Modularly structured integrated services digital network (ISDN) communication system

A modularly-structured integrated services digital network communication system comprising at least one switching unit including a switching network to which apparatus and networks can be connected via peripheral modules, and of the type comprising a memory-programmed system processor, that comprises a real-time operating system for controlling subsystems provided for execution of communication technology-associated functions and for control of an idle task subsystem that is fashioned as a branch subsystem for a transition to a time sharing operating system, a system memory that comprises memory segments addressable via descriptors, a system table memory that contains system table descriptors referring to memory segments of the real-time operating system, a further system table memory that contains real-time interrupt descriptors for events pertaining to the real-time operating system, a module table memory that can be selected alternatively to the system table memory and that contains module table descriptors referring to memory segments of the time sharing operating system, and further contains communication descriptors referring to intercommunication segments of the real-time operating system that can be interpreted identically with the system table descriptors contained in the system table memory and referring to the intercommunication segments and among which, at least one code segment descriptor for access onto the code segment of the branch subsystem, and one status segment descriptor for access onto a status segment assigned to the branch subsystem in the real-time operating system, and one data segment descriptor for access onto an intercommunication data segment storing input information required by each of the two operating systems, when a transition to the other respective operating system is given, a further module table that can be selected alternatively to the further system table memory and that contains interrupt descriptors for events pertaining to the time sharing operating system, control elements in the branch subsystem that, when activation of the branch subsystem is given, trigger an allocation change of the system processor from the system table memories to the module table memories and, calling of a status segment descriptor, subsequently effects a process change into the interrupt transition routine that is subsequently ended, and control statements in the interrupt transition routine that, when activation of the interrupt transition routine is given, initiate a process change into the branch subsystem upon calling the communication status segment descriptor, and further control statements in the branch subsystem that effect an allocation change of the system processor from the module table memories to the system table memories, the improvement wherein:

• a subsystem having comparatively high priority is constructed in the real-time operating system as a further branch subsystem;
• said further branch subsystem when an arrival of a first periodic event is given, assumes a ready status in order to be assigned to the system processor in the real-time operating system for execution;
• said further module table memory contains a further interrupt descriptor for at least a second periodic event following said first periodic event, said further interrupt descriptor contains address information for execution of the interrupt transition routine;
• said further module table memory contains interrupt descriptors for events pertaining to the real-time operating system, said interrupt descriptors each containing address information for execution of an interrupt rescue routine in the time sharing operating system;
• and
• following the allocation change of the system processor from the module table memories to the system table memories, said further branch subsystem assumes a dormant condition which is provided as a non-ready task status in the real-time operating system.