Abstract: Apparatus for the flexible coupling of peripheral units to a digital computer system, wherein any input/output port of the system is capable of communicating with a peripheral unit over various forms of interface.

Abstract: In a data processing system having a plurality of input/output channels, a first plurality of physical resources is dedicated to each of the channels. Said first plurality is sufficient only to enable standard data character exchanges between each of the channels and the central processing unit in a time-sharing manner. A second plurality of physical resources (including registers and logic) is provided which is dedicated to a corresponding plurality of service classes. The service classes are ordered according to a priority criterion and selectively assigned to different channels in accordance with the priority level of the interrupt which has requested the service. In this manner, while services of the same class are not reciprocally interruptible, services of different classes on different channels are hierarchically interruptible.

Abstract: Apparatus for providing notification of and distinguishing among various kinds of errors in information transferred between the central processor and the peripheral units of a digital computer system by isolating the elements of such system in which certain kinds of errors can occur.