Bus Speeds and Multipliers

Math?

Up until the 486DX2, all Intel CPUs ran at bus speed. This means that the chipset, the RAM, the cache and the processor all ran at the same speed. The 486 DX2 introduced the "clock-multiplier", meaning that the processor would run at a certain number times the clock speed. The speed of the processor is the product of the bus speed and the multiplier (bus speed x multiplier = processor speed). DX2s typically ran at 50MHz, 66MHz or (AMD's flavor) 80MHz while bus speeds were only 25MHz, 33MHz, and 40Mhz respectively. With a 2X multiplier, the processor could continue doing instructions, but would be unable to access memory or the chipset half the time.

Nowadays, we have multipliers as high as 10X, meaning that the processor is running 10 times as fast as its bus speed. For every 10 cycles of the processor, only during one is it able to access memory or the chipset. Athlons have the benefit of the EV6 bus which runs at 200MHz, meaning that the fastest (1GHz) Athlon has access to the system 1/5 of the time, though memory is still only running at one-tenth the speed.

Clock Locked!

Up until the introduction of the Deschutes Pentium II core, people were able to set both their bus speed and multiplier by changing jumpers on the motherboard. Unscrupulous dealers took advantage of this and started remarking chips: overclocking a chip and selling it as a higher speed grade. Intel has chosen to combat chip-remarking by locking multipliers: limiting a CPU to a single multiplier setting. Intel's decision had the side effect of putting overclockers in a hot spot. No longer could we simply change the multiplier to 5x from 5.5x to get from 333 to 366 on a 66MHz bus. Now we'd have to increase bus speed, and with that came certain hazards. With a multiplier stuck at 5x, you would have to increase the bus speed to about 73MHz in order to reach the same 366MHz mark, but most motherboards only offered two or three bus speeds at the time. Today's motherboards now offer many bus speeds. For example, Abit's SoftMenu III gives users access to FSB speeds from 100MHz to 183MHz in 1MHz steps.

When you increase bus speeds, you also have to worry about overclocking other devices on the system bus. Some PCI/AGP devices don't like being overclocked, and since the PCI/AGP buses are tied into/multiples of the system bus, PCI and AGP slots would overclock whether you want them to or not. Thankfully, more motherboards are offering more PCI/AGP system bus ratio selections that allow users to overclock the system bus without overclocking the PCI and AGP buses.