Transport mode data transfer between a channel subsystem and input/output devices

A computer program product is provided for performing an input/output (I/O) operation at a host computer system configured for communication with a control unit. The computer program product is configured to perform: sending a transport mode command message from a channel subsystem to the control unit, the command message including a command for data to be transferred to an I/O device controlled by the control unit; and sending a data transfer message to the control unit, the data transfer message having an amount of the data to be transferred, the amount of the data being less than or equal to a maximum amount of data, the maximum amount of data corresponding to a number of buffers associated with the control unit and a size of each of the number of buffers, the number and the size indicated by a value maintained in the host computer system.