Procedures

Using Procedures in MASM (Microsoft Macro Assembler)

In MASM, a procedure (or function) is a block of code that performs a specific task and can be called from different parts of the program. Procedures help to modularize code, making it easier to read, maintain, and reuse.

Below, I'll cover how to define and use procedures in MASM, including examples of passing parameters, returning values, and calling procedures from the main code.

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1. Basic Procedure Example (No Parameters)

In this example, we'll define a simple procedure that prints a string message.

MASM Code:

; basic_procedure.asm
.model small
.stack 100h

.data
    message db 'Hello from the procedure!', 0

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

    ; Call procedure to print message
    call PrintMessage

    ; Exit program
    mov ah, 4Ch
    int 21h

; Procedure to print a message
PrintMessage proc
    mov ah, 09h         ; DOS function to print string
    lea dx, message     ; Load address of message into DX
    int 21h             ; DOS interrupt to print the string
    ret                 ; Return from procedure
PrintMessage endp
end main

Explanation:

• PrintMessage proc: Declares the start of the procedure PrintMessage.
• mov ah, 09h and int 21h: These are DOS interrupt calls to print a string.
• ret: This instruction returns control to the caller (main program) when the procedure is finished.

To assemble and run:

1. Write the program in a text file named basic_procedure.asm.
2. Assemble using MASM: ml basic_procedure.asm
3. Link the program: link basic_procedure.obj
4. Run the resulting executable.

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2. Procedure with Parameters

In this example, we will pass parameters to the procedure. We'll pass two numbers to a procedure that will add them together and print the result.

MASM Code:

; procedure_with_params.asm
.model small
.stack 100h

.data
    num1 dw 5           ; First number
    num2 dw 3           ; Second number
    result db 'Result: $'

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

    ; Call AddNumbers procedure with parameters
    mov ax, num1        ; Load num1 into AX
    mov bx, num2        ; Load num2 into BX
    call AddNumbers     ; Call procedure

    ; Exit program
    mov ah, 4Ch
    int 21h

; Procedure to add two numbers and display result
AddNumbers proc
    add ax, bx          ; AX = AX + BX (num1 + num2)
    ; Print the result message
    mov ah, 09h         ; DOS function to print string
    lea dx, result      ; Load address of result message
    int 21h             ; Call DOS interrupt to print message

    ; Convert result to ASCII and print it
    add al, '0'         ; Convert to ASCII (assuming result is between 0-9)
    mov dl, al          ; Load result into DL for printing
    mov ah, 02h         ; DOS function to print character
    int 21h             ; Call DOS interrupt to print character

    ret                 ; Return to main program
AddNumbers endp
end main

Explanation:

1. Main program: We load the values of num1 and num2 into AX and BX, then call the AddNumbers procedure.
2. AddNumbers procedure:
   • add ax, bx: Adds the values of AX (num1) and BX (num2).
   • The result is stored in AX and is printed by converting the result into an ASCII character.
   • The result is printed using DOS interrupt 21h (int 21h) to display the message and the sum.
3. The procedure does not return any complex data types but simply modifies the contents of the AX register.

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3. Procedure with Return Values

In this example, we'll create a procedure that calculates the sum of two numbers and returns the result in a register (specifically AX).

MASM Code:

; procedure_with_return_value.asm
.model small
.stack 100h

.data
    num1 dw 10          ; First number
    num2 dw 15          ; Second number
    resultMessage db 'The sum is: $'

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

    ; Call AddNumbers procedure with parameters
    mov ax, num1        ; Load num1 into AX
    mov bx, num2        ; Load num2 into BX
    call AddNumbers     ; Call procedure

    ; Print result message
    mov ah, 09h         ; DOS function to print string
    lea dx, resultMessage ; Load address of result message
    int 21h             ; Call DOS interrupt to print message

    ; Print the result (in AX)
    add ax, '0'         ; Convert result to ASCII
    mov dl, al          ; Load result into DL
    mov ah, 02h         ; DOS function to print character
    int 21h             ; Call DOS interrupt to print result

    ; Exit program
    mov ah, 4Ch
    int 21h

; Procedure to add two numbers and return the result in AX
AddNumbers proc
    add ax, bx          ; AX = AX + BX
    ret                 ; Return to caller with result in AX
AddNumbers endp
end main

Explanation:

1. Main program:
   • Loads the values of num1 and num2 into AX and BX.
   • Calls the AddNumbers procedure.
2. AddNumbers procedure:
   • add ax, bx: Adds the values in AX and BX and stores the result in AX.
   • The result is returned in AX, which is then printed in the main program.
3. We use the add ax, '0' instruction to convert the result (a number) into an ASCII character before printing it.

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4. Procedure with Multiple Parameters

This example demonstrates how to pass multiple parameters to a procedure and handle more complex calculations.

MASM Code:

; multiple_params.asm
.model small
.stack 100h

.data
    num1 dw 10          ; First number
    num2 dw 5           ; Second number
    num3 dw 2           ; Third number
    resultMessage db 'The sum of the numbers is: $'

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

    ; Call AddThreeNumbers procedure with multiple parameters
    mov ax, num1        ; Load num1 into AX
    mov bx, num2        ; Load num2 into BX
    mov cx, num3        ; Load num3 into CX
    call AddThreeNumbers ; Call procedure

    ; Print result message
    mov ah, 09h         ; DOS function to print string
    lea dx, resultMessage ; Load address of result message
    int 21h             ; Call DOS interrupt to print message

    ; Print the result (in AX)
    add ax, '0'         ; Convert result to ASCII
    mov dl, al          ; Load result into DL
    mov ah, 02h         ; DOS function to print character
    int 21h             ; Call DOS interrupt to print result

    ; Exit program
    mov ah, 4Ch
    int 21h

; Procedure to add three numbers and return the result in AX
AddThreeNumbers proc
    add ax, bx          ; AX = AX + BX
    add ax, cx          ; AX = AX + CX
    ret                 ; Return with result in AX
AddThreeNumbers endp
end main

Explanation:

1. Main program:
   • Loads three numbers (num1, num2, and num3) into AX, BX, and CX respectively.
   • Calls the AddThreeNumbers procedure.
2. AddThreeNumbers procedure:
   • Adds the values of AX, BX, and CX together.
   • The result is returned in AX, which is then printed in the main program.

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5. Using the Stack for Local Variables (Push and Pop)

In MASM, you can use the stack to allocate local variables in procedures. This allows each procedure to have its own private workspace, independent of other parts of the program.

MASM Code:

; stack_local_variables.asm
.model small
.stack 100h

.data
    resultMessage db 'The result of the operation is: $'

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

    ; Call Calculate procedure
    call Calculate

    ; Exit program
    mov ah, 4Ch
    int 21h

; Procedure to perform calculations with local variables
Calculate proc
    push ax              ; Save AX to stack (local variable)