DOS Fn 30H: Get DOS Version Number

                                                         Compatibility: 2.0+ 
 Expects: AH    30H
 Returns: AL    major version number
          AH    minor version number (note: binary value -- x.20 is 14H)
          BX,CX (OEM info, see below) DOS 2.0+
    Info: Returns AX with the value of the current DOS version.  For
          instance, if DOS 3.20 is active, AL returns 3 and AH returns 14H
          (20 decimal).

          Use this function early in your program to find out what
          capabilities are present.

Versions: ■ 5.0+
            Setver intercepts this call and may return a cooked value if
            the caller is on its hit list.  Use Fn 3306H to obtain the
            actual DOS version number.
          ■ 1.x
            Since this function does not exist with DOS 1.x, you should set
            AL to 0 before calling it.  If AL then returns with 0, then you
            can assume that you are running DOS 1.x.
          ■ 2.0+
            Values returned in BX and CX may vary from OEM to OEM.  BH may
            return holding an OEM ID number and BL:CX may be set to a
            24-bit value used by the OEM to identify sub-versions.

See Also: Fn 3306H (get DOS version info)
          DOS Function Index - by DOS Version
          DOS Versions (overview and history)
          System Information Functions
          DOS Functions