"Overclocking" is a slang term, and not an engineering or scientific term. The correct technical terms are "speed-margining" (more common) and "undertiming" (less common). One can also "overclock" the computer's bus. The 'overclocking' describes the process of running your CPU at a clock and/or bus speed that the CPU hasn't been specified for - logically, that speed is usually higher.
The tempting idea behind overclocking is to increase system performance at very little or no cost. In many cases you only need to change a few settings on your motherboard to make your system run faster. In other cases you only have to add a few components (usually for cooling) to achieve the performance increase.
In the past, overclocking was usually nothing more than increasing a CPU's clock speed to that of the next higher model, e.g. a Pentium 120 to a Pentium 133. Now, with new bus speeds available on several motherboards, you can change the clock and bus speed of a CPU to values that don't officially exist. This new way of overclocking is yielding an even higher performance increase than the classic one. It even gives you the ability to increase the performance of the fastest model of a particular CPU production line (e.g. P200 to 250 MHz, Pentium Pro 200 to 233 MHz).
Basics of CPU Speed
Two variables determine the speed at which your CPU runs: the frontside bus speed and the clock multiplier. By manipulating these variables you determine the CPU's clock speed.
Understanding Bus Speed : The CPU uses the FSB (frontside bus, also called the system or external bus) to communicate with system memory and peripherals.
FSB Speed x Clock Multiplier = CPU Speed
(This is what publicised speed of CPU e.g. Pentium 233 MHz MMX here FSB speed is 66 MHz and internal speed of CPU is 233 MHz using the clock multiplier of 3.5 i.e. 66 x 3.5 = 233 MHz approx).
The motherboard chipset controls the clock multiplier, which, in conjunction with the FSB speed, determines the core speed of the CPU. Multiply the FSB speed by the clock multiplier to get the CPU speed. By manipulating the clock multiplier and/or the FSB speed, you can increase the core speed at which the CPU runs.
Let's start by discussing a 486 processor (these have been around long enough that most people are familiar with how they work). There are various breeds of 486 cpu's. The major designations are SX, SX2, DX, DX2, and DX4. All 486 processors use a 32 bit data bus. The external data bus is how the CPU communicates with the motherboard. On SX and DX chips the speed of this bus (FSB Speed) is the same speed as the internal workings of the processor. On SX2 and DX2 chips the internal speed of the CPU is twice that of the external data bus i.e. using clock multiplier of 2. On DX4 CPUs the internal chip speed is three times that of the external bus. The difference between an SX and a DX chip is the fact that DX 486 chips have a math co-processor whereas the SX chips do not. Ok, so if you have a 486DX33 that means the external bus is running at 33mhz. Morover, the internal processing speed of this chip is 33mhz. A 486dx2/66 also has an external speed of 33, but the internal workings of the chip double that speed to attain 66mhz. The "dx2" chips use a technology that takes the external clock speed at which the motherboard is operating at and doubles it. "SX" and "DX" chips which do not have the "2" at the end of the name (as in "DX2") do not support a 2x mode of operation. This should dispel any rumors that one can take a 486dx33 and turn it into a 486dx2/66 like magic. Similarly, the "dx4" chips have clock tripling technology built in. This means that again, one may not simply turn a 486dx33 into a 486dx100.
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