INT 7F - Alloy NTNX, MW386 - GET CHANNEL STATUS
        AH = 15h
        CL = 00h
        AL = channel number
        DX:DI -> status structure (see below)
Return: AL = status
            00h successful
            01h busy
            02h channel range error (not 00h-3Fh)
            03h invalid subfunction

Format of status structure:
Offset  Size    Description
 00h    BYTE    channel status
                bit 0: channel open
                    1: channel buffer contains received data
                    7: channel locked
 01h    BYTE    sender ID