Computer Operating System Paging Memory Management Program Computer Science Essay

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In the computer operating system, paging memory management program, which a computer can store and retrieve data from main memory used to store one secondary school. In the paged memory management scheme, the operating system retrieves the size of the block from the same secondary storage of data called pages. The main advantage is to provide paging, which allows the physical address space of a process is not continuous. Time spent before the paging, the system must continuously adapt to the program storage, resulting in different storage and dispersion problems. A virtual memory paging is implemented in most modern general-purpose operating system, an important part of allowing them to use the data, is not suitable to the physical random access memory (RAM) of disk storage.

Strategy for page replacement is the optimal page replacement is the best page replacement algorithm is easy to describe, but impossible to implement. Currently if a page fault occurs, the number of pages set in memory. These pages will be quoted on the very next instruction that the page contains the instructions. Other pages may not refer to 10,100, or 1,000 years later instructions. Each page can be executed before the page with the first number of instructions to be referenced. Algorithm is simply the best web page; the page with the highest tag should be removed. If a page will not be used for 800 million instructions, another page will not be used for six million instructions, the first push to eliminate the error page put it back to get as far as possible in the future. Computers and people are trying to delay as long as unpleasant as they can. The only problem with this algorithm is that it is impossible to achieve. Wrong time on the page, the operating system cannot know every page, the reference for the future. Running the program on the simulator and keeping track of all the page references, it is possible to achieve optimal page replacement in the second run using the first page of the reference information collected during the operation. In this way one can compare with the best, realization algorithm. If the operating system implementation, performance, that is, only 1 percent or more of the optimization algorithm to find a better algorithm will yield a maximum cost increase of 1 percentage point energy difference. To avoid any possible confusion, it should be clear, this page references refer only to a due process logs, and then only to a specific measured input. The page replacement algorithm derived from it that is so specific to a program and input data.

Another strategy for page replacement is the least recently used (LRU) algorithm replaces the page which has not been accessed for the longest time. It performs very well, but it is difficult to implement. There are several Implementations of it. Every time a page is read, it is moved to the head of the list. The next page to replace is the one at the tail of the list. It is very hard to perform this task so fast that it does not slow down the whole system, because memory accesses occur very frequently. We use a bit matrix consisting of n * n bits, where n is the number of physical pages in memory referring to [Tanenbaum, 2001] for details on this implementation. If there are many pages, the bit matrix grows very big. Every time a page is read, the current time is stored in a variable of this page. The next page to swap out is the one with the lowest time of last access. This is hardly implementable because the timer needs more than 32 bit to be fine-granular enough and finding the page with the lowest access time is very expensive.

More strategy for page replacement is the FIFO (First-in-First-out) algorithm. The FIFO (First-in-First-out) is extremely simple: It removes the page with the earliest creation time. This can be implemented using a list: New pages are inserted at the head of the list, and the page at the tail is swapped out. Another implementation is using a ring. Every time a page has to be replaced, the page the pointer points at is swapped out and at the same place the new page is swapped in. After this, the pointer moves to the next page. The FIFO algorithm's performance is rather bad. It is a bad choice, because the page would have to be recalled to main memory instantly, which results an increased rate of page fault.

There another strategy for page replacement is the second-chance algorithm. The second-chance is very similar to FIFO. However, it interferes with the accessing process every page has, in addition to its "dirty bit", a "referenced bit" (r-bit). Every time a page is accessed, the r-bit is set. The replacement process works like FIFO, except that when a page's r-bit is set, instead of replacing it, the r-bit is unset, the page is moved to the list's tail or the pointer moves to the next page is examined. Second Chances performs better than FIFO, but it is still far from optimal.

Strategy for page replacement is the aging algorithm. The aging is somewhat tricky. It uses a bit field of w bits for each page in order to track its accessing profile. Every time a page is read, the first bit of the page's bit field is set. Every n instructions all pages' bit fields are right-shifted by one bit. The next page to replace is the one with the lowest numerical value of its bit field. If there are several pages having the same value, an arbitrary page is chosen. The aging algorithm works very well in many cases, and sometimes even better than LRU, because it looks behind the last access. It furthermore is rather easy to implement, because there are no expensive actions to perform when reading a page. However, finding the page with the lowest bit field value usually takes some time. Thus, it might be necessary to predetermine the next page to be swapped out in background.

Lastly strategy for page replacement is the NRU (not-recently-used) algorithm. The NRU uses an r-bit for every page. Every time a page is read, the r-bit is set. Periodically, all r-bits are unset. When a page fault occurs, an arbitrary page with r-bit unset is swapped out. NRU is actually aging with a bit field width of 1, and it does not perform very well. It's a page at random from the lowest numbered nonempty class. Implicit in this algorithm is that it is better to remove a modified page that has not been referenced in at least one clock tick typically 20m sec than a clean page that is in heavy use. The main problem with NFU is that it never forgets anything. For example, in a multipass compiler, pages that were heavily used during pass 1 may still have a high count well into later passes. In fact, if pass 1 happens to have the longest execution time of all the passes, the pages containing the code for subsequent passes may always have lower counts than the pass 1 pages. Consequently, the operating system will remove useful pages instead of pages no longer in use. The main attraction of NRU is that it is easy to understand, moderately efficient to implement, and gives a performance that, while certainly not optimal, may be adequate.

In Conclusions, there are many types of page replacement strategic like optimal page replacement, the least recently used (LRU) replacement, FIFO (First-in-First-out) page replacement, second-chance page replacement, aging page replacement and NRU (not-recently-used) page replacement. Each of them has their own advantages and disadvantages. The main advantage is to provide paging, which allows the physical address space of a process is not continuous. Time spent before the paging, the system must continuously adapt to the program storage, resulting in different storage and dispersion problems.


