Paging For Virtual Memory Management Computer Science Essay

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Page replacement is where the system must decide which page in main memory should be replaced or removed in order to make room for new pages. This can be done by over-writing / modifying the memory space. List and explain in detail all the strategies used for page replacement.

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.


In a computer operating system, we uses paging for virtual memory management, page replacement algorithms decide which memory pages to page out (swap out, write to disk) when a page of memory needs to be allocated. Paging happens when a page fault occurs and a free page cannot be used to satisfy the allocation, either because there are none, or because the number of free pages is lower than some threshold.

When the page that was selected for replacement and paged out is referenced again it has to be paged in (read in from disk), and this involves waiting for I/O completion. This determines the quality of the page replacement algorithm: the less time waiting for page-ins, the better the algorithm. A page replacement algorithm looks at the limited information about accesses to the pages provided by hardware, and tries to guess which pages should be replaced to minimize the total number of page misses, while balancing this with the costs (primary storage and processor time) of the algorithm itself.

The best possible page replacement algorithm is easy to describe but impossible to implement. It goes like this. At the moment that a page fault occurs, some set of pages is in memory. One of these pages will be referenced on the very next instruction (the page containing that instruction). Other pages may not be referenced until 10, 100, or perhaps 1000 instructions later. Each page can be labeled with the number of instructions that will be executed before that page is first referenced.

Operating systems provide the fundamental mechanisms for securing computer processing. In the 1960s, the operating system designers need to explore how to create more secure operating system security-The operating system mechanism, actively prevent confrontation system environment. Recently, to ensure the safety security has become the mainstream of the importance of all the problems. The security system is designed for the design of these requirements, we may see, the software system often results in implementation challenges, designers are still exploring the complexities of the software system. However, if the system is not designed to achieve the security requirements of the operating system, its security feature a myriad of ways the system failed.

In Operating System, security is a free access control, mandatory access control computer information systems, signs, labels, object reuse, audit, data integrity, covert channel analysis, trusted path, and trusted recovery to meet the appropriate security of the ten aspects of Requirements. The main features of secure operating system:

Least privilege principle that each privileged user only has the power began his work.

Independent of the access control, mandatory access control, including confidentiality and integrity of the Access Control Access Control.

Safety audits.

Security domain isolation. With these security features as long as the end of Ministers, all applicants to the virus mixed, Trojans, network intrusion and illegal operations, to make it as real resistance, because they are contrary to the operating system's security rules, will lose the run Basis.

Define Question 1

Most virtual memory systems use paging, where the virtual address space is divided into pages and the corresponding units in physical memory are called frames. When a page fault occurs and physical memory is full, the operating system removes a page from memory and moves in a new one. But which page should be chosen for removal? The optimal replacement algorithm would want us to remove the page that will not be used for the longest period of time. This algorithm guarantees the lowest possible page-fault rate for a fixed number of frames. But this algorithm is unrealizable since it would require from the operating system to have a crystal ball to be able to see the future and to determine which page will remain unreferenced for the longest period of time. Most of the existing page replacement algorithms are approximations of the optimal algorithm.

When a page fault occurs, the operating system has to choose a page to remove from memory to make room for the page that has to be brought in. If the page to be removed has been modified while in memory,it must be rewritten to the disk to bring the disk copy up to date. If, however, the page has not been changed, the disk copy is already up to date, so no rewrite is needed. The page to be read in just overwrites the page being evicted. While it would be possible to pick a random page to evict at each page fault, system performance is much better if a page that is not heavily used is chosen. If a heavily used page is removed, it will probably have to be brought back in quickly, resulting in extra overhead. It have to do a lot of work on the page, replace algorithm both theory and experimental.

Answer for Question 1

There are several types of page replacement:

Random Page

Random replacement algorithm replaces a random page in memory. This eliminates the overhead cost of tracking page references. Usually it fares better than FIFO, and for looping memory references it is better than LRU, although generally LRU performs better in practice. OS/390 uses global LRU approximation and falls back to random replacement when LRU performance degenerates and the Intel i860 processor used a random replacement policy.


