The Central Processing Unit

This part of the computer system collects the raw data from the input devices and converts it to useful information which can then be used by the output devices. On some computers, the CPU can be a single microchip. On bigger systems, the CPU can be formed from a number of chips working together.

The CPU is made up of three main parts:

  • Control Unit this part controls the input and output devices
  • Arithmetic Logic Unit this is the part that does all the working out: it does all the maths and makes the decisions
  • Immediate Access Store this is the memory available for programmes and data. The more memory the CPU has - the more programs it can run at the same time; and the more data it can deal with in one go
the 3 main parts of the CPU

Data that is held in the Immediate Access Store (IAS) can be used immediately by the Arithmetic Logic Unit (ALU), if the data is held in the backing store then it may take a little longer for the CPU to find it and copy it into memory. In 2002 a powerful microcomputer has 256 Mb (megabytes) of memory ... that is 256 million bytes.

The Control Unit is connected to a clock (a piece of crystal that 'ticks' very fast) and it issues a command at each 'tick' of the clock. If your computer has a very fast clock it means that the control unit is able to issue a lot of commands in a short time. So the computer carries out its jobs in a shorter time: it is faster.

In 2002 a fast microcomputer has a clock speed of 1.8 GHz (gigahertz) - this means that the control unit can issue about 1.8 billion instructions in a second ... every second.

Answer these questions:

  1. What does ALU stand for ?
  2. What does IAS stand for ?
  3. What does CPU stand for ?
  4. What does Mb stand for ?
  5. What does GHz stand for ?

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