Modern CPUs run at two separate speeds: the core speed and the bus speed. The core speed is the number you probably associate most strongly with the CPU, and is the speed at which most of the CPU's internal circuitry runs. A Pentium-III 700 runs at a core speed of 700MHz. The core of the CPU can run at a higher speed than the motherboard can. The bus speed is the speed of the interface between the CPU and the motherboard. An 80486 motherboard typically ran at a bus speed of only 33MHz, although a few models went as high as 50MHz. Most motherboards used with Intel Pentiums run at 66MHz, although some CPU speeds require bus speeds of 60MHz or 50MHz. Super7 motherboards boost the bus speed to as high as 133MHz.
Among slotted designs, bus speed ranges from 66MHz up to a theoretical maximum of 400MHz for Slot A. In practice most Slot 1 and 2 designs in early 2000 run at 133MHz or less and most Slot A designs run at 200MHz.
Whatever interface type you choose, it's critical that the CPU and motherboard share a bus speed. In some cases, a CPU can run at several different bus speeds, but requires a particular speed to achieve its best performance. If you're building a new computer, you should be sure that the motherboard you purchase supports operation at the CPU's best speed.
CPU bus speed has an impact on other system components, most notably RAM. The PC100 and PC133 forms of RAM are designed to operate on 100MHz and 133MHz motherboard busses, respectively. You can safely run faster RAM on a slower bus (PC133 RAM on a 100MHz bus, for instance), but don't try to run slower RAM on a faster bus. In some cases it's possible to use slower memory on a faster bus, however, by inserting wait states—pauses in the motherboard's RAM access cycle. Slot A motherboards running at 200MHz use PC100 or PC133 memory in early 2000; they run the memory bus at 100MHz or 133MHz and the CPU/chipset bus (the front-side bus) at 200MHz. Faster motherboard speeds, expected to appear in 2000, may require faster types of RAM, such as RDRAM. For more information on RAM, see Chapter 3, "Memory."
In order to set the CPU core speed, motherboards include jumpers or BIOS settings to specify a clock multiplier. The CPU runs its core at the bus speed multiplied by the clock multiplier. For instance, a 100MHz bus combined with a 4x clock multiplier produces a 400MHz core speed. The exact meanings of the clock multiplier settings can vary from one brand or model CPU to another, however, so it's important that you consult your motherboard documentation to set the clock multiplier appropriately for whatever CPU you use.
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