Abstract: Apparatus and a method by which the flow of commands to an input/output device may be halted when the device is unable to respond to a command decoded to its address space. The apparatus includes circuitry for ascertaining whether the input/output device is able to respond to a command decoded by the decoding circuit, a circuit for storing the data and address of a command transferred to the input/output device to which the input/output device is unable to respond, and circuitry for generating a signal to disable immediately the flow of commands to the input/output device and an interrupt to assure that the unimplemented command is handled in an expeditious manner.

Abstract: A method and apparatus for implementing a class hierarchy of objects in a hierarchical file system is disclosed, which does not require the support of additional file attributes by the hierarchical file system, and has particular application to object oriented programming in window-based computer systems. The class hierarchy comprises a root class, a plurality of classes and a plurality of class instances. The root class is implemented with a hierarchy of root class directory and root class files. Each class is implemented with a hierarchy of class directories and class files comprising the class methods, and the initial values of the class instance variables. Each class instance is implemented with a hierarchy of class instance directory and class instance files comprising the class instance variables. Each hierarchy of directories and files also comprises a path file. The content of these path files are logically related to each other, based on their class' relationships.

Abstract: The system of the present invention provides for the context switching of devices connected through the system's memory management unit and is particularly useful in a multi-tasking computer system in which multiple processes access the same device. In the method and apparatus of the present invention, devices that are connected to the system through the MMU are controlled using the page fault mechanism of the MMU and the page fault handler in each segment. Addresses are allocated in the process address space for each process to provide for the addressing of the devices and device queues connected through the MMU, such that one device or one device queue is mapped into one segment of each process address space that will access the device. The "valid bits" associated with each page in a segment are turned on/off by the process or operating system in order to control the device.

Abstract: The method of rendering an X Windows server system running on a server and at least one host computer terminal secure including the steps of allowing users to view only resources of the X Windows server system the use of which has been specifically authorized to that user, and allowing users to manipulate only resources of the X Windows server system the use of which has been specifically authorized to that user.