Question 4.

Explain using an example how a simple assembly language program is executed.

The example given below illustrates/draw the execution of a simple code.

You need to add two numbers that are in memory locations FF0 and FF1, and finally store the result in memory location FF2.

To perform the addition following operations have to be accomplished.

1. Contents of memory location FF0 has to be loaded into accumulator. Let us assume that 5 is present in FF0.
2. Contents of memory location FF1 have to be read and should be added to value in accumulator. Result of the addition should be stored back in accumulator. Let us assume that 7 is present in FF1. So, 5 and 7 have to be added and the result should be stored in accumulator.
3. The result of the addition which is stored in accumulator must be written into memory location FF2. In this example, result of addition (12) have to be written into memory location FF2.

The assembly language program is as follows:

1^st Instruction LDA FF0:

The contents in memory location FF0 are loaded into accumulator. After the

instruction is executed accumulator stores value 5.

2^nd Instruction ADD FF1:

The contents in memory location FF1 is added to accumulator. The final result is stored in accumulator. So 5+7 addition is performed and result 12 is stored in accumulator.

3^rd Instruction STA FF2:

This instruction stores the contents of accumulator in memory location FF2.