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[ The PC Guide | Systems and Components Reference Guide | Motherboard and System Devices | System Chipset and Controllers | Chipset Functions and Features | Chipset Processor Support ]

Processor Speed Support

Faster processors require chipset control circuitry capable of handling them. The specification of the processor speed is done using two parameters: the memory bus speed, and the processor multiplier. These are discussed in detail in the section on processor architecture. In short, the processor bus and memory bus connect the processor, chipset and memory together.

The memory bus speed is the processor's "external" speed, the speed it talks to the rest of the computer at (as opposed to its internal speed). The memory bus speed also (normally) dictates the speed of the PCI local bus, which in most motherboards runs at half the memory bus speed. Typical modern memory bus speeds are 50, 60, 66 and 75 MHz. The multiplier represents the factor that the processor multiplies the memory bus speed in order to obtain its internal speed. Multipliers on modern PCs are normally 1.5x, 2x, 2.5x, 3x, 3.5x, 4x, 4.5x or 5x.

The range of the processor speeds supported by the chipset is indicated, generally, by looking at the range of supported memory bus speeds and multipliers. For example, a typical classic Pentium chipset will support bus speeds of 50 to 66 MHz with a multiplier range of 1.5x to 3x. This yields speeds of 75, 90, 100, 120, 133, 150, 166 and 200 MHz (along with some in-between values that don't correspond to any processors actually on the market; for example, 50 MHz and 2.5x yields 125 MHz, a pair of settings not normally used.)

Note: In some cases the limiting factor on processor speed is not the chipset, but the physical socket. A Pentium 166 MHz will not fit into a Socket 5, so a motherboard using that socket is limited to lower-speed Pentiums (or OverDrive processors).

Note: Some Intel-compatibles use "equivalence ratings" in naming their chips, which do not represent the true speed of the processor. The Cyrix 6x86-PR166+ chip, for example, actually runs at 133 MHz. See here for details on "P ratings".

Next: Multiple Processor Support


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