Friday, 8 April 2011

Parts of computer


The computer is a programmable finite state machine which can perform precise arithmetic and logic operations. A programmable finite state machine is one that can take on only one of a fixed range of values. 


A list of instructions can be submitted to the computer in the form of a program. A task is performed by executing the corresponding program.

A collection of related programs are known as the Software.
The physical circuitry and components are known as Hardware.

Hardware can be categorized into five blocks; they are:

  1. Input Unit (I/P)
  2. Arithmetic and Logic Unit (ALU)
  3. Control Unit (CU)
  4. Memory
  5. Output Unit (O/P)

The input unit is the device used to enter data and instructions into the computer. The data read from the input device is stored in the computer’s memory. E.g. Keyboard, Joystick, Mouse, Light Pen .etc.

The Arithmetic and Logic Unit (ALU) performs arithmetic and logic operations. The arithmetic operations include addition, subtraction, multiplication and division. Logical operations include the Boolean functions such as AND, OR and NOT. These operations are used during conditional branching.

3. Control Unit (CU)

The Control Unit (CU) coordinates the activities of the various components of a computer. The Arithmetic and Logic Unit (ALU) and Control Unit (CU) together comprise the Central Processing Unit (CPU). The CPUs are called Microprocessors. The speed of a computer depends of the clock frequency of the microprocessors (Execution speed of instructions per seconds).

In addition to the main processor (CPU), there are coprocessors to speed up the operations of the main processor of a computer (basically the chip-set used in the motherboard of a computer is the coprocessor). They perform activities like floating point operations, memory management, and input-output management .etc.

4. Memory

The information (instruction and data) required is stored in memory. The Computer’s memory is constructed out of semi-conducting material and stores information in binary form. Binary information is composed of two symbols ‘0’ and ‘1’ called binary digits (bits). The memory is organized into equal sized unit (usually a collection of 8bits, called a byte). These units are arranged in a sequence and are defined by numbers called address (memory address). The memory of a computer can be divided into distinct parts as below.


Registers are locations within the microprocessor where data is stored temporarily during processing. These internal high speed registers are used in Arithmetic and Logic operations for holding data and operands. Some registers are accessible by the user through instructions. Others are reserved for the use of the CPU to perform its activities.


Cache is a small high speed memory that contains frequently used data. The use of cache avoids repeated reading of data from the slower main memory. Internal cache is located within the microprocessor (indicated as L1, L2 and L3 Cache).


External cache is used to supplement the internal cache. It is used when an internal cache is not available or not present. It is placed between the CPU and Main Memory (The size of the cache memory is indicated in the specification of the microprocessor). Cache size increases the performance of the microprocessor will also increases.


Main Memory stores the data and instructions required by the microprocessor. The main memory is also called RAM. Microprocessor instructions can directly access main memory locations. Main memory is fast but expensive. However, it is volatile; the contents stored will be lost when the power supply is cut off.

E.g.: SDRAM, DDR1 RAM, DDR2 RAM, DDR3 RAM (Main Memory types used in today’s Computer) .etc.



All the data and programs required by the computer cannot be stored in the main memory because it is small in size and volatile. Secondary memory is needed to store data and programs which is not currently required by the microprocessor. Secondary memory is slower but less expensive than the main memory. It is also non-volatile and larger in size. The processor can access the various type of memory in the memory hierarchy up to the main memory. The microprocessor cannot access the secondary memory directly. Therefore, data from the secondary memory has to be brought to the main memory so that the processor can use it. Memory management techniques are required to transfer information between the main memory and secondary memory, and this function is performed by the operating system.

E.g.: Hard Disk Drives, CD/DVD ROMs, Floppy Disk .etc.
Typical Modern Computer Memory Hierarchy Diagram

5. Output Unit (O/P)

Just as input devices are used to supply the computer with data, there should be some means for the computer to communicate with the user. The information generated by the computer is displayed using an output device.

E.g.: Printers, Monitors, Plotters .etc.

Typical Personal Computer with Peripherals (Input and Output Devices)


Typical Computer Storage Types 




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