Basic Structure of Assembly Language Program

YASH PAL,

Basic Structure of Assembly Language Program – A program is a collection of instructions written in a sequence to perform a specific task.

Basic Structure of Assembly Language Program

A basic assembly language program can have two types of structure.

  1. Sequential Program Structure
  2. Iterative Program Structure

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 Figure 1.

Flowchart for a sequential program
Figure 1: Flowchart for a sequential program
Memory
Address		Hex
Code		Instruction
Opcode		Instruction
Operand		Comments
XX00H
XX01H		06
53		MMB, 53HLoad 53H to register B
XX02H
XX03H		0E
58		MMC, 58HLoad 58H to register C
XX04H78MOVA, BCopy the contents of register B to A
XX05H81ADDCAdd the contents of register C to A
XX06H
XX07H
XX08H		32
70
XX		STAXX70HStore the result stored in register A to memory location XX70H
XX09H76HLTStop the execution
Assembly Language Program

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 Figure 2.

Flowchart for a lterative program
Figure 2: Flowchart for an iterative program
Memory
Address		Hex
Code		LabelInstruction
Opcode		Instruction
Operand		Comments
XX00HDBSRTIN01HRead the input port with the address 01H.
XX01H01IN01HRead the input port with the address 01H.
XX02HFECPI00HCompare the input data from input port with the immediate data 00H.
XX03H00CPI00HCompare the input data from the input port with the immediate data 00H.
XX04HC2JNZSRTIf the zero flag Z = 0, jump to the label SRT; else execute the next instruction.
XX05H00JNZSRTIf the zero flag Z = 0, jump to the label SRT; else execute the next instruction.
XX06HXXJNZSRTIf the zero flag Z = 0, jump to the label SRT else execute the next instruction.
XX07H76HLTStop the execution.
Assembly Language Program
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