Header File 

dos.h 

Syntax 

extern unsigned char _osmajor; 

Description 

The major version number of the operating system is available individually through _osmajor. For example, if you are running DOS version 3.2, _osmajor will be 3. 

This variable can be useful when you want to write modules that will run on DOS versions 2.x and 3.x. Some library routines behave differently depending on the DOS version number, while others only work under DOS 3.x and higher. For example, refer to creatnew and _rtl_open.