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.



Here is a list of the bus and processor speeds of the most common 486 chips.

||Proce Name:||FSB (ext) Speed:|| Int (proc) Speed:|| Clock Mltplier||
486SX20             20MHZ                   20MHZ                    1x
486SX25             25MHZ                   25MHZ                    1x
486SX33             33MHZ                   33MHZ                    1x
486DX40             40MHZ                   40MHZ                    1x
486DX50             50MHZ                   50MHZ                    1x
486sx2/50          25MHZ                   50MHZ                    2x
486dx2/50          25MHZ                   50MHZ                    2x
486dx2/66          33MHZ                   66MHZ                    2x
486dx2/80          40MHZ                   80MHZ                    3x
486dx4/75          25MHZ                   75MHZ                    3x
486dx4/100        33MHZ                   100MHZ                   3x
486dx4/120        40MHZ                   120MHZ                   3x
486dx4/133        33MHZ                   133MHZ                   4x
Pent. Ovdrive63  25MHZ                   63MHZ                     2.5x
Pent. Ovdrive83  33MHZ                   83MHZ                     2.5x




So what does all this mean?
Well, the fact is that on many motherboards the bus clock speed of the motherboard may be set using jumpers or dip switches. I suppose one day someone asked, "What will happen if I set the bus speed of my motherboard to 40MHZ despite the fact that I'm using a 486dx33 chip?" This is one of the types of overclocking; setting the bus speed faster than it really ought to be in relation to the intended speed of the CPU.


What improvements will I see?
By setting a 486dx33 to a 40MHZ bus speed you will find that the internal processing speed of the processor directly follows this modification. The chip will try to operate at 40MHZ. By speeding up the bus speed on a DX2 chip the internal workings of the chip will still try to double the external bus speed. That means that a 486dx2/66 operating on a motherboard whose bus speed is set to 40MHZ will try to operate at 80MHZ. A 486dx4/100 will try to operate at 120MHZ if the bus speed is increased to 40MHZ.


What else can be done?
Some motherboards also allow you to select whether the processor runs internally at 1x, 2x, 2.5x, 3x, etc. the external bus speed. Additionally, the BIOS of many systems allows you to modify the number of DRAM wait states, the cache type and wait states, the ISA bus speed, etc. All of these tinkering may prove to be of some benefit to overclockers.