INT 2F U - DOS 3.3+ internal - IOCTL
        AX = 122Bh
        BP = 44xxh
        SS = DOS DS (must be using a DOS internal stack)
        additional registers as appropriate for INT 21/AX=44xxh
Return: as for INT 21/AH=44h
Notes:  equivalent to INT 21/AH=44h, but may only be called when already inside
          a DOS function call
        sets user stack frame pointer to dummy buffer, moves BP to AX, performs
          IOCTL, and restores frame pointer