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 having both input and output data, wherein a location of the output data and a location for storing the input data is specified by a single TCW; forwarding the I/O operation to the control unit for execution; gathering the output data responsive to the location of the output data specified by the TCW; forwarding the output data to the control unit for use in the execution of the I/O operation; receiving the input data from the control unit; and storing the input data at the location specified by the TCW for storing the input data.