Assembly Language Program

Assembly Language Program

Let us define a program statement as 'write an assembly language program to add two numbers'. The three tasks are involved in this program :

• Load two hex numbers

• Add numbers and

• Store the result in the memory

These tasks can be symbolically presented as flow chart, as shown in the Fig. 2.4.1.

Next job is to find the suitable 8085 assembly language instruction/s for each task. These instructions are as follows :

Task 1 instructions :

MVI A, 20H         ; Load 20H as a first number in register A

MVI B, 40H         ; Load 40H as a second number in register B

Task 2 instruction :

ADD B ; Add two numbers and save result in register A

Task 3 instruction :

STA 2200H ; Store the result in memory location 2000H

HLT ; Stop the program execution

We want to execute three tasks in a sequence, thus writing corresponding instructions in the same sequence constitutes program.

1. Assembly Language Program to Machine Language Program

Once the assembly language program is ready, it is necessary to convert it in the machine language program. It is possible to do this by referring the proper hex code for each assembly instruction from the 8085 instruction set manual. This process is known as hand assembly and the resulted machine language program is also known as hex code. Let us see the hex code for our program.

Mnemonics          Hex code

MVI A, 20H              3EH ←  Opcode

                        20H ← Operand

MVI B, 40H             06H ←  Opcode

                         40H ←  Operand

ADD B            80H ← Opcode

STA 2200H               32H ←  Opcode

                      00H ←  Operand (lower byte of address)

                      22H ← Operand (higher byte of address)

HLT              76H ← Opcode

Review Questions

1. Explain the process of writing assembly language program with the help of example.

2. What do you mean by hand assembly ? Explain with the help of example.