So you’re looking into overclocking your Intel powered computer, but aren’t sure how the FSB, system, CPU, and memory speeds all relate? Well here’s some Speed 101 to get you going.
Let’s start with the motherboard bus- the bus speed is simply the speed at which information can pass between the CPU and the system memory (RAM). This speed is dictated by your motherboard, since it acts as the information highway (both the CPU and the RAM are connected via the motherboard circuitry). So if your CPU has a FSB speed of, say 1066MHz (really a base 266MHz x 4, since Intel CPUs can perform 4 operations per clock cycle, also known as quad pumping), then you’ll need a motherboard with electronics and circuitry (aka the chipset) to support that type of CPU potential, namely a motherboard with a system clock speed of 266MHz.
Let’s say you got all that. Now what about the memory speed?
The most common RAM these days is DDR2 or DDR3. DDR stands for double data rate, meaning it performs two operations per clock cycle. So if you have DDR2-800MHz memory, the true memory clock speed is 400MHz, but since it can do two things per clock cycle, the effective speed is 800MHz. The thing we need to take care of when matching RAM and CPU is really bandwidth, the amount of data transfer per second, and not just raw advertised speeds.
For example, if you merge a 266MHz system clock motherboard with a 1066MHz FSB CPU and then put in one stick of DDR2-800MHz memory, this is what happens:
With one stick of DDR2 RAM transfering 64 bits per clock cycle (meaning 800MHz effective speed x 64 = 51200 bits per cycle), we have a bandwidth of 6400MB/s. We know our CPU can perform 1066MT/s (million transfers per second), based on its FSB, and because each transfer involves 64 bits, the total bandwidth is about 8500MB/s.
Now you see there is a mismatch: a stick of DDR2-800MHz RAM can only handle 6400MB/s while the CPU is spitting out data at the rate of 8500MB/s. What’s the fix?
Most motherboard chipset memory controllers (that the CPU and RAM really interface through) has dual channel technology, meaning it can read/write to two (2) memory modules at once. Thus, 2 sticks of DDR-800MHz RAM has, effectively, 12,800MB/s of bandwidth (6400MB/s x 2). With this much bandwidth, it can handle the CPU data spit rate of 8500MB/s and then some.
And as for CPU speed, it’s simply the system clock speed of 266MHz x a hardwired multiplier value (usually a value between 5 thru 10).
If you’ve got different system clock, FSB, memory, and CPU speeds, you can work the numbers similarly.
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