Abstract: Internal states of storage devices within a modular digital data processing system are made accessible using variable mode multi-bit shift register storage devices in each module. The inputs and outputs of the shift register storage devices in each module are interconnected by gating means within the module which may be selectably controlled to form separately selectable serial strings along which the bits of a selected string of a selected module may be caused to be shifted to a monitoring unit for monitoring, diagnosing and/or correcting purposes and then returned, via the string, to their original locations in the shift register storage devices of the selected module, after which normal data processing operations can be resumed, or other appropriate action taken. At least one string in each module is formed of monitoring storing devices which are not involved in data processing operations so as to permit the module to be monitored simultaneously with the performance of data processing operations.

Abstract: Internal states of storage devices within a digital data processing system are made accessible using variable mode multi-bit shift register storage devices. The inputs and outputs of these shift register storage devices are interconnected by gating means which may be selectably controlled to form separately selectable serial strings along which the bits of a selected string may be caused to be shifted to a monitoring unit for monitoring, diagnosing and/or correcting purposes and then returned, via the string, to their original locations in the shift register storage devices, after which normal data processing operations can be resumed, or other appropriate action taken.

Abstract: The internal states of a memory and other storage devices within a digital data processing system are made accessible for monitoring purposes using variable mode multi-bit shift register storage devices which are also used for the input-output register of the memory to be monitored. The inputs and outputs of the shift register storage devices are interconnected by gating means which may be selectably controlled to form separately selectable serial strings along which the bits of a selected string may be caused to be shifted to a monitoring unit for monitoring, diagnosing and/or correcting purposes and then returned, via the string, to their original locations in the respective shift register storage devices of the selected string, after which normal data processing operations can be resumed.