Saturday, March 9, 2013

Digital Computers

Digital Computers

Practically all modern computers:

1 Are electronic.

2 Are digital.

3 Are two-state.

4 Have a stored program


The layout of a typical digital computer is as shown in Fig 2.

Digital Computers

Fig 2 Layout or a typical digital computer

The central processing unit is the main part of the computer. It operates at a higher speed than the rest of the computer system and (in a modern computer) has no moving parts.

It is sometimes called the central processor and consists of the control unit, the arithmetic and logic unit (ALU) and the main store.

The devices outside the central processing unit but controlled by it are called peripheral devices or peripherals (e.g. input devices, output devices and backing storage devices are peripherals).

Backing store is a means of storing large amounts of data outside the central processing unit

The control unit

When the computer is running, the control unit repeatedly interprets (decodes) instructions rapidly, one after another. After each instruction is decoded it is carried out (executed). If the instruction involves a peripheral the control unit sends signals to operate the peripheral.

Arithmetic and logic unit

If an instruction involves carrying out an arithmetic operation on data, the data is transferred to the arithmetic and logic unit and the operation is carried out there.

Main store

The main store of a computer is the fast access store in the central processing unit. It is also called the main memory or the immediate access store (or IAS).

The most important function of the main store is to hold programs and data while the programs are being executed.

The main store has the following characteristics:

1 It can be written to and read from at very high speed. This is done without any mechanical movement.

2 It is divided up into small equal sized units called locations. Each location can be accessed by the computer using a number called its address. Each location has a different address.

3 It usually consists of two types of store or memory:

ROM-read only memory, which can be read but not written to and

RAM-random access memory, which can be read from or written to .


1 It is important to use the words 'location' and 'address' accurately. It is not correct to say that data is stored in an address- it is stored in a location.

2 Each location may contain as few as 8 bits (in some microcomputers) but often there are 16, 32 or more.

The clock

Most digital computers can carry out a million or more operations a second (see fetch-execute cycle). These operations occur in a regular cycle. The changes in the cycle from one state to the next are timed using a pulse produced by a circuit called a clock.