Computer is a device that operates upon information or data. It is an electronic device which accepts input data, stores the data, does arithmetic and logic operation and outputs the information in desired format. Even though the size, shape, performance, reliability and cost of computers have been changing over the years, the basic logical structure proposed by Von Neumann has not change. The internal architecture of computers differs from one system model to another.
INPUT UNIT
Input unit accepts coded information from human operators through electromechanical devices such as the keyboard or from other computers over digital communication lines. The information received is either stored in the memory for later reference or immediately used by the Arithmetic and Logic circuitry to perform the desired operation. Finally the result is sent back to the outside through the output unit. The keyboard is wired so that whenever a key is pressed, the corresponding letter or digit is automatically translated into its corresponding code and sent directly to either the memory or the processor. Other kinds of input devices: Joy stick, track ball, mouse (pointing devices), scanner etc.
MEMORY UNIT
The memory unit stores program and data. There are two classes of memory devices– Primary memory and Secondary memory.
Primary memory (Main memory):
- Contains a large number of semiconductor cells each capable of storing one bit of information
- These cells are processed in group of fixed size called words containing ‘n’ bits. The main memory is organized such that the contents of one word can be stored or retrieved in one basic operation.
- For accessing data, a distinct address is associated with each word location.
- Data and programs must be in the primary memory for execution.
- Number of bits in each word is called the word length and it may vary from 16 to 64 bits.
- Fast memory
- Expensive
- Time required to access one word is called Memory Access Time – 10nS to 100nS. This time is fixed and independent of the location.
E.g. Random Access Memory (RAM)
Secondary storage:
- They are used when large amount of data have to be stored (also when frequent access is not necessary) E.g. Hard Disk, Compact Disk, Floppy Disk, Magnetic Tapes etc.
- PROCESSOR UNIT
- The heart of the computer system is the Processor unit.
- It consists of Arithmetic and Logic Unit and Control Unit.
- Arithmetic and Logic Unit (ALU)
- Most computer operations (Arithmetical and logical) are executed in ALU of the processor.
- For example: Suppose two numbers (operands) located in the main memory are to be added. These operands are brought into arithmetic unit – actual addition is carried. The result is then stored in the memory or retained in the processor itself for immediate use.
- Note that all operands may not reside in the main memory. Processor contains a number of high speed storage elements called Registers, which may be used for temporary storage of frequently used operands. Each register can store one word of data.
- Access times to registers are 5 to 10 times faster than access time to memory.
Control Unit
- The operations of all the units are coordinated by the control unity (act as the nerve centre that sends control signal to other units)
- Timing signal that governs the I/O transfers are generated by the Control Unit.
- Synchronization signals are also generated by the Control Unit
- By selecting, interpreting and executing the program instructions the program instructions the control unit is able to maintain order and direct the operation of the entire system.
- The control unit and ALU’s are usually many times faster than other devices connected to a computer system. This enabled a single processor to control a number of external devices such as video terminals, magnetic taped, disk memories, sensors, displays and mechanical controllers which are much slower than the processor.
OUTPUT UNIT
- Counter part of input unit
- Output devices accept binary data from the computer – decodes it into original form and supplies this result to the outside world.
E.g. Printer, Video terminals (provides both input & output functions), graphic displays etc.