2.0 CHARECTERISTICS AND STRUCTURES OF OPERATING SYSTEM
Main Characteristics
The operating system characteristic can be divided into two which is the main and the supporting characteristic. For this unit, we will explain about the main characteristic that involves concurrency, sharing, long term storage and non determinacy.
Concurrency
In a single multiprocessor multiprogramming system, processes are interleaved in time to yield the appearance of simultaneous execution. Even though actual parallel processing is not achieved, and even though there is a certain amount of overhead involved in switching back and forth between processes, interleaved execution provides major benefits in processing efficiency and program structuring in a multiple processors systems, it is possible not only to interleave processes but to overlap them.
Figure 4.1 The execution of concurrent processes
(Source: Stalling(1995) Operating System)
At first glance, it may seem that interleaving and overlapping represent fundamental different modes of execution and present different problems. In fact, both techniques can be viewed as the example of concurrent processing, and both present the same problems. In the case of a single processors system, the problems produce by multiprogramming flow from the fact that the relative speed of execution of processors cannot be predicted. It depends on the activities of other processes, the way in which the operating system handles interruptions, and the scheduling policies of the operating system. The following difficulties arise:
I. The sharing of global resources is fraught with peril. For example, if two processes both make use using the same global variable, and both perform reads and writes on the variable, than the order in which the various reads and writes are
executed is critical.
II. It is difficult for the operating system to manage the allocation of resources optimally. For example, process A may request use of a particular I/O channel and then be suspended before using that channel. If the operating system locks the channel and prevents its use by other processes, inefficiency results.
III. It becomes difficult to locate a programming error because results are typically not reproduced.
Sharing
The introduction of multiprogramming brought the ability to share the resources among users. Sharing involves not only the processors but also the following:
- Memory
- Input/ output devices, such as discs and printers
- Programs
- Data
I. Sharing the resources such as disc and printers.
The ability to share the process allows cost reduction because it can be used by more than one system users. It’s could happen since
the system resources are not being used all the time by the user.
II. Program and routine sharing
Program and routine sharing will help the programmer to simplify
their job for example the use of sorting routine that is being shared.
III. Data sharing
File which contains global data in database can be shared by
various programs, this can saves the use of storage and prevent the
file duplication and data.
However the sharing ability introduced the need of protection for:
a. Data access
b. Concurrent access of program
c. Data protection from breakdown system
Long term storage
Many users and applications required means for storing information for extended periods, since they are in need for data, program and routine sharing which are stored in Ram or secondary storage. The things to be considered are:
a. Assessment to the data/ easy programs
b. Security from any interference
c. Protection from any system breakdown
In contemporary usage, memory usually refers to a form of semiconductor storage known as random access memory typically DRAM (Dynamic-RAM) but memory can refer to other forms of fast but temporary storage.
The distinctions also reflect an important and significant technical difference between memory and mass storage devices, which has been blurred by the historical usage of the term storage. Many different forms of storage, based on various natural phenomena, have been invented. So far, no practical universal storage medium exists, and all forms of storage have some drawbacks.
No determinacy
The result of a particular program should depend only on the input of the program and not on the activities of other programs in shared systems. But when programs share memory and their execution is interleaved by the processor they may interfere each other by over writing common memory areas in unpredictable ways. Thus the order in which various programs are scheduled may affect the outcome of any particular program.
