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 and a flag; extracting the specified location and the flag from the TCW; obtaining a message from the specified location responsive to the flag having a first value, 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 to form the message responsive to the flag having a second value, the message comprising the gathered one or more I/O commands; and forwarding the message to the control unit for execution.