Abstract: In an improved method for accessing files in a computer system, file open requests are intercepted at the level of the operating system. The identity of the file to be opened is extracted from the request and a determination is made as to whether the identity of the requested file should be replaced with the identity of a substitute file to be opened instead of the requested file. If it is determined that the identity should be so replaced, the file open request is amended by substituting the identity of the substitute file for the identity of the requested file, and the request is passed on to its original destination for execution. This technique can be used to cause execution of a substitute program without the knowledge of the user.

Abstract: A backup program for a computer system with a hard disk maintains a record of changes made to the data in a file being backed up. During backup both the original and update file portions or records are stored, and the backup program reads the original data while other applications read the updated data. To identify whether a read operation is initiated by the backup program or by another application, the system determines, using the stack, the source of the read request. Alternatively it interrogates the user, process or task id. Access to the file by the backup program is guaranteed by manipulating the System File Table. It is ensured that the file is in a `good` state by delaying the start of the backup operation for a predetermined period.