1. A computer program product for facilitating input/output (I/O) processing for an I/O operation at a host computer system configured for communication with a control unit, the computer program product comprising: a tangible storage medium readable by a processing circuit and storing instructions for execution by the processing circuit for performing a method comprising: obtaining a transport command word (TCW) for an I/O operation, the TCW specifying a location address and a flag, the flag indicating whether the TCW directly or indirectly addresses a message for transmitting one or more commands to the control unit; extracting the specified location address and the flag from the TCW; obtaining the message from the specified location address based on the flag having a first value indicating direct addressing, the message comprising one or more I/O commands; gathering one or more I/O commands from command locations specified by a list of addresses identified by the specified location address to form the message based on the flag having a second value indicating indirect addressing, the message comprising the gathered one or more I/O commands; and forwarding the message to the control unit for execution.