Abstract: An in-storage table pair structure is disclosed which extends to the user of the data processing system the internal structures of a component of the operating system. The table pair structure includes pairs of tables that are pointed to by a table pair pointer. A table pair pointer is a double word of virtual storage in which the first word is the address of a user-defined table and the second word is the address of a developer-supplied table. Both tables form a table pair. The table pairs permit functional routines (IETDFs) to reference data in both developer-supplied tables as well as user-defined tables in order to tailor a component of the operating system. The IETDF locates a particular table, or set of tables, by first referencing a global control block which contains addresses of two router control blocks.

Abstract: An apparatus and method employs dual checkpoint data sets for communicating system status. A journal of changed data is implemented to reduce I/O to a subsystem's shared data area on a non-volatile shared storage device. The journal provides for an increase in the amount of time that a processor may have access to the shared data area. Also, two versions of the data area are implemented in order to insure the integrity of the continuously updated data area. The two versions flip-flop depending upon which one has the most recent updates. That is, the version that has the most recent updates becomes the to-be-read-from data area and is read by the processor that currently has access to the shared data area during this series of I/O operations. The other version becomes the to-be-written-to data area and is written to by the processor that currently has access to the data area in order to update the to-be-written-to version to the current level.