Q59541: How to Determine the Operating System Mode and Version

Article: Q59541
Product(s): See article
Version(s): 4.x 5.00 5.10 6.00 | 5.10 6.00
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | s_quickc s_quickasm | mspl13_c
Last Modified: 18-APR-1990

There are four variables defined by the start-up source code for the
Microsoft C compilers that can be used to determine the version of the
operating system the program is running on as well as whether the
program is running in DOS or OS/2 mode. Those variables are _osmajor,
_osminor, _osversion, and _osmode.

The four variables are prototyped in the DOS.H and STDLIB.H header
files, declared in the CRT0DAT.ASM start-up source file, and defined
in both CRT0DAT.ASM and STDARGV.ASM files. They are defined as
follows:

   Variable     Definition
   --------     ----------

   _osmajor     Holds the major operating system version number.

   _osminor     Holds the minor operating system version number.

   _osversion   Holds the complete version number, as follows:
                the high-order byte contains the minor version
                number and the low-order byte holds the major
                version number.

   _osmode      Defined as DOS_MODE or OS2_MODE for DOS or OS/2
                operating systems. The variable is set to
                DOS_MODE if you are running OS/2, but in the DOS
                compatibility box.

The _osmajor is set to 10 for Versions 1.00, 1.10, and 1.20 of OS/2,
and 20 for Version 2.00.

Note that _osmode is not defined for DOS-only versions of the compiler
(for example, QuickC, C 5.00, etc.).

Sample Program
--------------

The following program prints the version and mode of the operating
system:

#include <stdlib.h>
#include <dos.h>
#include <stdio.h>

void main ( void )
{

   printf ( "_osmajor = %d\n", (int)_osmajor ) ;
   printf ( "_osminor = %d\n", (int)_osminor ) ;
   printf ( "_osversion=%u\n", _osversion ) ;

   if (_osmode == DOS_MODE)
   {
      if ( (int)_osmajor < 10 ) /* We're under DOS */
         printf ("\nDOS version %d.%d\n",
                 (int) _osmajor, (int) _osminor );

      else  /* We're running OS/2 DOS compatibility box */
         printf ("\nOS/2 version %d.%d running"
                 " DOS compatibility \n", ( (int) _osmajor ) / 10,
                 (int) _osminor  ) ;
   }
   else
      printf ("\nOS/2 version %d.%d\n",
                ((int)_osmajor)/10, (int)_osminor );
}

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