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Unit 2: Evolution of Operating System

Sensors bring data to the computer. The computer must analyze the data and possibly adjust Notes
controls to modify the sensor inputs. Systems that control scientific experiments, medical imaging
systems, industrial control systems, and some display systems are real-time systems. Also
included are some automobile-engine fuel-injection systems, home-appliance controllers, and
weapon systems.
A real-time operating system has well-defined, fixed time constraints. Processing must be done
within the defined constraints, or the system will fail. For instance, it would not do for a robot
arm to be instructed to halt after it had smashed into the car it was building. A real-time system
is considered to function correctly only if it returns the correct result within any time constraints.
Contrast this requirement to a time-sharing system, where it is desirable (but not mandatory) to
respond quickly, or to a batch system, where there may be no time constraints at all.

There are two flavors of real-time systems. A hard real-time system guarantees that critical tasks
complete on time. This goal requires that all delays in the system be bounded, from the retrieval
of stored data to the time that it takes the operating system to finish any request made of it. Such
time constraints dictate the facilities that are available in hard real-time systems. Secondary
storage of any sort is usually limited or missing, with data instead being stored in short-term
memory, or in read-only memory (ROM). ROM is located on nonvolatile storage devices that
retain their contents even in the case of electric outage; most other types of memory are volatile.
Most advanced operating-system features are absent too, since they tend to separate the user
further from the hardware, and that separation results in uncertainty about the amount of time
an operation will take. For instance, virtual memory is almost never found on real-time systems.
Therefore, hard real-time systems conflict with the operation of time-sharing systems, and the
two cannot be mixed. Since none of the existing general-purpose operating systems support
hard real-time functionality, we do not concern ourselves with this type of system in this text.

A less restrictive type of real-time system is a soft real-time system, where a critical real-time
task gets priority over other tasks, and retains that priority until it completes.

Self Assessment

Fill in the blanks:
1. The system of the 50's which usually ran one job at a time were known as ...........................
batch processing systems since programs and data were submitted in groups or batches.
2. The main function of a ........................... is to automatically keep executing one job to the
next job in the batch.
3. Time sharing, or multitasking, is a logical extension of ...........................
4. A ........................... is used when there are rigid time requirements on the operation of a
processor or the flow of data, and thus is often used as a control device in a dedicated
application.

2.2 Operating System Architecture

The operating system structure is a container for a collection of structures for interacting with
the operating system's file system, directory paths, processes, and I/O subsystem. The types and
functions provided by the operating system substructures are meant to present a model for
handling these resources that is largely independent of the operating system. There are different
types of structure as described above:

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