1. An I/O system for controlling transfer of data between a memory of a host processor and a plurality of different types of device controllers, said I/O system comprising: a plurality of first programmed means, within an other processor, for determining if a plurality of respective device controller types support specific I/O commands; second programmed means, within said other processor and responsive to an I/O command to perform an I/O operation on a specific device controller, for identifying and invoking execution of a program within the corresponding first programmed means associated with a type of said specific device controller and, if said specific device controller supports the command, invoking execution of a program within said specific device controller; third programmed means, within said other processor, for receiving I/O operation state information from said specific device controller and signalling the state information to said second programmed means; and multitasking operating system means for controlling execution of the program within said corresponding first programmed means and a program within said second programmed means on said other processor in a common task, and a program within said third programmed means on said other processor in a different task.