CPU (central processing unit)

clock speed

Computers can only do one thing at a time. It may appear that many things are happening simultaneously, for example you may be listening to music while writing an essay and also downloading some software in the background. In reality, the computer can only process one instruction at a time. It is just that the machine is so fast, to you, everything seems to happen at once. A CPU processes digital data by taking each piece of data one-at-a-time and doing something with it. The amount of time that it has to process each piece of data is controlled by a quartz clock inside the CPU. With every tick of the clock, the CPU is able to process one piece of data or execute clockone instruction. The CPU clock speed is measured in cycles per second. 1 cycle per second is also known as 1 Hertz A computer running at 1GHz can carry out a thousand million instructions per second. The clock on a modern desktop computer runs extremely quickly, typically three thousand million times a second (3 GHz). It is hard to imagine just how fast this is, but it does mean that a CPU running at this clock speed can handle a huge amount of data in a very short time. This is why you can listen to music on the computer while you are also browsing the web and doing some work.

Fetch, decode, execute cycle

Fetch: CPU fetches data and instructions from the main memory and stores it in it's registers. To do this, the CPU makes use of the the address bus, which fetches the data/instructions from the address that the CPU has given it. The data/instructions then moves from the main memory tom the CPU, via the data bus. Decode: The next step is for the CPU to make sense of the instruction it has just fetched. The CPU is designed to understand a specific set of commands. These are called the 'instruction set' of the CPU. Each make of CPU has a different instruction set. The CPU decodes the instruction and prepares various areas within the chip in readiness of the next step. Execute: This is the part of the cycle when data processing actually takes place. The instruction is carried out upon the data (executed). The result of this processing is stored in yet another register. Once the execute stage is complete, the CPU sets itself up to begin another cycle once more.

Number of cores

In an ideal world, running two CPUs would give you twice the performance. In real life this is not quite the case because many applications have not been written to take advantage of extra processors. However, by having more than one processor, you would certainly see an improved performance. A modern processing device may contain two or even four CPUs. Some chip-making companies call these CPUs 'cores'. So a dual-core device means it contains two CPUs and a quad-core contains four.

The control unit

The Control Unit has three main jobs: It controls and monitors the hardware attached to the system to make sure that the commands given to it by the application software are used. For example, if you send something to print, the control unit will keep a check that the instructions are sent to the printer correctly. It controls the input and output of data so that the signals go to the right place at the right time It controls the flow of data within the CPU - which is the Fetch-Execute cycle described above.

Purpose of the CPU

The purpose of the CPU is to process data. It is where all the searching, sorting, calculating and decision making takes place in the computer.

The immediate access store

This holds the data and programs needed at that instant by the Control Unit. The CPU reads data and programs kept on the backing storage and stores them temporarily in the IAS's memory. The CPU needs to do this because Backing Storage (e.g. the hard disk) is much too slow to be able to run applications directly. So data and programs are first of all loaded into main memory then the CPU fetches just the part it immediately needs and loads it into its own internal registers as part of the Fetch-Execute cycle.

The arithmetic and logic unit

This is where the computer processes data either by manipulating it or acting upon it. It consists of two parts: Arithmetic part - which does exactly what you think it should - it performs the calculations on the data e.g. 3 + 2 = 5 Logic part - this deals with logic and comparisons. For example, it works out if one value is greater, less than or equal to another.