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.
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.
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 Versions (overview and history)
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DOS Fn 30H: Get DOS Version Number