Synchronization under a Commercial Operating System

The principles of mutual exclusion and semaphore synchronization of concurrent processes ('tasks') have been widely known for years, and the use of these principles for constructing reliable real-time systems has been demonstrated several times.1• 2• 4 • 8 • 9 Yet, most commercially available real-time operating systems do not provide tools to match the principles. When designing a real-time system for a particular computer, you then have three choices: 1. To develop an operating system of your own. 2. To use a commercially available operating system and try to implement the wellknown principles. 3. To replace the well-known principles by ad hoc programming. The authors have had long experience with the first choice, which gave high-quality dedicated systems. However, the systems required by customers tended to become less and less 'dedicated': customers wanted to develop modules of their own on the 'dedicated' system. In order to follow choice (1), we would have to develop or maintain several compilers, editors, etc. for our own operating systems. And that is extremely more costly than developing an operating system for a dedicated system. Consequently we had to consider the second choice more seriously. The computer in question was the PDP-11 and the real-time operating system supplied by the manufacturer was RSX-11M. With this given, we tried to implement mutual exclusion and semaphore synchronization among tasks (jobs) running under RSX. If it could be done efficiently, a major problem would be solved. RSX provides a large set of 'synchronization' operations. Two operations provide simple stop and start of tasks: suspend Stops the calling task temporarily until it is started by another task. resume(Q) Starts the task Q, provided that it is already suspended.