A Program in Execution Processes

By: Jesse Miller

The concept of process is fundamental to the structure of operating systems. This term was first used by the designers of Multics in the 1960s. It is general term than job. Many definitions have been given for the term process, they are a program in execution, the "animated spirit" of a program, and that entity that can be assigned to and executed on a processor.

Three major lines of computer system development created problems in timing and synchronization that contributed to the development of the concept of the process multi programming batch operation, time sharing, and real time transaction systems. Multi programming was designed to keep the processor and input/output devices, including storage devices, simultaneously busy to achieve maximum efficiency. In response to signals indicating the completion of I/O transactions, the processor is switched among the various programs residing in main memory.

A second line of development was general purpose time sharing. The rationale for such systems is that computer users are more productive if they can interact directly with the computer from some sort of terminal. The key design objective is that the system be responsive to the needs of the individual user and yet, for cost reasons be able to simultaneously support many users. These goals are compatible because of the relatively slow reaction time of the user.

If a typical user needs an average of 2 seconds of processing time per minute, then close to 30 such users should be able to share the same system without noticeable interface. The over head of the operating system must be allowed for.

Another important line of development has been real-time transaction processing systems. In this case, a number of users are entering queries or updates against a database. This phenomenon is widely observed during airline reservation system. The key difference between the transaction processing system and the time sharing system is that the former is limited to one or a few applications, whereas users of a time sharing is that the former is limited to one or a few applications, whereas users of a time sharing system can engage in program development, job execution, and the use of various applications. System response time is paramount in both the cases.

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The principal tool available to system programmers in developing the early multiprogramming and multiuser interactive systems was the interrupt. The activity of any job could be suspended by a defined event.

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