COMBINATION OPERATING SYSTEMS

As presented here, different types of operating systems are optimized, or at least largely geared toward serving the needs of certain specific environments. In practice, however, a given environment may not exactly fit any of the described molds. For instance, both interactive program development and lengthy simulation are often encountered in university computing centers. For this reason, some commercial operating systems provide a combination of the described services.

For Example, a time-sharing system may support interactive user while also incorporating a full-fledged batch monitor. This allows computationally intensive non interactive programs to be run concurrently with interactive programs. The common practice is assign low priority to batch jobs and thus to exccute batched programs only when the processor would otherwise be idle. In other words, batch processing may be used as a filter to improve processor utilization while accomplishing a useful service of its own. Similarly, some time-critical events, such as receipt and transmission of network data packets, may be handled in real-time fashion on systems that otherwise provide time-sharing services to their terminal user.  

EVOLUTION OF OPERATING SYSTEMS

An Operating system may process its workload serially or concurrently. That is, resourses of the computer system may be dedicated to single program until its completion, or they may be dynamically reassigned among a collection of active programs in different stage of execution. Because of their ability to execute multiple programs in interleaved fashion, such operating systems are often reffered to as multi programming systems exist. In order to motivate the need for the types of services that each of these varieties provide, we briefly and informally sketch the evolutionary path of operating-system development. In particular, we describe serial processing, batch processing, and multi programming. No specific dates are attached to some of the events because we are interested in the progression of ideas and not in attempting to chart the history of oprating systems. Moreover, much of the historical development of operating systems for mainframes was first retraced by minicomputers and then again by microcomputers and personal computers. Thus many of the concepts and ideas recur are applicable to different systems at deferent times.

The mechanics of program execution and brief sketch of the process of program development, namely, the edit-compile-execute cycle, are described for each type of operating system. While execution of programs is an essential activity in all computer systems, productivity of program preparation is important only in program-development environments, such as university computing centers.