INT 21 - DOS 2+ - GET DOS VERSION
        AH = 30h
---DOS 5+ ---
        AL = what to return in BH
            00h OEM number (as for DOS 2.0-4.0x)
            01h version flag
Return: AL = major version number (00h if DOS 1.x)
        AH = minor version number
        BL:CX = 24-bit user serial number (most versions do not use this)
---if DOS <5 or AL=00h---
        BH = MS-DOS OEM number (see below)
---if DOS 5+ and AL=01h---
        BH = version flag
            bit 3: DOS is in ROM
            other: reserved (0)
Notes:  the OS/2 v1.x Compatibility Box returns major version 0Ah (10)
        the OS/2 v2.x Compatibility Box returns major version 14h (20)
        the Windows/NT DOS box returns version 5.00, subject to SETVER
        DOS 4.01 and 4.02 identify themselves as version 4.00; use
          INT 21/AH=87h to distinguish between the original European MS-DOS 4.0
          and the later PC-DOS 4.0x and MS-DOS 4.0x
        IBM DOS 6.1 reports its version as 6.00; use the OEM number to
          distinguish between MS-DOS 6.00 and IBM DOS 6.1 (there was never an
          IBM DOS 6.0)
        generic MS-DOS 3.30, Compaq MS-DOS 3.31, and others identify themselves
          as PC-DOS by returning OEM number 00h
        the version returned under DOS 4.0x may be modified by entries in
          the special program list (see AH=52h); the version returned under
          DOS 5+ may be modified by SETVER--use AX=3306h to get the true
          version number
SeeAlso: AX=3000h/BX=3000h,AX=3306h,AX=4452h,AH=87h,INT 15/AX=4900h
SeeAlso: INT 2F/AX=122Fh,INT 2F/AX=E002h

Values for DOS OEM number:
 00h    IBM
 01h    Compaq
 02h    MS Packaged Product
 04h    AT&T
 05h    Zenith
 06h    Hewlett-Packard
 0Dh    Packard-Bell
 16h    DEC
 23h    Olivetti
 29h    Toshiba
 33h    Novell (Windows/386 device IDs only)
 34h    MS Multimedia Systems (Windows/386 device IDs only)
 35h    MS Multimedia Systems (Windows/386 device IDs only)
 4Dh    Hewlett-Packard
 66h    PhysTechSoft (PTS-DOS)
 99h    General Software's Embedded DOS
 EEh    DR-DOS
 EFh    Novell DOS
 FFh    Microsoft, Phoenix

.