The first-in, first-out (FIFO) page replacement algorithm is a low-overhead algorithm that requires little book-keeping on the part of the operating system. The operating system maintains a list of all pages currently in memory, with the page at the head of the list the oldest one and the page at the tail the most recent arrival. On a page fault, the page at the head is removed and the new page added to the tail of the list.

It is not strictly necessary to record the time when a page is brought in. We can create a FIFO queue to hold all the pages in memory. We replace the page at the head of the queue. When a page is brought into memory, we insert it at the tail of the queue.

There are two common types of FIFO, there are:

Replace the oldest page

The latest replacement pages


The NRU (Not Recently Used) algorithm removes 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 20 msec) than a clean page that is in heavy 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.

The Second Chance algorithm is a modification of FIFO and checks the referenced bit of the page before swapping it out of memory. If the referenced bit is set (=l), then the bit is cleared (=O), the page is put at the end of the FIFO queue, and its load time is updated as though it had just arrived in memory. In other words, the page is given a second chance. To avoid moving too many pages, the Second Chance algorithm can be implemented by using a circular list.

Least Recently Used

A good approximation to the optimal algorithm is based on the observation that pages that have been heavily used in the last few instructions will probably be heavily used again in the next few. Conversely, pages that have not been used for ages will probably remain unused for a long time. This idea suggests a realizable algorithm: when a page fault occurs, throw out the page that has been unused for the longest time.

Least Frequently Used

The Least Frequently Used (LFU) replacement policy selects a page for replacement if the page had not been used often in the past. This policy keeps count of the number of times that page is accessed. Pages with the lowest counts are replaced while pages with higher counts remain in primary memory

Far page

The far strategy creates an access graph that characterizes a process's reference patterns. The algorithm chooses to replace the unreferenced page that is furthest away from any referenced page in the access graph. Although the strategy performs at near-optimal levels, it has not been implemented in real systems, because the access graph is complex to search and manage without hardware support.

Define Question 2

When we start our computer (in BIOS loading), are we interact with the operating system. This software component defines what we can do with computer systems, and how to do it. Whether we are, and file systems interaction or and in chatting tools people chat, the operating system is behind the scenes to provide a (hope), because it perfect experience, we may explain our behaviour and what others have converted into the computer can process. Although in many aspects of the operating system on different, the most common operating systems offer than the interface between the user and the machine more simple. Plan includes numerous extras, provides users from simple screensaver, to complex file encryption scheme. However, it is important to understand that these solutions are added to the operating system, and no necessary extras in computer operation. Many users to be close and operating systems accessories (for example, but forget Solitaire) familiar with the security is also included in order to help users maintain a safe and reliable running environment characteristics. As a result, many information system is not safety status, leaf infected risk, or by an attacker completely to damage the system. This is partly specializes in the system's security problems. From build a safe home network to create strong password, it is important to know the safety and reliable way to use the details of the operating system. In today's interconnected world, irresponsible establishment does not consider safe computer. It only needed a virus or Trojan creates an infected computer systems chain effect and compromise.

Answer for Question 2

Below are some objectives about the security in operating system:


Cryptography can be defined as the conversion of data into a scrambled code that can be deciphered and sent across a public or private network. Cryptography uses two main styles or forms of encrypting data; symmetrical and asymmetrical. Symmetric encryptions, or algorithms, use the same key for encryption as they do for decryption. Other names for this type of encryption are secret-key, shared-key, and private-key. The encryption key can be loosely related to the decryption key; it does not necessarily need to be an exact copy. Authentication

Computer Security Authentication means of verifying a network to log on to a user's identity. Passwords, digital certificates, smart cards and biometrics can be used to prove the identity of network users. Computer Security Authentication involves verifying the integrity of the message, e-mail authentication and MAC (message authentication code), checking the integrity of a transmitted message.

