INT 21 - DOS 5.0 - GET TRUE VERSION NUMBER
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 in in HMA
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
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
SeeAlso: INT 2F/AX=112Fh

See also: 2130

213306 - DOS 5.0 - GET TRUE VERSION NUMBER