Returns the version number of the host MS-DOS operating system.
This function is used by application programs to determine the
capabilities of their environment.
Input Output
AH = 30h If running under MS-DOS version 1.0
AL = 00h
If running under MS-DOS version 2.0 and later
AL = Major version number (MS-DOS 3.1 = 3,...)
AH = Minor version number (MS-DOS 3.1 = 0Ah,...)
BH = Original Equipment Manufacturer's (OEM's)
serial number (OEM dependent--usually 00h for
IBM's PC-DOS, 0FFh or other values for MS-DOS)
BL:CX = 24-bit user serial number (optional, OEM
dependent)