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PCI Bus Performance

The PCI bus provides superior performance to the VESA local bus; in fact, PCI is the highest performance general I/O bus currently used on PCs. This is due to several factors:

• Burst Mode: The PCI bus can transfer information in a burst mode, where after an initial address is provided multiple sets of data can be transmitted in a row. This works in a way similar to how cache bursting works.
• Bus Mastering: PCI supports full bus mastering, which leads to improved performance.
• High Bandwidth Options: The PCI bus specification version 2.1 calls for expandability to 64 bits and 66 MHz speed; if implemented this would quadruple bandwidth over the current design. In practice the 64-bit PCI bus has yet to be implemented on the PC (it does exist in non-PC platforms such as Digital Equipment's Alpha and is also found now on servers) and the speed is currently limited to 33 MHz in most PC designs, most likely for compatibility reasons. For mainstream PCI, we may be limited to 32 bits and 33 MHz for some time to come. However, it appears that the higher-performance PCI options are going to live on, albeit in modified form, through the new Accelerated Graphics Port.

The speed of the PCI bus can be set synchronously or asynchronously, depending on the chipset and motherboard. In a synchronized setup (used by most PCs), the PCI bus runs at half the memory bus speed; since the memory bus is usually 50, 60 or 66 MHz, the PCI bus would run at 25, 30 or 33 MHz respectively. In an asynchronous setup the speed of the PCI bus can be set independently of the memory bus speed. This is normally controlled through jumpers on the motherboard, or BIOS settings. Overclocking the system bus on a PC that uses synchronous PCI will cause PCI peripherals to be overclocked as well, often leading to system stability problems.

Next: PCI Expansion Slots

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