for high-speed devices.
A bus that runs on schedule
The PCI local bus was jointly developed by Intel(R) and other industry leaders including IBM in order to bring current and next-generation PCs to new levels of system performance.
The PCI bus is well-equipped to handle newer, more demanding applications. PCI boasts a 32-bit data path, 33MHz clock speed and a maximum data transfer rate of 132MB/sec. A 64-bit, 66MHz specification exists for PCI designs, but it is not expected to be introduced until costs can be justified to support the faster, wider path. The 64-bit PCI bus will double data transfer performance to 264MB/sec. The PCI bus includes support for PCI to PCI bridge chips, allowing for more than three slots. Plug-and-play drivers are configured to determine automatically the level of chip in the system and load the appropriate drivers for you.
The key to PCI technology is that it supplements rather than replaces the traditional I/O bus. PCI is standard on selected models of PCs, IntelliStations and PC Servers and is the standard interface bus for high-speed devices, including LANs, SCSI, and video technology. Existing ISA cards work with PCI. Most PCI systems will support three to five performance-critical peripherals, whether integrated directly onto the motherboard or added through PCI expansion cards, such as multimedia, graphics, disk drives and LAN cards.
How PCI works
PCI is not a true local bus; instead, it occupies an intermediate level between the CPU local bus (processor/memory/cache subsystem) and a standard expansion bus (ISA). The PCI bus is isolated from the CPU local bus by a PCI bridge/controller. The CPU can write data to PCI peripherals, such as a hard drive, and the PCI bridge/controller can immediately store the data in its buffer. This lets the CPU go on to its next operation rather than waiting for the transfer to complete. The buffer then feeds the data to the peripheral at the most efficient rate possible.
PCI also supports busmasters, intelligent devices that, when attached to a system bus, can gain control of the bus and perform tasks independent of the CPU. The CPU can also run concurrently with busmaster peripherals. And it can operate independently when a PCI peripheral is active because of its buffered design.
The result is a previously unheard of 132MB per second data transfer rate. This blazing speed helps ensure you won't have to wait endlessly for your screen to be redrawn or for data to be retrieved from your hard drive.
Comparison of bus architectures
|Data Path Width||8/16||32||32/64||32/64|
|Data Bus Speed (MHz)||5.33/8.33||8.33||33/50||33|
|Data Transfer Rates (MB/sec)||5.33/8.33||33||132/264||132/264|
|Data Rates Implemented (MB/sec)||5.33/8.33||33||132||132|
|Number of Slots||0-8||0-8||0-2||0-4|
|Bus Masters Supported||No||Yes||Yes||Yes|
|Sync, Channel Checks||No||No||No||Yes|
|Works with ISA/EISA||N/A||N/A||Yes||Yes|