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.


There are quite a number of security measures to protect our computer data. Make sure our computer's security level is on comprehensive strategy to protect our core from online fraud and treatment and maintenance of data integrity. Consequences of data loss will affect the company to lost customers. This is usually because the company's database contains records, contracts, or customer information and employees. Not only that, but many businesses are dependent on the database to automate their daily work process. If the data lost without having any backup, daily process will be interrupted. If this incident happened, the employee will wasting their time to recreate the database, if there are any physical records being kept. Unfortunately if the customer knows about the data loss, they will lose confidence on the company. All these can lead to business failures.

There a thing that called malware that can steal or destroys important data. Malicious software includes computer viruses, worms, Trojan horses, spyware, adware, most root kits and other malicious and unwanted software or program. These are dangerous program. A virus is a unit of code which aims to reproduce the additional, similar code units. In order to let virus work, virus needs a host, such as documents, email, or program. A worm is a unit of code that is coded to reproduce additional, similar to the code units, but the difference is that the virus does not need a host. This mainly shows that between the clients or from other Web site users to send the file transfer. If a virus on your computer, it can be use your identity to send instant messages on your behalf.

Denials-of- service (DoS) attacks are also a treat to an organization. DoS are attacks that refuse the use of resources to legitimate users of the system, information, or capabilities. DoS attacks generally do not allow the attackers to access or modify information on the computer system or in the physical world. DoS attackers are nothing more than vandalism. Dos attacks are primarily attacks against computer systems and networks. It sends a large amount of traffic to overloading the system until it stops responding or responds slowly. It will also hide the evidence of the attacks and prevent all users from accessing network resources.

There has another thing called modification attack. It is an attempt to modify information that is not allowed to modify. This attack can take place wherever the information resides. It may also be attempted against information transit. This type of attack is against the integrity of the information. Changes are one type of modification attack is to change existing information, such as an attacker changing an existing employee's salary. The information already existed in the organization but it is now incorrect. Change attacks can be targeted at important information. Insertion is another type of modification attack is the insertion of information. When an insertion attack is done, information that did not suppose exist before is added. This attack may be mounted against information that is yet to be acted upon. For example, an attacker might choose to add a transaction in a banking system that moves funds from a customer's account to his own. Deletion attack is the removal of existing information. Examples, an attacker could remove the record of a transaction from a bank statement.

What security measures that can be taken to protect data and information in the computer or being exchanged in a network? The first step to protect our computer is to ensure that we have a fully functional suite of security software to be installed in our company. There are major options include Norton and MacAfee providing comprehensive security suite. In these software programs, including anti-virus scanning technology, anti-spyware scanning and firewall is the key every part of the solution.

First of all, anti-virus software helps to ensure that any of us download or upload files to our computer are free from viruses. In addition to potential harm to our computer hardware, viruses can also be programmed to implement nefarious programs within our machine. Class of viruses can be known as worm, it can provide a loophole that allows criminals the ability to access our machines. Spyware also are often an assisting part of free, downloaded software from internet which plants hidden pieces of information on our machine that may help criminals take advantage of our system. Spyware can be difficult to remove, so it is important to take all measures to avoid installing in the first place. Prevention is always better than cure.

Access controls are needed to be in an organization. Every computer system within an organization should have the capability to restrict access to files based on the ID of the user attempting the access. This can ensure that not everyone in the organization able to access any unauthorized file. If systems are properly configured and the file permissions set appropriately, file access controls can restrict legitimate users from accessing files they should not have access to. File access controls will not prevent someone from using a system vulnerability to gain access to the system as an administrator and thus see file on the system. Even access control systems that allow the configuration of access controls on systems across the organization cannot do this. To the access control system, such an attack will look like a legitimate administrator attempting to access files to which the account is allowed access.

Intrusion detection systems were once touted as the solution to the entire security problem. No longer would we need to protect our files and systems, we could just identify when someone was doing something wrong and stop them. In fact, some of the intrusion detection systems were marketed with the ability to stop attacks before they were successful. No intrusion detection system is foolproof, and they cannot replace a good security program or good security practice. Intrusion detection systems that support automatic protection features may be also used to generate additional security problems. We can imagine that a situation where the system is configured to block access from an attacking address. In the mean time, we able to find who are generating traffic that is falsely identified as an attack. Our company should have the intrusion detection system to protect our files and systems.

Firewall software provides the ability by limiting the external computer to access our documents or key features of the operating system. By limiting a limited basis for each of the software can only access the external verification machine known that we can prevent hackers penetrate our machines. Powerful firewall software will continuously to monitor our access points to ensure that there is no gap between our securities. In order to be safe, we should always leave our security software on while we having some games, but sometimes we may need to pause our security software, in the meantime we should not put it off or pause for a long time. One way to ensure our security is by hiding our true IP from external scans, because this is a way to avoid from hackers to identify our computer by using dynamic IP, these programs will help us invisible from those who might try to harm our computer.

There is a simple ways to ensure our computer security in high level by choosing difficult to guess the password and always clear our Internet browser's Cookie or completely disable the Cookie features. Cookie is a text file which stores our information so that website can remember who we are after we reopen the Internet browser again. Most are used by legitimate companies to help remember our preferences when we login, others may track or target us for less legitimate purposes. There are some physical methods that we can take by without leaving our computer alone in public places, never tries to log on to the important sites from the public computer and keep a close tab on our laptop when we are travel.  ­­­

Conclusions, Security measures are important to protect our computer and data from any security threats. Security measures must be updated if there got any new updates to avoid any latest security threats.