UNIX time-sharing system: The network operations center system

The Network Operations Center System (NOCS) is a real-time, multiprogrammed system whose function is to survey and monitor the entire toll network. While performing this function, the NOCS supports multiple time-shared user work stations used to interrogate the data base maintained by the real-time data acquisition portion of the system. The UNIX∗ operating system was chosen to support the NOCS because it could directly support the time-shared user work stations and could, with the addition of some interprocess communication enhancements and a fast access file system, support the real-time data acquisition. Features of the UNIX operating system critical to data acquisition include signals, semaphores, interprocess messages, and raw I/O. Two features were added, the Logical File System (LFS), which implements fast application process access to disk files, and the Multiply Accessible User Space (MAUS), which allows application processes to share the same memory areas. This paper describes these features, with emphasis on the manner in which UNIX operating system features are used to support simultaneously both real-time and time-shared processing and on the ease with which features were added to the UNIX operating system.