Operating Systems: Concurrent and Distributed Software Design

11.2. An example of semaphores in system design: The THE system

In the THE system, Dijkstra et al. designed a hierarchy of virtual resources managed by a strictly layered system of processes. A brief outline of its structure is as follows; see also Figures 11.1 and 11.2. The lowest level, level 0, creates virtual processors. Above this, processes exist and may communicate using a semaphore mechanism. The rest of the system may be written using these concurrency constructs. All interrupts enter at this level, and all but the clock interrupt are handled at higher levels. At level 1, one process provides a one-level virtual store. It synchronizes with drum interrupts and with requests for store from higher-level processes. At level 2, one process ...