Chapter 9: Control Structures in MASM

Introduction:

Importance of control structures in assembly language programming

Overview of control structures available in MASM

Conditional Branching:

Implementing conditional branching using jump instructions (JMP, JZ, JNZ, etc.)

Handling comparisons and conditional jumps

Constructing if-else and switch statements in assembly

Looping Structures:

Implementing loop structures using jump instructions (JMP, LOOP, etc.)

Constructing while, do-while, and for loops in assembly

Handling loop control variables and loop termination conditions

Example:

assembly

Copy code

.model small

.stack 100h

.data

    count dw 10

    sum dw 0

.code

main proc

    mov ax, @data

    mov ds, ax

    ; Conditional branching: Check if count is zero

    mov ax, count

    cmp ax, 0

    jz zeroCount

    ; Looping structure: Calculate the sum of numbers 1 to count

    mov bx, 1

    mov cx, count

    call calculateSum

    jmp endProgram

zeroCount:

    mov sum, 0

endProgram:

    mov ax, sum

    ; Display the sum

    ; (Displaying code goes here)

    mov ah, 4Ch

    int 21h

main endp

; Procedure to calculate the sum of numbers 1 to count

calculateSum proc

    push cx

    push bx

    push ax

    ; Clear the sum

    xor ax, ax

    mov sum, ax

    sumLoop:

        add sum, bx ; Add bx to the sum

        inc bx ; Increment bx by 1

        loop sumLoop ; Repeat for cx times

    pop ax

    pop bx

    pop cx

    ret

calculateSum endp

end main

In this example, we demonstrate control structures in MASM. We have a count variable that represents the number of iterations for the loop, and a sum variable to store the sum of numbers.

We start with conditional branching. We compare the value of count with zero using the cmp instruction and then use the jz instruction to jump to the zeroCount label if the comparison is true (count is zero).

Inside the zeroCount block, we set the sum variable to zero.

Next, we have a looping structure implemented using the calculateSum procedure. We initialize the bx register to 1 and use the mov instruction to set the value of cx to count. The calculateSum procedure calculates the sum of numbers from 1 to count using a loop structure. We use the add instruction to add the value of bx to the sum, the inc instruction to increment the value of bx by 1, and the loop instruction to repeat the loop until cx becomes zero.

Finally, we display the value of sum (displaying code not shown in the example) and end the program.

By understanding control structures in MASM, readers gain the ability to control the flow of execution in their assembly language programs. They learn how to use conditional branching to make decisions based on conditions, and how to construct different looping structures to repeat a set of instructions. This knowledge allows them to create more complex and dynamic programs that respond to different situations and conditions.