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Unit 2: Evolution of Operating System

these media. For instance, it takes round about five milliseconds for a magnetic 'tape' and about Notes
one millisecond for a fast fixed-head disk in comparison to a card reader or printer that takes
around 50-100 milliseconds. Thus, if you use a disk or tape, it reduces the amount of time the
central processor has to wait for an input output operation to finish before resuming processing.
This would reduce the time taken to process a job which indirectly would bring down the
turnaround times for all the jobs in the batch.

Figure 2.2

Jobs/tasks
Jobs/tasks

Jobs/tasks

Jobs/tasks

operating system

hardware

Another term that is commonly used in a batch processing system is Job Scheduling.
Job scheduling is the process of sequencing jobs so that they can be executed on the processor. It
recognizes different jobs on the basis of First-Come-First-Served (FCFS) basis. It is because of
the sequential nature of the batch. The batch monitor always starts the next job in the batch.
However, in exceptional cases, you could also arrange the different jobs in the batch depending
upon the priority of each batch. Sequencing of jobs according to some criteria require scheduling
the jobs at the time of creating or executing a batch. On the basis of relative importance of jobs,
certain 'priorities' could be set for each batch of jobs. Several batches could be formed on the
same criteria of priorities. So, the batch having the highest priority could be made to run earlier
than other batches. This would give a better turn around service to the selected jobs.
Now, we discuss the concept of storage management. At any point of time, the main store of the
computer is shared by the batch monitor program and the current user job of a batch. The big
question that comes in our mind is-how much storage has to be kept for the monitor program
and how much has to be provided for the user jobs of a batch. However, if too much main
storage is provided to the monitor, then the user programs will not get enough storage. Therefore,
an overlay structure has to be devised so that the unwanted sections of monitor code don't
occupy storage simultaneously.
Next we will discuss the concept of sharing and protection. The efficiency of utilization of a
computer system is recognized by its ability of sharing the system's hardware and software
resources amongst its users. Whenever, the idea of sharing the system resources comes in your
mind certain doubts also arise about the fairness and security of the system. Every user wants
that all his reasonable requests should be taken care of and no intentional and unintentional acts
of other users should fiddle with his data. A batch processing system guarantees the fulfillment
of these user requirements. All the user jobs are performed one after the other. There is no
simultaneous execution of more than one job at a time. So, all the system resources like storage
IO devices, central processing unit, etc. are shared sequentially or serially. This is how sharing
of resources is enforced on a batch processing system. Now, arises the question of protection.

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