The function of
each of these units is described below:
Input Unit
Information is
entered into a computer through input device. An input device reads the data
and program into the computer. The program contains instructions about what has
to be done with the data. It provides a way of man to machine communication. An
input device converts input information into suitable binary form acceptable to
the computer. Some popular input devices are listed below:
§
Keyboard
§
Floppy
and Hard Disk
§
Mouse
§
Punched
cards
§
Joystick
§
Optical
Mark reader
The functions
performed by input unit are
1. It reads/accepts data and instructions
from outside world.
2. It converts these data and instructions
received from outside world.
3. It supplies these converted data and
instructions to CPU for further processing.
Output Unit
The output
devices receive results and other information from the computer and provide
them to the users. The computer sends information to an output device in the
binary form. An output device converts it into a suitable form convenient to
users such as printed form, display on a screen, voice output etc. Some of the
popular output units are:
§
Computer
Screen called VDU (Visual Display Unit)
§
Printer
§
Plotter
The functions
performed by an output unit are as follows:
1. The output unit accepts the coded result
from computer after the processing.
2. It converts these binary coded
results/information into a human readable form.
3. It supplies the converted results to the
external world in the user required form.
Storage Unit
The function of storage
unit is to store information. The data and instructions that are entered into
the computer system through input units have to be stored inside the computer
before the actual processing starts. Similarly, the results produced by
computer after processing must be kept somewhere before they are passed onto
the output unit for display. Moreover, the intermediate results produced by the
computer must also be preserved. The storage unit or the primary/main memory of
the computer provides support for these storage functions. The main memory is a
fast memory. It stores programs along with data. The main memory is directly
accessed by the CPU.
The secondary
memory also called the auxiliary memory is used to store the information, data
and program instructions permanently. These may be used later on or deleted
whenever not required.
The functions of
storage unit are as follows:
1. The data and instructions received from
input unit are stored in it before processing.
2. It stores the intermediate results of
processing.
3. It stores the final results of
computation and when required sends it to the output unit.
Central
Processing Unit
The CPU is the
brain of the computer. Its primary function is to execute programs. Besides
executing programs, the CPU also controls the operation of all other components
such as memory, input and output devices. The major sections of a CPU are:
v
Arithmetic
and Logic Unit (ALU)
v
Control
Unit (CU)
1.
Arithmetic and Logic Unit
The ALU is the
main component/unit of CPU which performs actual execution of instructions and
programs. As the name tells, it performs all arithmetical calculations and
logical comparisons. The control unit transfers data and instructions in ALU,
one by one for processing. For this temporary storage, ALU contains a group of
special purpose and general purpose registers in it. During processing, the
data transfer from memory to ALU and ALU to memory is done many times. It
depends on the circuit designing of ALU that what type of arithmetic and logic
operations it can perform. The efficiency of ALU may vary from computer to
computer but still every ALU performs at least four basic operations i.e.
addition, subtraction, multiplication and division.
The main
functions performed by ALU are as follows:
1. It accepts data and decoded instructions
from main memory and control unit.
2. When it gets signal from CU, it performs
arithmetical calculations and logical operations according to current
instructions in it.
3. It returns the final result to main
memory for storage.
4. All instructions are executed in it and
it is main processing unit of the computer.
2.
Control Unit
The
control unit acts as a central nervous system for other components of computer.
Though, it does not perform anything in actual processing but it controls all
the actions of ALU and main memory. Not only this, it is the control Unit which
tells the input unit that this is the time to feed data into the memory, to
output unit, that get ready and accept data from memory and supply it to user.
It also tells ALU when and what is to be done with data i.e. during processing,
it fetches the instructions after selection, interprets it by decoding and
tells the ALU what is to be done with that. It maintains order of execution and
directs the operation of the entire system. It sends signals to various units
for performing jobs. Normally, all of them are control signals and hence we say
that control unit is the major controller circuit of the CPU.
The
functions of control unit are as follows:
1. It controls the operations of all other
blocks and units of the computer system by issuing them signals time to time.
2. It fetches the instructions of program
stored in main memory one by one in a step wise manner.
3. It decodes/interprets the instruction by
separating the operation code and operands.
4. It issues proper signals for data
transfer in ALU and instructs ALU to perform required job.
5. It sends intermediate results and final
results in main memory for storing it.
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