Computer Organization

Article No: 4

The modern computer is a computer system that is made up of hardware and software. The hardware has five different types of units like: input unit, memory unit, ALU, CU and output unit. A computer as shown in FIGURE 1.4 performs basically five major operations or functions irrespective of their size and make. These are 1) it accepts data or instructions by way of input, 2) it stores data, 3) it can process data as required by the user, 4) it gives results in the form of output, and 5) it controls all operations inside a computer. We discuss below each of these operations.

FIGURE 1.4 Functional units in computer

Input Unit

Data and instructions must enter the system before any computation can be performed on the supplied data. This task is performed by the input unit that links the external environment with the computer system. Data and instructions enter input units in forms that depend upon the particular device used. For example, data are entered from a keyboard in a manner similar to typing, and this differs from the way in which data are entered through a scanner, which is another type of input device. However, regardless of the form in which they receive their inputs, all input devices must provide a computer with data that are transformed into the binary codes that the primary memory of a computer is designed to accept. This transformation is achieved by units called input interfaces. Input interfaces are designed to match the unique physical or electrical characteristics of input devices to the requirements of the computer system.

In short, the following functions are performed by an input unit:

1.      It accepts (or reads) the list of instructions and data from the outside world.

2.      It converts these instructions and data in computer acceptable form.

3.      It supplies the converted instructions and data to the computer system for further processing.

Memory Unit

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 the computer after processing must also be kept somewhere inside the computer system before being passed on to the output units. Moreover, the intermediate results produced by the computer must also be preserved for ongoing processing. The storage unit of a computer system is designed to cater to all these needs. It provides space for storing data and instructions, space for intermediate results, and also space for the final results.

In short, the specific functions of the storage unit are to hold (store):

1.      All the data to be processed and the instructions required for processing (received from input devices).

2.      Intermediate results for processing.

3.      Final results of processing before these results are released to an output device.

Primary storage

The primary storage, also known as main memory, is used to hold pieces of program instructions and data, intermediate results of processing, and recently produced results of processing of the job(s) that the computer system is currently working on. These pieces of information are represented electronically in the main memory chip’s circuitry, and while it remains in the main memory, the central processing unit can access it directly at a very fast speed. However, the primary storage can hold information only while the computer system is on. As soon as the computer is switched off or reset, the information held in the primary storage disappears. Additionally, the primary storage normally has limited storage capacity because it is very expensive. The primary storage of modern computer system is made up of semiconductor devices.

Secondary storage

The secondary storage, also known as auxiliary storage, is used to take care of the limitations of the primary storage. That is, it is used to supplement the limited storage capacity and the volatile characteristics of primary storage. This is because secondary storage is much cheaper than primary storage and it can retain information even when the computer system is switched off or reset. The secondary storage is normally used to hold the program instructions, data, and information of those jobs on which the computer system is not working on currently, but needs to hold them for processing at a later time.

Control Unit

How does the input device know that it is the time to feed data into the storage unit? How does the ALU know what should be done with the data once they are received? And how is that only the final results are sent to the output device and not the intermediate results? All this is possible because of the control unit of the computer system. By selecting, interpreting, and seeing the execution of the program instructions, the control unit is able to maintain order and direct the operation of the entire system. Although, it does not perform any actual processing on the data, the control unit acts as a central nervous system for the other components of the computer system. It manages and coordinates the entire computer system. It obtains instructions from the program stored in main memory, interprets the instructions, and issues signals that cause other units of the system to execute them.

ALU (Arithmetic Logical Unit)

The arithmetic logical unit (ALU) of a computer system is the place where the actual execution of the instructions takes place during the processing operation. To be more precise, calculations are performed and all comparisons (decisions) are made in the ALU. The data and instructions, stored in the primary storage prior to processing, are transferred as and when needed to the ALU where processing takes place. No processing is done in the primary storage unit. Intermediate results generated in the ALU are temporarily transferred back to the primary storage until needed at a later time. Data may thus move from primary storage to ALU and back again to storage many times before the processing is over. After the completion of processing, the final results, which are stored in the storage unit, are released to an output device.

The type and number of arithmetic and logic operations that a computer can perform is determined by the engineering design of the ALU. However, almost all ALUs are designed to perform the four basic arithmetic operations (add, subtract, multiply, and divide), and logic operations or comparisons such as less than, equal to, and greater than.

Output Unit

The job of an output unit is just the reverse of that of an input unit. It supplies the information obtained as a result of data processing to the outside world. Thus it links the computer with the external environment. As computers work with binary code, the results produced are also in the binary form. Hence, before supplying the results to the outside world, it must be converted to human acceptable (readable) form. This task is achieved by units called output interfaces. Output interfaces are designed to match the unique physical or electrical characteristics of output devices (terminals, printers, etc.) to the requirements of the external environment.

In short, the following functions are performed by an output unit:

1.      It accepts the results produced by the computer, which are in coded form and hence cannot be easily understood by us.

2.      It converts these coded results to human acceptable (readable) form.

3.      It supplies the converted results to the outside world.

Compiled By: Chaudhary Amit V.