INT 21 - DOS 2+ - GET DOS VERSION
        AH = 30h
---DOS 5.0---
        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 = OEM number
            00h IBM
            05h Zenith
            16h DEC
            23h Olivetti
            29h Toshiba
            4Dh Hewlett-Packard
            99h STARLITE architecture (OEM DOS, NETWORK DOS, SMP DOS)
            FFh Microsoft, Phoenix
---if DOS 5.0 and AL=01h---
        BH = version flag
            08h DOS is in ROM
            10h DOS is in HMA
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 major version 1Eh (30)
        DOS 4.01 and 4.02 identify themselves as version 4.00; use
          INT 21/AH=87h to distinguish between the original European MSDOS 4.00
          and the later PCDOS 4.0x and MSDOS 4.0x
        generic MSDOS 3.30, Compaq MSDOS 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 5.0 may be modified by SETVER; use
          AX=3306h to get the true version number
SeeAlso: AX=3306h,AX=4452h,AH=87h,INT 15/AX=4900h