Q39520: Passing an Integer from C to MASM, Returning a Double

Article: Q39520
Product(s): Microsoft Macro Assembler
Version(s): 5.1,6.0,6.0a,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 5.1, 6.0, 6.0a, 6.0b 
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SUMMARY
=======

The following MASM program shows how to receive an integer from a C program,
then pass the value back to the C program as a double. Note: This routine was
composed for the large-memory model and assumes that a coprocessor exists on the
system.

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

Below is the C program that calls the MASM routine. It should be compiled for
large-memory model and either 8087 or emulator-math library.

Also shown is the MASM routine. The main piece to this program is the FILD
instruction that transforms the integer to a floating-point real, then pushes
the value on the co-processor stack. The other point is to obtain the segment
and offset of the __fac variable. A value requiring more than 4 bytes cannot be
stored in the DX and AX registers for return, so a double is returned by storing
its address in DX:AX.

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

  /* Compile options needed: /AL /FPi87 (or /FPi)
  */ 

  #include <stdio.h>

  extern double abc( int x );

  void main()
  {
     int x;
     double y;

     x = 7;
     y = abc( x );
     printf( "%The value of y is %lf.\n", y );
  }

  ----------------------------------------------------------------------

  ; Assemble options needed: /Mx

          .MODEL LARGE,C    ; so Masm will use C naming conventions
          .DATA
          .CODE
  EXTRN _fac:QWORD          ; __fac is the global variable used by C
                            ; for storing floating point accumulations
  PUBLIC  abc
  abc     PROC    FAR
          push    bp
          mov     bp,sp
          fild    WORD PTR [bp+6]
          mov     dx,SEG _fac
          mov     es,dx
          fstp    QWORD PTR es:_fac  ; this pops the value off of
          mov     ax,OFFSET _fac     ; the co-processor and puts it
          fwait                      ; in __fac
          mov     sp,bp
          pop     bp
          ret

  abc     ENDP
          END

Program Output:

The value of y is 7.000000.

Additional query words: kbinf 5.10 6.00 6.00a 6.00b

======================================================================
Keywords          :  
Technology        : kbMASMsearch kbAudDeveloper kbMASM510 kbMASM600 kbMASM600a kbMASM600b
Version           : :5.1,6.0,6.0a,6.0b

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