Operating Systems And Page Replacement Computer Science Essay

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Operating system is a connection between users of computers with computer hardware. Before the operating system introduced, people just use the computer by using analog signals and digital signals. Along with the development of knowledge and technology,now there are a variety of operating systems with many advantages.General function of operating system is managing all the resources contained on a computer system and provides a set of services called as system calls to the user. This will make it easier and freshen the use and utilization of computer system resources. Basic Functions of computer system basically consists of four main components, namely hardware device, application programs, operating systems, and users. The operating system serves, regulate and supervise the use of hardware and application programs by various users.

Page replacement aims to produce the lowest error rate pages. There are several different algorithms for the page. Selection page will be replaced in the use of these algorithms can be done in various ways, such as by selecting at random, optimal, First In First Out(FIFO),Not Recently Used(NRU) and Least Recently Used(LRU).Selection of a less precise algorithm can lead to increased levels of error pages so that the process will run slower.

Information security is the process of protecting information. It protects the availability, privacy and integrity of the information. Access of information on computer databases has increased greatly. More companies store business, corporate and personal computer information on computer. Many of the information Stored information is highly confidential and not for public viewing. Many businesses are solely based on information stored in computers such as personal staff details, client lists, salaries, bank account details, marketing, sales information and many more.Without this information, it would often be very hard for a business to operate. Information security systems need to be implemented to protect this information. Effective information security system uses a series of policies, security products, technology and the scope. A set of procedures and systems needs to be applied to effectively access the information. The examples of information security can be Firewall, Encryptions/Decryption, Access Control

List, Authentication and Backup.

Main body

Page replacement aims is to produce the lowest error rate pages. The first strategy use to page replacement is First In First Out(FIFO) is the simplest algorithm. The principle of this algorithm is the principle of a queue means no-priority queue, the incoming first page will come out ahead as well. These algorithms use a stack data structure. If no empty frames during a page fault, then the victim selected is located in the stack frame at the bottom on pages that are the most long in the memory. With only that information in the memory this algorithm can move a page that is often used. It may be a page that is kept in memory because it is always used. Page was due to follow a pattern based on the length of the queue in the memory for the leading element, replaced, and soon have to go back to that memory if page fault occurs again.

Secondly is optimal page replacement.This algorithm is the most optimal algorithm based on its name.The principle of this optimal algorithm is to replace the pages will not be used again in a long time, thus increasing the efficiency of the turn of a page to avoid page fault that occurred and free of anomaly Belady. This strategy will yield a total of at least page fault. This algorithm has lowest a page fault rate among all algorithms in all cases. However, the optimal algorithm is not perfect because it is very difficult to apply. The system cannot find any pages that will be used next. This approach can be done by simulation. But the simulation is only specific to a program. If the best is not possible, then it try to approach it. Page replacement algorithm developed near optimal performance. Each page replacement algorithm collect and use information to determine the page is changed so that the optimal approach.

The next strategy is Least Recently Used(LRU) algorithms replace the oldest page is not needed. The assumption was that the pages had not been used is not needed, a new page is loaded will be used again.Just as the optimal algorithm, LRU algorithm does not have an anomaly Belady. This algorithm uses a linked list to record which pages the longest unused. Linked list that makes larger cost, because it must update the linked list each time a page is accessed. The page is located at the front is a linked list of recently used pages. The longer is used, the page will be getting back and in the last position is the oldest page is not used and ready for swap. Although theoretically realizable LRU, not cheap. To implement the LRU, it is necessary to maintain a list of links from all pages in memory, with the current site and used on the front page recently used at least in the back. The difficulty is that the register should be updated on every memory reference. Find a page in the list, delete, and then move to the next is a very time consuming operation, even in the hardware with the assumption that the hardware can be built.

However, there are other ways to implement the LRU with special hardware. Let us consider first the simplest way. This method requires a complete hardware with 64-bit counter, C, which will automatically increase after each command. Furthermore, each page table entry must also have a field large enough to accommodate a table. After each memory reference, the current value of C is stored in the page table entry for page reference only. When the error page, the operating system to check all the counters in the schedule page to find the lowest. That is the most recent site is used.

The fourth strategy is Random algorithm is quite simple algorithm, which is also in addition to the FIFO algorithm. In this algorithm, the selected site is chosen at random victims.The algorithm is relatively low cost, because it does not require a stack, queue, or counter. Compared to FIFO, average page fault rate cases showed random algorithm is lower than the FIFO algorithm. While compared with LRU, random algorima is superior in terms of looping reference memory, because totally random algorithm does not require looping.

The fifth strategy Not Recently Used(NRU) algorithm. In this algorithm it is given a two page status register bit that is namely bit of R and M. Then the pages is divided into 4 classes, namely

Class 0: not being referenced, not modified (R = 0, M = 0)

Class 1: Not being referenced, has been modified (R = 0, M = 1)

Class 2: Medium referenced, not modified (R = 1, M = 0)

Class 3: Medium referenced, has been modified (R = 1, M = 1)

figure 1 shows the four classes of NRU

Selecting a page to replace the lowest numbered class if there are pages in that class at random.

When the class was empty, the selected page in the higher classes, and so on.This algorithm assumes that the classes be numbered lower will be used again in the relatively long time.This algorithm is easily understood and implemented. The implementation of this algorithm is very efficient because not many steps in the selection page. This algorithm is not optimal, but in normal conditions are adequate.

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