Let's attempt to answer a fundamental question: 'How does the Computer execute a Program?' Let's describe this with the help of an example from higher level language domain.

Problem:  Write a program to add two numbers.

A sample C program (Assuming two fixed values of numbers as a = 5 and b = 2)

1.  #include < stdio.h >

2. main ()

3.    {

4.  int a =5, b=2, c;

5. c= a+b;

6. printf ("\n The added value is: % d", c);   

7.           }

Program at line 4 states variables which will be equal to 3 memory locations that are a, b and c.  At line 5 these variables are added along with at line 6 value of c is printed.

However how will these instructions be executed by CPU? First you have to compile this program to convert it to machine language. Though how will the machine instructions look like? Let's suppose a hypothetical instruction set of machines of a size of 16 binary digits (bits) data and instructions. Each one of the instruction of machine comprises two components: (a) Operation code which specifies the operation which is to be performed by instruction and (b) Address of operand in memory on that given operation is to be performed.

Let's further presume that size of operation code is supposed to be of six bits consequently rest 10 bits are for address of operand. Also the memory word size is presumed to be of 16 bits. Figure below shows instruction and data formats for this machine.  To simplify our discussion let's present operation code employing Mnemonics such as ADD, LOAD, STORE and signed decimal values for data and decimal values of operand addresses.