A computer program product is provided for performing: sending, by a channel subsystem, a process login (PRLI) request message to the control unit that indicates whether the channel subsystem supports bi-directional data transfer; receiving a PRLI response message from the control unit that indicates whether the control unit supports bi-directional data transfer; gathering a plurality of commands, at least one which specifies an input data transfer and at least one specifying an output data transfer; sending at least one output data message to the control unit including output data to be transferred to the control unit, the output data message associated with the at least one of the plurality of commands specifying an output data transfer; and receiving at least one input message from the control unit including input data to be stored in a main storage of the host computer system.