Practice on MASM

Practice on MASM (Microsoft Macro Assembler)

MASM (Microsoft Macro Assembler) is a powerful tool for writing and assembling low-level assembly language code for x86 and x64 architectures. Below are various practice examples that demonstrate common concepts and instructions in MASM.

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1. Simple "Hello World" Program

Let's start with a simple MASM program that outputs "Hello World" to the console.

MASM Code:

; hello.asm
.model small
.stack 100h

.data
    helloMessage db 'Hello, World!', 0

.code
main:
    mov ax, @data       ; Initialize data segment
    mov ds, ax

    ; Print "Hello, World!"
    mov ah, 09h         ; DOS function to print string
    lea dx, helloMessage ; Load the address of the string
    int 21h             ; Call DOS interrupt

    ; Exit the program
    mov ah, 4Ch         ; DOS function to terminate program
    int 21h             ; Call DOS interrupt
end main

Explanation:

1. .model small: Defines a small memory model where both data and code fit within a single segment.
2. .stack 100h: Reserves 256 bytes of stack memory.
3. .data: Declares the data segment, where we define the helloMessage string.
4. .code: Marks the start of the code segment.
5. mov ax, @data and mov ds, ax: Initializes the data segment.
6. mov ah, 09h: Prepares to call DOS interrupt 21h to display a string.
7. lea dx, helloMessage: Loads the address of the string into the DX register.
8. int 21h: Calls DOS interrupt 21h to print the string.
9. mov ah, 4Ch and int 21h: Terminates the program.

To assemble and run:

1. Write the program in a text file named hello.asm.
2. Use MASM to assemble: ml hello.asm
3. Link the program: link hello.obj
4. Run the resulting executable.

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2. Simple Arithmetic Operations

This example performs basic arithmetic operations like addition, subtraction, multiplication, and division.

MASM Code:

; arithmetic.asm
.model small
.stack 100h

.data
    num1 dw 10         ; Define first number
    num2 dw 5          ; Define second number
    result dw 0        ; Variable to store result

.code
main:
    mov ax, @data      ; Initialize data segment
    mov ds, ax

    ; Load numbers
    mov ax, num1       ; AX = num1 (10)
    mov bx, num2       ; BX = num2 (5)

    ; Addition
    add ax, bx         ; AX = AX + BX (10 + 5)
    mov result, ax     ; Store result (15)

    ; Subtraction
    mov ax, num1       ; Load num1 again
    sub ax, bx         ; AX = AX - BX (10 - 5)
    mov result, ax     ; Store result (5)

    ; Multiplication
    mov ax, num1       ; Load num1 again
    mov bx, num2       ; Load num2 again
    mul bx             ; AX = AX * BX (10 * 5)
    mov result, ax     ; Store result (50)

    ; Division
    mov ax, num1       ; Load num1 again
    mov bx, num2       ; Load num2 again
    div bx             ; AX = AX / BX (10 / 5), result in AX, remainder in DX
    mov result, ax     ; Store result (2)

    ; Exit the program
    mov ah, 4Ch        ; DOS function to terminate program
    int 21h            ; Call DOS interrupt
end main

Explanation:

1. mov ax, num1 and mov bx, num2: Loads num1 and num2 into the registers AX and BX.
2. add ax, bx: Adds the contents of BX to AX (10 + 5).
3. sub ax, bx: Subtracts BX from AX (10 - 5).
4. mul bx: Multiplies AX by BX (10 * 5), storing the result in AX.
5. div bx: Divides AX by BX (10 / 5). The quotient is in AX, and the remainder is in DX.
6. The result of each operation is stored in the result variable.

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3. Using Loops in MASM

This example demonstrates how to use loops in MASM. The loop will count from 1 to 10 and print the number each time.

MASM Code:

; loop.asm
.model small
.stack 100h

.data
    msg db 'The number is: $'
    counter db 1          ; Counter starts at 1

.code
main:
    mov ax, @data         ; Initialize data segment
    mov ds, ax

start_loop:
    ; Print message
    mov ah, 09h           ; DOS function to print string
    lea dx, msg           ; Load the address of the message
    int 21h               ; Call DOS interrupt

    ; Print counter
    mov al, counter       ; Load the current counter value
    call PrintNum         ; Call the PrintNum procedure

    ; Increment counter
    inc counter           ; Increment counter by 1
    cmp counter, 11       ; Check if counter is 11
    jl start_loop         ; Jump to start_loop if counter < 11

    ; Exit program
    mov ah, 4Ch           ; DOS function to terminate program
    int 21h               ; Call DOS interrupt

; PrintNum procedure
PrintNum:
    ; Print the number in AL (assuming AL holds a number < 10)
    add al, '0'           ; Convert number to ASCII
    mov dl, al            ; Move number to DL register
    mov ah, 02h           ; DOS function to print a character
    int 21h               ; Call DOS interrupt
    ret                   ; Return from procedure
end main

Explanation:

1. counter db 1: Initializes a counter variable starting at 1.
2. lea dx, msg: Loads the address of the string into the DX register.
3. int 21h: Uses DOS interrupt 21h to print the string.
4. The loop is controlled using the cmp instruction (cmp counter, 11), checking if counter is less than 11. If true, it loops again.
5. The PrintNum procedure is used to print a single number by converting it into ASCII.

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4. Conditional Jumps (If-Else Simulation)

This example simulates an "if-else" block using conditional jumps.

MASM Code:

; if_else.asm
.model small
.stack 100h

.data
    msg_true db 'The condition is true.$'
    msg_false db 'The condition is false.$'
    condition db 1        ; Set the condition to 1 (True)

.code
main:
    mov ax, @data         ; Initialize data segment
    mov ds, ax

    mov al, condition     ; Load the condition value into AL
    cmp al, 1             ; Compare condition with 1
    je true_condition     ; Jump if condition is true (AL == 1)

    ; Else part
    mov ah, 09h           ; DOS function to print string
    lea dx, msg_false     ; Load the "false" message
    int 21h
    jmp end_program       ; Jump to end

true_condition:
    ; If part
    mov ah, 09h           ; DOS function to print string
    lea dx, msg_true      ; Load the "true" message
    int 21h

end_program:
    ; Exit the program
    mov ah, 4Ch           ; DOS function to terminate program
    int 21h               ; Call DOS interrupt
end main

Explanation:

1. condition db 1: This is a variable that controls the conditional flow. You can change it to 0 for the "false" branch.
2. cmp al, 1: Compares the value of AL (which contains the condition) with 1.
3. je true_condition: If AL == 1, jump to the true_condition label to print the true message.
4. If the condition is false (AL != 1), the program prints the false message.

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5. Function Call Example

In this example, we'll define a function that adds two numbers and returns the result.

MASM Code:

; function_call.asm
.model small
.stack 100h

.data
    num1 dw 5         ; First number
    num2 dw 3         ; Second number
    result dw 0       ; Store result

.code
main:
    mov ax, @data     ; Initialize data segment
    mov ds, ax

    ; Call AddNumbers function
    mov ax, num1      ; Load num1 into AX
    mov bx, num2      ;