Microsoft KB Archive/100290

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Detecting Windows NT from an MS-DOS-Based Application


The information in this article applies to:

  • Microsoft Win32 Software Development Kit (SDK), versions 3.1, 3.5, 3.51, 4.0


There are calls that MS-DOS-based applications can make that are not supported under Windows NT. For example, calling Interrupt 25h to read the disk is not supported under Windows NT. Therefore, in some cases MS-DOS- based applications will need to know whether or not they are running under Windows NT.

Interrupt 21h, function 3306h can be used by MS-DOS-based applications to detect whether or not they are running under Windows NT. On return, registers BL and BH will contain the operating system major and minor numbers, respectively. If your application is running under Windows NT, the return will be:

   BL = 5
   BH = 50 


Note that it is important to check both BL and BH, because MS-DOS 5.0 will also return a 5 in BL.

The following code demonstrates how to detect the operating system version from an MS-DOS-based application:

Sample Code

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

void main()
   unsigned char cbh = 0;
   unsigned char cbl = 0;

   _asm {
      mov ax, 3306h
      int 21h
      mov cbh, bh
      mov cbl, bl
   printf( "After int 21h\n" );
   printf( "%u, %u (bh, bl)\n", cbh, cbl );

Additional query words: 3.10 3.50 4.0 detect NT MS-DOS

Keywords :
Issue type :
Technology : kbWin32SDKSearch kbAudDeveloper kbSDKSearch kbWin32sSearch kbWin32SDK400 kbWin32SDK310 kbWin32SDK350 kbWin32SDK351

Last Reviewed: January 16, 2000
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