Basic Working Principle of a Computer | Computer Architecture & Organisation (CAO) - Computer Science Engineering (CSE) PDF Download

Basic Working Principle of a Computer
Before going into the details of working principle of a computer, we will analyse how computer works with the   help of a small hypothetical computer.

In this small computer, we do not consider about Input and Output unit. We will consider only CPU and memory module. Assume that somehow we have stored the program and data into main memory. We will see how CPU can perform the job depending on the program stored in main memory.

P.S.  - Our assumption is that students understand common terms like program, CPU, memory etc. without knowing the exact details.

Consider the Arithmatic and Logic Unit (ALU) of Central Processing Unit :

Consider an ALU which can perform four arithmatic operations and four logical operations

To distingish between arithmatic and logical operation, we may use a signal line,

0  -  in that signal, represents an arithmatic operation and
1  -   in that signal, represents a logical operation.

In the similar manner, we need another two signal lines to distinguish between four arithmatic operations.

The different operations and their binary code is as follows:

Consider the part of control unit, its task is to generate the appropriate signal at right moment.

There is an instruction decoder in CPU which decodes this information in such a way that computer can perform the desired task

The simple model for the decoder may be considered that there is three input lines to the decoder and correspondingly it generates eight output lines. Depending on input combination only one of the output signals will be generated and it is used to indicate the corresponding operation of ALU.

In our simple model, we use three storage units in CPU,
                  Two  --  for storing the operand and
                  one  --  for storing the results.
These storage units are known as register.

But in computer, we need more storage space for proper functioning of the Computer.

Some of them are inside CPU, which are known as register. Other bigger junk of storage space is known as primary memory or main memory. The CPU can work with the information available in main memory only.

To access the data from memory, we need two special registers one is known as Memory Data Register (MDR)and the second one is Memory Address Register (MAR).

Data and program is stored in main memory. While executing a program, CPU brings instruction and data from main memory, performs the tasks as per the instuction fetch from the memory. After completion of operation, CPU stores the result back into the memory.

In next section, we discus about memory organization for our small machine.