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An operating system is a program designed to run other programs on a computer.A computer's operating system is its most important program.It is considered the backbone of a computer,managing both software and hardware resources.Operating systems are responsible for everything from the control and allocation of memory to recoginizing input from external devices and transmitting output to computer displays.They also manage files on computer hard drivers and control peripherals,like printers and scanners.
The operating systemof a large computer system has even more work to do.Such operating systems monitor different programs and users,making sure everything runs smoothly,without interference,despite the fact that numerous devices and programs are used simultaneously.An operating system also has a vital role to play in security.Its job includes preventing unauthorized users from accesing the computer system.Examples of operating system are LINUX,Window xp and Window 7.
1)Page replacement is where the system must decide which page in main memory should be replaced or removed in order to make room for few new pages.This can be done by over-writting modifying the memory space.List and explain in details all the strategies used for page replacement.
2)Name and explain all the security measures that can be taken to protect data and information in the computer or being exchanged in a network.
1.Page replacement is where the system must decide which page in main memory should be replaced or removed in order to make room for few new pages. This system done by using few strategies known as optimal, least recently used(LRU), First-in First out(FIFO), and clock.
In most operating system texts,the treatment of memory management includes a section entitled "replacement policy", which deals with the selection of a page in memory to be replaced when a new page must be brought in. For example how many page frames are to be allocated to each active process. The set of pages to be sonsidered for replacement should be limited to those of the process that caused the page fault or encompass all the page frames in main memory. Among the set of pages considered, which particular page should be selected for replacement.
When all of the frames in main memory are occupied and it is necessary to bring in a new page to satisfy a page fault, the replacement policy determines which page currently in memory is to be replaced. All of the policies have as their objective that the page that is removed should be the page least likely to be referenced in the near future. Because of the principle of locality, there is often a high correlation between recent referencing history and near-future referencing patterns. Thus, most policies try to predict future behavior on the basis of past behavior. One trade-off that must be considered is that the more elaborate and sophisticated the replacement policy, the greater the hardware and software overhead to implement it.
Frame locking is one restriction on replacement policy needs to be mentioned before looking at various algorithms. Some of the frames in main memory may be locked. When a frame is locked, the page currently stored in that frame may not be replaced. Much of the kernel of the operating system is held on locked frames, as well as key control structures. In addition, I/O buffers and other time critical areas may be locked into main-memory frames. Locking is achieved by associating a lock bit with each frame. This bit may be kept in a frame table as well as being included in the current page table.
The optimal policy selects for replacement that page for which the time to the next reference is the longest. It can be shown that this algorithm results in the fewest number of page faults[BELA66]. Clearly, this algorithm is impossible to implement, because it would require the operating system to have perfect knowledge of future events. However, it does serve as a standard against which to judge other algorithms. The execution of the process requires reference to five distinct pages. The page address stream formed by executing the program is 2 3 2 1 5 2 4 5 3 2 5 2 which means that the first page referenced is 2, the second page referenced is 3, and so on. The optimal policy produces three page faults after the frame allocation has been filled.
The least-recently-used (LRU) policy replaces the page in memory that has not been referenced in the longest time. By the principle of locality, this should be the page least likely to be referenced in the near future. And in fact, the LRU policy does nearly as well as the optimal policy. The problem with this approach is the difficulty in implementation. One approach would be to tag each page with the time of its last reference this would have to be done at each memory reference, both instruction and data. Even if the hardware would support such a scheme, the overhead would be tremendous. Alternatively, one could maintain a stack of page references, again an expensive prospect.
An interesting refinement of LRU, referred to as SEQ, was proposed in [GLAS97]. The authors examined a number of applications that are "memory intensive" that is, that make many memory references. They found that while many applications exhibit strong locality, there were also a number of applications that exhibited clear nonlocal reference patterns that could be exploited for better replacement decisions. For these latter programs, ranges of address space are traversed in the same pattern repeatedly. Typically, the applications are array based. Some of the programs repeatedly traverse ranges of memory in one direction only, while others traverse the range in one direction and then reverse direction.
