Computers can only do one thing at a time.
It may appear that many things are happening simultaneously, for example you may be reading emails whilst you also view the latest music video streaming off the internet. In reality, the computer is following one instruction at a time.
It is just that the machine is so fast, everything seems to happen at once.
CPU CLOCK
Inside the CPU there is a 'clock' - the CPU clock 'ticks' by switching from high to low and then back again in a very precise time:
Every time the clock cycles, an instruction may be carried out.
Clock speed is measured in cycles per second. Since "1 cycle per second" is also known as 1 Hertz (after a famous scientist http://www.ideafinder.com/history/inventors/hertz.htm if you want to know), computer speed is often specified in 'Gigahertz' (GHz).The CPU clock speed on personal computers runs from less than 1GHz to about 3.8GHz
This means that the machine can carry out a thousand million instructions per second (in theory) when running at 1GHz.
If it runs at 3.8GHz then three thousand eight hundred million instructions per second could be carried out.
In practice, there are bottlenecks in the way, such as having to wait for data to be read from memory. This alone can take many clock cycles.
BUS CLOCK
Whilst the CPU is busy cycling through its program, data is being moved around the computer by means of data buses (see the page on buses). Each of these buses are also running with their own clocks. Each time the clock 'ticks' a data transfer can take place.
If you wish to improve the performance of a computer you select a machine that will run on a faster clock. As the clock is actually running as part of the CPU, you would not go to a computer supplier and say 'I want a new clock', instead you would upgrade your CPU.
Some enthusiasts try to get the most extreme performance out of their existing CPU. They do this by 'overclocking'. This means they adjust the clock to run faster than the CPU was really designed for. However, this makes the CPU run hotter and so extra cooling fans also have to be purchased.
Some overclocked machines have water cooling installed!
Recently (in 2004), the two main suppliers of CPUs have declared that they will no longer be pushing for ever higher speeds, so 3 to 4 Gigahertz is as fast as single CPUs will go for the forseeable future. Even now, the silicon chip running at top speed inside the CPU is nearly as hot an iron - any hotter and it is likely to burn itself out.
So computer makers are turning to other ideas to improve performance, for example parallel processing and better use of memory.
