Abstract: A method for ensuring data integrity in a computer system having a primary logical device and one or more alternate logical devices. These logical devices have substantially identical data stored in them and have the capability of responding to requests. The system duplicates device access to the alternate logical devices. The logical devices are identified to the computer system as the devices on which duplicating or mirroring operations are to be performed. A read operation or a write operation is performed on the primary logical device. In the case of a write operation, it is also performed simultaneously on the alternate logical devices. The system waits for a response from the primary logical device. If the response indicates that the primary logical device has failed, the alternate logical devices are relied on to complete the operation.

Abstract: A method for extracting and replacing Control Block information in an operating system. An extract replace table is provided to permit application programmers to locate and in certain instances replace items contained in operating system Control Blocks. The user of the application program need not know the precise location of information contained in operating system Control Blocks. The extract/replace table will, upon formulating a request for either extracting or replacing Control Block items, find the requested items and read or replace them. Revisions of operating system programs may be made without regard to the new location of control items. The system user will locate and replace Control Block items by addressing the updated extract/replace tables.