INT 62 - Cswitch - SEND MESSAGE
        AH = 0Bh
        CX = number of bytes to write
        DS:SI -> buffer
        DX = queue number (0-63)
Return: AX = FFFEh triggered by something arriving, redo the call
           = FFFFh bad queue number
           = 0000h no message was on queue
             else  number of bytes in message
SeeAlso: AH=0Ch