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: the host computer system obtaining a transport command word (TCW) for an I/O operation having both input and output data, the TCW specifying a location of the output data, and a location for storing the input data; the host computer system forwarding the I/O operation to the control unit for execution; the host computer system gathering the output data responsive to the location of the output data specified by the TCW; the host computer system forwarding the output data to the control unit for use in the execution of the I/O operation; the host computer system receiving the input data from the control unit; and the host computer system storing the input data at the location specified by the TCW for storing the input data.