The first-in, first-out (FIFO) policy treats the page frames allocated to a process as a circular buffer, and pages are removed in round-robin style. All that is required is a pointer that circles through the page frames of the process. This is therefore one of the simplest page replacement policies to implement. The logic behind this choice, other than its simplicity, is that one is replacing the page that has been in memory the longest. A page fetched into memory a long time ago may have now fallen out of use. This reasoning will often be wrong, because there will often be regions of program or data that are heavily used throughout the life of a program. Those pages will be repeatedly paged in and out by the FIFO algorithm. Note that LRU recognizes that pages 2 and 5 are referenced more frequently than other pages, whereas FIFO does not. Whereas the LRU policy does nearly as well as an optimal policy, it is difficult to implement and imposes significant overhead. On the other hand, the FIFO policy is very simple to implement but performs relatively poorly.
The security measures that can be taken to protect data and information in the computer or being exchanged in a network are protect the data and information from hackers, password protection, and antiviruses.
2.1 PROTECT THE DATA FROM HACKERS
We need to protect our personal data and informations from hackers. So that, we can prevent our data being hacked and the best way is install a firewall to keep intruders out. A firewall can protects our computer from unauthorized person or attackers on the internet. Moreover, if you use public computer don't send any personal information or don't store anything personal information in the hard drive. Beside that, even we saved our personal information to a secured site, there is also a chance of getting the data hacked.
If you are using your personal PC, to prevent the program from entering it, don't open unsolicited mails and attachments, keep away from strange sites with alot of spelling errors and mistakes. Before that, don't visit the open links to your mailbox from person you don't know before. This may leads to be a spy-ware storehouse. In addition, don't leave your personal details into the PC because the hackers can hacked your data easily.
2.2 PASSWORD PROTECTION
The second best way to protect our data and information by password protection. The font line of defense against intruders is the password system. Virtually all multiuser systems require that a user provide not only a name or identifier (ID) but also a password. The password serves to authenticate the ID of individual logging on to the system. In turn, the ID provides security in the following ways.
The ID determines whether the user is authorized to gain access to a system. In some systems, only those who already have an ID filed on the system are allowed to gain access. The ID also determines the privileges accorded to the user. A few users may have supervisory or "superuser" status that enables them to read files and perform functions that are especially protected by the operating syste. Some systems have guest or anonymous accounts, and users of these accounts have more limited privileges than others. Beside that, the ID is used in what is referred to as discretionary access control. For example, by listing the IDs of the other users, a user may grant permission to them to read files owned by the user.
2.3 PROTECT DATA AND INFORMATION FROM VIRUSES
The third best way to prevent data and information to the threat of viruses is do not allow a virus to get into the system in the first place. This in general, impossible to achieve, although preventation can reduce the number of successful viral attacks. The next best approach is to be able is detection. Once the infection has occured, determine that it has occured and locate the virus. Once detection has been achieved, identify the specific virus that has infected a program. Remove the virus from all infected systems, so that the disease cannot spread further. Once the specific virus has been identified, remove all traces of the virus from the infected program and restore it to its original state. If detection succeeds but either identification or removal is not possible, then the alternative is to discard the infected program and reload a clean backup version. Advances in virus and antivirus technology go hand in hand. Early viruses were relatively simple code fragments and could be identified and purged with relatively simple antivirus packages. As the virus arms race evolved, both viruses and, necessarily, antivirus software has grown more complex and sophisticated.
Here i would like to share my experience during i m doing my operating system assingment. Firstly, i would like to say that operating systemis a tough subject and it is not easy to understand but when i started to do this assingment and i went through for more research about this assignment questions i can understand alot and its make my work easier. I also can understand what is operating system. This is because of my lecturer, co-ordinator, and my friend. For that, i would like to thank Mr. Alex who is my lecturer taugh me and guide me how to do my assignment in easy way. The second person i would like to thank is Mr. Viswa who is my course cordinator because he let me use his IT lab computers and other staffs for my assignment research this helped me alot to get points for the questions. Finally, i also thank my best friend Miss. Vidyamani who helped me to get more information about the assignment and also spent time with me to complete the assignment. Thats all i can conclude and thank you.
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