Q34377: How to Implement a Function Pointer in MASM

Article: Q34377
Product(s): Microsoft Macro Assembler
Version(s): 5.0,5.1,5.1a,6.0,6.0a,6.0b
Operating System(s): 
Keyword(s): 
Last Modified: 06-MAY-2001

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The information in this article applies to:

- Microsoft Macro Assembler (MASM), versions 5.0, 5.1, 5.1a, 6.0, 6.0a, 6.0b 
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SUMMARY
=======

The following sample program illustrates how to implement a function pointer
using the Microsoft Macro Assembler. It takes the address of the function (in
this example, the address is loaded into ES:DX) and moves it into a 4-byte
variable; it then does a far call through the pointer.

In this example, it may seem odd to call a function in this manner because the
function is defined locally and its name is known; however, what if the name of
the function was not known? What if you were programming a device driver and all
that was know was the entry point of the function? Using the following
technique, a name could be given to the address of the function and the function
could be called like any other function.

MORE INFORMATION
================

This program illustrates how to implement a function pointer in MASM. This code
is similar to the code that the C compiler would generate for a C program that
used a pointer to a function.

Sample Code
-----------

  ; Assemble options needed: none

     .model small

     .data
  fptr dd 1 dup(?)   ; set aside four bytes for function address

     .code
  ;
  ;  MACRO Definitions
  ;
  FARCALL MACRO func_ptr
     CALL dword ptr func_ptr  ; call by 4-byte far function reference
     ENDM

  DosExit MACRO
     MOV  ax, 4C00h    ; ah = 4Ch ( dos exit interrupt) al = 0
     INT  21h
     ENDM
  ;
  ;  FUNCTION Definition: function uses int 10h, function 07h to
  ;                       initialize a window
  ;
     PUBLIC _ClrScr

  _ClrScr PROC FAR

     PUSH bp          ; save bp
     MOV  bp, sp      ; get sp
     PUSH bx          ; save registers
     PUSH cx
     PUSH dx

     MOV ax, 0700h    ; ah = 7,  al = 0
     MOV bx, 0700h    ; bh = 7,  bl = 0
     XOR cx, cx       ; cx = 0
     MOV dx, 184Fh    ; dh = 24, dl = 79, decimal
     INT 10h

     MOV ax, 0200h    ; ah = 2, al = 0
     XOR bx, bx       ; bx = 0
     XOR dx, dx       ; dx = 0
     INT 10h

     XOR  ax, ax      ; function returns void
     POP  dx          ; restore registers
     POP  cx
     POP  bx

     MOV  sp, bp      ; reset sp
     POP  bp          ; restore bp
     RET              ; return
   _ClrScr ENDP

  BEGIN:              ; main part of the program
  ;
  ;  Get address of the function, put in es:dx
  ;
     mov  dx, SEG _ClrScr
     mov  es, dx
     mov  dx, OFFSET _ClrScr
  ;
  ;  Load function address into fptr
  ;
     mov  WORD PTR fptr, dx    ; low word of fptr is the
                               ; offset of the function
     mov  WORD PTR fptr+2, es  ; high word of fptr is the
                               ; segment of the function
  ;
  ;  Call the function via a function pointer
  ;
     FARCALL fptr              ; call function. FARCALL is a macro
                               ; defined above
  ;
  ;  Exit to DOS
  ;
     DosExit                   ; Exit to DOS. DosExit is a macro
                               ; defined above

     END BEGIN

Additional query words: kbinf 5.00 5.10 5.10a 6.00 6.00a 6.00b

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Keywords          :  
Technology        : kbMASMsearch kbAudDeveloper kbMASM510 kbMASM600 kbMASM500 kbMASM600a kbMASM510a kbMASM600b
Version           : :5.0,5.1,5.1a,6.0,6.0a,6.0b

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