The main unit of a PC is the central processing unit or CPU. This unit is built into a microprocessor chip - an integrated circuit - which executes program instructions and supervises the computer's operation. The unit consists of three main parts.

The control unit interprets each instruction and activates the circuits and other components - disk drives, monitor, etc. to execute the necessary functions.

The arithmetic logic unit (ALU) performs mathematical calculations (+, -, etc.) and logical operations (and, or, etc.).

The registers store information. One of them is the program counter (PC) which keeps track of the next instruction to be performed in the main memory. Another is the instruction register (IR) which holds the instruction that is currently being used at the moment.

Microprocessors differ in the amount of data - the number of bits – they can work with at a time. There are 16, 32 and 64-bit processors. For example, 64-bit processors are able to address 4 billion times more

information than a 32-bit system.

The programs and data which pass through the central processor must be loaded into the main memory (also called internal memory). When the user runs an application, the microprocessor looks for it on secondary storage devices (disks) and transfers a copy of the application into the RAM (random access memory). RAM is temporary, i.e. its information is lost when the computer is turned off. However, the ROM (read only memory) is permanent and contains instructions needed by the processor.

The power of a computer partly depends on the speed of its microprocessor.

A clock provides pulses at fixed intervals to measure and synchronize circuits and units. The clock speed is measured in MHz (megahertz) or GH (gigahertz) and depends on the frequency at which pulses are emitted.

Answer the questions:

1.   What are the main parts of the CPU?

2.   What is RAM?

3.    What memory section is permanen and contains instructions needed by the CPU?

4.   What information is lost when the computer is switched off?

5.   What is the typical unit used to measure RAM memory and storage memory?

6.   What is a megahertz?

7.   What is the abbreviation for 'binary digit'?

8.    How can we store data and programs permanently?