Int 21 Fn 3306  - DOS 5+ - Get True Version Number                         [D]

   AX = 3306h

Return: BL = major version
   BH = minor version
   DL = revision (bits 2-0, all others 0)
   DH = version flags
       bit 3: DOS is in ROM
       bit 4: DOS is in HMA
   AL = FFh if true DOS version < 5.0

Notes: this function always returns the true version number, unlike AH=30h,
     whose return value may be changed with SETVER
   because of the conflict from the CBIS PowerLAN redirector (see next
     entry), programs should check whether BH is less than 100 (64h)
     and BL is at least 5 before accepting the returned BX as the true
     version number; however, even this is not entirely reliable when
     that redirector is loaded
   fully reentrant
   OS/2 v2.1 will return BX=0A14h (version 20.10)
   the Windows NT DOS box returns BX=3205h (version 5.50)
BUG:   DR-DOS 5.0 and 6.0 return CF set/AX=0001h for INT 21/AH=33h
     subfunctions other than 00h-02h and 05h, while MS-DOS returns AL=FFh
     for invalid subfunctions

See Also: AH=30h,INT 2F/AX=122Fh