Chapter 7: Procedures and Parameter Passing in MASM

Introduction:

Importance of procedures and parameter passing in assembly language programming

Overview of procedures and the stack frame in MASM

Procedures:

Defining and calling procedures in MASM

Understanding the structure of a procedure (prologue, body, epilogue)

Managing local variables and parameters within procedures

Parameter Passing:

Passing parameters to procedures using registers and the stack

Understanding the calling convention and parameter order

Handling different types of parameters (integer, floating-point, structures)

Example:

assembly

Copy code

.model small

.stack 100h

.data

    message db 'Hello, World!', 0

.code

main proc

    mov ax, @data

    mov ds, ax

    ; Call the displayMessage procedure

    lea dx, message

    call displayMessage

    mov ah, 4Ch

    int 21h

main endp

; Procedure to display a message

displayMessage proc

    push bp

    mov bp, sp

    ; Parameters: dx - pointer to the message

    mov dx, [bp + 4]

    ; Display the message

    mov ah, 9

    int 21h

    pop bp

    ret

displayMessage endp

end main

In this example, we demonstrate procedures and parameter passing in MASM. We have a message string, message, that we want to display using a procedure called displayMessage.

In the main procedure, we load the address of the message string into the dx register and then call the displayMessage procedure using the call instruction.

Inside the displayMessage procedure, we use the push and pop instructions to set up and restore the stack frame. The mov dx, [bp + 4] instruction retrieves the parameter (pointer to the message) from the stack frame.

We then use the mov ah, 9 instruction to set up the appropriate DOS service to display the message, and the int 21h instruction to invoke the DOS interrupt.

Finally, we restore the stack frame and return from the procedure using the pop bp and ret instructions.

By understanding procedures and parameter passing in MASM, readers can modularize their code and reuse common functionality. They learn how to define and call procedures, manage local variables and parameters, and understand the stack frame. This knowledge enables them to write more structured and organized assembly language programs, improving code maintainability and reusability.