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Operating Systems: History

 


Operating Systems: History

Today’s operating systems are large, complex software packages that have grown from
humble beginnings. The computers of the 1940s and 1950s were not very flexible
or efficient. Machines occupied entire rooms. Program execution required
significant preparation of equipment in terms of mounting magnetic tapes,
placing punched cards in card readers, setting switches, and so on.

The execution of each program, called a job, was handled as an isolated
activity—the machine was prepared for executing the program, the program was
executed, and then all the tapes, punched cards, etc. had to be retrieved
before the next program preparation could begin. When several users needed to
share a machine, sign-up sheets were provided so that users could reserve the
machine for blocks of time.

During the time period allocated to a user, the machine was totally under that user’s control. The session usually began with program setup, followed by short periods of program execution. It was often completed in a hurried effort to do just one more thing (“It will only take a minute”) while the next user was impatiently starting to set up. In such an environment, operating systems began as systems for simplifying program setup and for streamlining the transition between jobs.

One early development was the separation of users and equipment, which eliminated the physical transition of people in and out of the computer room. For this purpose, a computer operator was hired to operate the machine. Anyone wanting a program run was required to submit it, along with any required data and special directions about the program’s requirements, to the operator and return later for the results. The operator, in turn, loaded these materials into the machine’s mass storage where a program called the operating system could read and execute them one at a time.

This was the beginning of batch processing—the execution of jobs by collecting them in a single batch, then executing them without further interaction with the user. In batch processing systems, the jobs residing in mass storage wait for execution in a job queue (Figure 50). A queue is a storage organization in which objects (in this case, jobs) are ordered in first-in, first-out (abbreviated FIFO and pronounced “FI-foe”) fashion.

That is, the objects are removed from the queue in the order in which they arrived. In reality, most job queues do not rigorously follow the FIFO structure, since most operating systems provide for consideration of job priorities. As a result, a job waiting in the job queue can be bumped by a higher-priority job.


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