Following figure shows the functional units of a computer it consists of five main parts.
(1) INPUT UNIT
(2) OUTPUT UNIT
(3) MEMORY
(4) ARITHMETIC AND LOGIC UNIT
(5) CONTROL UNIT
The input unit accepts the coded information from the keyboard of a video terminal or from other computer over digital communication line. ALU and control unit together called as CPU or simply a processor.
(1) INPUT UNIT
Computer accepts the coded information through the input unit. It has the capability or reading the instructions and data to be processed the most commonly used input devices are keyboard of a video terminal. This is electronically connected to the processing part of a computer. The keyboard is wired such that whenever a key is pressed the corresponding letter or digit is automatically translated into its corresponding code and is directly sent either to memory the processor.
(2) OUTPUT UNIT
Output unit displays the processed result. Examples are video terminals and graphic displays. Input and output unit is usually combined under the term I/O unit for example considers the keyboard of a video terminal. This consists of keyboard for input and a cathode ray tube display for output. I/O devices do not alter the information. Content or meaning of the data some devices can be used as output only.
(3) MEMORY UNIT
Memory unit is an integral part of a computer system. Main function of a memory unit is to store the information needed by the system. Typically it stores programs and data. The system performance is largely dependent on the organization. Storage capacity and speed of operation of the memory system can be broadly classified into four groups.
I. Internal memory
II. Primary memory
III. Secondary memory
IV. Cache memory
(4) ARITHMETIC AND LOGIC UNIT
The arithmetic and logic unit is the ‘CORE’ of any processor its unit that performs the calculations. A typical ALU will have two inputs ports and a result port. It will also have a control input telling it which operation to perform and additional outputs for condition codes.
Arithmetic unit
The arithmetic unit performs typical arithmetic operations such as addition subtraction. Increment and decrement usually by unity. This unit includes the necessary hardware required for operation on signed and unsigned integers. Floating point numbers and BCD numbers etc. this multiplayer selects either Y OR X depending on the selection line so which is also the carry-in input of the addle.
If so = 0 output is f = x + y
If so = 1 output is f = x + y + 1 were y + 1 is 5
Hence the output is f = x – y
(Compliment of Y)
Consider a simple logic unit which can perform two logic operation AND or EX- OR here also multiplexer selects either or these operations depending on the state of so.
If so = o output G = X AND Y
If so = 1 output G = X CD Y
Some additional hardware can be added to the above basic logic unit perform the Boolean operations.
For example
X = X + 1
Y + Y + Y * Y = X OR Y
Arithmetic and logic units are combined together to form ca complete ALU. Outputs F and G are multiplexed with a nibble multiplexer to get a single output depending on the selection lines of the nibble multiplexer the selection line is sometime known as mode selection line since it selects the desired mode either arithmetic or logic.
(5) CONTROL UNIT
The purpose of control unit is to control the system operations by routing the selected data items to the selected processing hardware at the original time control unit acts as nerve centre for the other units. This unit decodes and translators each instruction and generates the necessary enable signals for ALU and other units. Control unit has two responsibilities. I.e. Instructions interpretation and instruction sequencing
The operation of the computer can be summarized as below.
v The computer accepts in formation through the input unit and transfers it to the memory.
v Information stored in the memory is fetched into arithmetic and logic unit to perform the desired operations.
v Processed information is transferred to the output unit.
v All activities inside the machine are controlled by a control unit.
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