INT 2F U - DOS 3+ internal - SET DRIVE???
        AX = 1219h
        SS = DOS DS (must be using a DOS internal stack)
        STACK: WORD drive (0 = default, 1 = A:, etc)
Return: ???
        STACK unchanged
Notes:  calls AX=1217h
        builds a current directory structure if inside server call
          (INT 21/AX=5D00h)
SeeAlso: AX=121Fh