Access Control

Access control is a system, make this power to control in a given physical facilities or computer based information system access areas and resources. Access control system of personal safety field, is often as a physical structure of the safety of the second. Access control is, in reality, our daily life phenomenon. A car door lock is basically an access control form. At a bank ATM system password is another kind of access control method. Trampoline in standing in front of a nightclub is perhaps a more primitive access control mode (because involves technical information obvious lack of). Access control is the most important owner, seek the security of important, confidential or sensitive information and equipment.

Security Attack

A security attack is threatening the security agencies have information on any behaviour. 

It can be divided into several types like security attacks interruption, interception, modification, and fabrication. Threatening to interrupt the availability of availability, such as the failure of information systems or resources consumption make the denial of service, information systems, the file is damaged due to virus infection, etc.; intercepted compromise the confidentiality of information, such as analysis of communication lines or facilities on the traffic, leaking the message content; tampering with critical information integrity, without the authorization of unauthorized modification of the message content; forged compromise the authenticity of the information, disguised as another network entity, such as access to some extra privileges, or a legitimate message intercepted then re-transmit the information to achieve the illegal intrusion into the system or gain unauthorized access to information and other purposes. Not invade the letter intercepted information systems, information transmission only way to attack, so called passive attacks, other types of attacks is attributable to active attacks category. In the way of defences against security attacks can use encryption technology and physical security policy, software or hardware control methods achieved.

Attack prevention and security solutions

Our security management solutions for mainframe deploy increased automation, reducing administration time. They also simplify the monitoring and management of security resources and applications. The result is a more productive way to manage security that minimizes data risks, as it maximizes infrastructure investment and improves mainframe security operations.

Attack prevention may be attempted at several levels. These include individual screening and physical controls in operations.

Individual screening would require that users are screened to authenticate themselves and be responsible individuals.

Physical controls involve use of physical access control. Finally, there are methods that may require configuration controls. We shall begin the discussion on the basic attack prevention with the defenses that are built in Unix. These measures are directed at

user authentication

Security Policy and Access Control

Secure communication

When two entities are communicating with each other, and they do not want a third party to listen to their communication, then they want to pass on their message in such a way that nobody else could understand their message. This is known as communicating in a secure manner or secure communication. Secure communication includes means by which people can share information with varying degrees of certainty that third parties cannot know what was said. Other than communication spoken face to face out of possibility of listening, it is probably safe to say that no communication is guaranteed secure in this sense, although practical limitations such as legislation, resources, technical issues (interception and encryption), and the sheer volume of communication are limiting factors to surveillance. Secure communication products and solutions are increasingly required to provide Privacy, Authentication, data Integrity, and Non-Repudiation (PAIN) services to communication and data transmission system.

Conclusion & Recommendation

Here are some Windows Memory Management Secrets and memory optimization tool scam exposed. When we are browsing the web, we may often see some pop-up ads, such as "organize memory fragmentation, improve system performance, "or" greatly reduce the potential collapse of systems and processes, recycling garbage memory "and so on. If you click on these links pop-up ads, you will see some of the so-called optimization software, claiming that only takes 9.95, 14.95 or $ 29.95; you can easily achieve all of the features. Well, these tools may seem like really good, but in fact the best case, these so-called memory optimization tools without any effect; the worst case can seriously degrade system performance.

In the password security, with an "out-of-box" operating system, re-usable passwords are entered each time a login is performed. The biggest problem with reusable passwords is that they are susceptible to a variety of attacks that have a single goal: to uncover the user's password. In the preceding chapter, this kind of security boundary was cited as a source of concerns. The login boundary is affected by three factors: the strength of the data provided by the user, the integrity of the data transmitted at the boundary, and safe communications at the boundary. A weak password is an example of the first factor. If the password is transmitted across a network, an adversary might be able to intercept and modify the value, validating the second type of concern. Finally, if the login programs are tampered with, and the boundary between the user and the computer becomes untrustworthy, the third factor is attacked. These three factors are targeted to varying degrees by the attacks described next.