Basic Structure of Assembly Language Program

Basic Structure of Assembly Language Program – A program is a collection of instructions written in a sequence to perform a specific task. A basic assembly language program can have two types of structure.

1. Sequential Program
2. Iterative Program

Sequential Program Structure

The instructions of a sequential program are executed one after another without any change in the sequence. Meaning that there is no branch instruction in the program. The program counter register is continuously incrementing until the instruction HLT is executed.

Let’s write a sequential assembly language program to perform the following operations:

1. Load the number 53H in register B and 58H in register C.
2. Add the contents of register B and register C.
3. Save the sum at memory location XX70H.

Solution: A flowchart of the given problem is shown in the figure below.

Memory Address	Hex Code	Instruction Opcode	Instruction Operand	Comments
XX00H XX01H	06 53	MM	B, 53H	Load 53H to register B
XX02H XX03H	0E 58	MM	C, 58H	Load 58H to register C
XX04H	78	MOV	A, B	Copy the contents of register B to A
XX05H	81	ADD	C	Add the contents of register C to A
XX06H XX07H XX08H	32 70 XX	STA	XX70H	Store the result stored in register A to memory location XX70H
XX09H	76	HLT		Stop the execution

Since there is no repeated task or instruction, this is an example of a sequential program.

Iterative Program Structure

The program in which a segment of the program is repeated to complete a task or to solve a typical problem is known as an iterative program. An iterative program structure can be further classified into two groups:

1. Unconditional
2. Conditional

Unconditional Iterative Program – In this type of program, the segment is repeated without any condition.

Conditional Iterative Program – In conditional iterative programs, the segment of the program is repeated after checking the condition. If the condition is satisfied, the repetition is done else not.

Let’s write an assembly language program to continuously read the status of eight switches connected at the input port. Input port address is 01H. Stop the execution of the program if all the input switches are OFF(0).

Solution: Reading operation, in this problem, is a task to be repeated until all the input switches are OFF(0). It means this is an example of a conditional iterative program. The flow chart for this is shown in the figure below.

Memory Address	Hex Code	Label	Instruction Opcode	Instruction Operand	Comments
XX00H	DB	SRT	IN	01H	Read the input port with the address 01H.
XX01H	01		IN	01H	Read the input port with the address 01H.
XX02H	FE		CPI	00H	Compare the input data from input port with the immediate data 00H.
XX03H	00		CPI	00H	Compare the input data from the input port with the immediate data 00H.
XX04H	C2		JNZ	SRT	If the zero flag Z = 0, jump to the label SRT; else execute the next instruction.
XX05H	00		JNZ	SRT	If the zero flag Z = 0, jump to the label SRT; else execute the next instruction.
XX06H	XX		JNZ	SRT	If the zero flag Z = 0, jump to the label SRT else execute the next instruction.
XX07H	76		HLT		Stop the execution.