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intdos()                 Invoke DOS Function, Long Form

 #include   <dos.h>

 int          intdos(inregs,outregs);
 union REGS   *inregs;                   Register values going into call
 union REGS   *outregs;                  Register values on return

    intdos() invokes a DOS system call (Interrupt 21h) after setting the
    registers to the values in 'inregs'.  'outregs' is set to the value
    of the registers after the system call.  The 'cflag' field of
    'outregs' is set to the status of the system carry flag; a non-zero
    value indicates a DOS error. 'inregs.h.ah' is the DOS function
    number. (The structure REGS is defined in dos.h.)

    Returns:    The value of the AX register after the system call. If
                the 'cflag' field in 'outregs' is non-zero, an error has
                occurred and '_doserrno' (defined in <stdlib.h>) is set
                to the error code.

      Notes:    intdos() is intended to invoke DOS system calls that
                expect arguments in registers other than DX or AL, or
                that indicate errors by setting the carry flag. Services
                that use only DX and AL and don't use the carry flag, can
                be invoked via the bdos() function.

                If 'far' or 'huge' addresses are passed to DOS, you may
                need to use intdosx() instead of intdos().

  -------------------------------- Example ---------------------------------

    The following statements print statistics about disk usage for drive
    A:.

         #include <dos.h>            /* for intdos() and union REGS */
         #include <stdio.h>          /* for printf() */

         union REGS inregs, outregs;

         main()
         {
               inregs.h.ah = 0x36;  /* get disk free space function number */
               inregs.h.dl = 1;       /* drive A: */
               intdos(&inregs, &outregs);
               printf("%d sectors/cluster, %d clusters,
                       %d bytes/sector, %d total clusters",
                       outregs.x.ax, outregs.x.bx, outregs.x.cx,
                       outregs.x.dx);
         }