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Name: Matthew F.
Status: Student
Age: 19
Location: N/A
Country: N/A
Date: October 2002


Question:
I am wondering just how a computer's processor speed is determined. They give measurements like 500 MHz. But what really is a MHz. How much information does that really process? I understand how the measurement for bits to bytes to kilobytes and so forth works. Is there a direct relation to these sizes?



Replies:
MHz stands for mega-Hertz, which means million cycles per second.

What this tells you is the speed of the clock driving the computer's processor. All computers use a timing clock, actually a crystal, to drive them. This clock or crystal has high and low voltage changes, and changes at an exact frequency. Each time the clock changes, the computer's processor processes some part of an instruction.

So, a computer that has a 500MHz clock is doing something 500 million times per second. A 1GHz computer does something 1 Billion times per second.

Now in comparing two computers, it gets a little tricky. If the two computers have the same type of processor, for example they are both Pentium 4 computers, you can take the numbers and compare them directly, and you will get the difference in processing speed of the computers. Other things can affect the speed as well, but the clock speed will give you the difference in total speed of the processors.

Comparing different processors is much more difficult. The clock still makes each processor do something each cycle, but what that something is can be very different. For example, on old computers, it would take several cycles for the processor to complete one operation. On a P4, however, it usually completes two operations per cycle.

This is not too bad if the processors are of the same brand. Intel processors for example, roughly doubled the amount of work each processor was able to do each cycle. So a 80286 ran at a faster clock speed and completed more each cycle than an 8086.

Different brands of processors use different instructions, and this makes it more difficult. PowerPC processors, used in the iMac, usually run at slower clock speeds than Intel processors, but usually do more per instruction.

So, in the case of different brands of processors, the clock speed will give you a rough idea of the speeds, but will not give you a precise difference. For a precise difference, you probably should run the applications you are interested in using on both computers and see the difference for yourself. This is because, since the instructions are different, it is entirely possible some applications will run faster on one computer and other programs will run faster on the other.

I hope this explains a bit of what you wanted to know. Basically MHz gives you the clock speed of the computer, which in turn gives you some idea of how fast it is.

Eric Tolman
Computer Scientist


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