Operating system is software that controls the hardware. There is a significant trend for functions to migrate from software into firmware. That a view of operating system as the programs, implemented in either software or firmware, those make the hardware usable. Operating systems make this computing power conveniently available to users, and they manage the hardware carefully to achieve a good performance.
Operating system are primarily resource managers the main resource they manage is computer hardware in the form of processors, storage, input or output devices, communication devices and data. Operating system is an integral part of the computing environment, and they must be understood to some degree by every computer user.
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.
ANSWER QUESTION 1
The page replacement strategies are common in paging system for all page frames to be in use. The operating system storage management routines must decide the pages in primary storage to displace to make room for an incoming page. That considers each of the following page replacement strategies. The main topic is virtual storage management.
The principle of optimality
The principle of optimality state that to obtain optimum performance the page to replace is the one that will not be used again for the furthest time into the future. It is possible to demonstrate the optimality of this strategy and the strategy is not realizable since that cannot predict the future. This optimal replacement strategy is called OPT or MIN.
Random page replacement
If a seek a low overhead page replacement strategy that does not discriminate against particular users, one simple technique to choose is random page replacement. All pages in main storage have an equal likelihood of being selected for replacement. This strategy could select any page for replacement, including the next page to be referenced. Random page replacement decisions can be made quickly and with any significant number of page frames to choose from would actually have only a small probability of replacing a page likely to be referenced again almost immediately.
FIFO Page replacement
In first in first out page replacement, the time-stamp each page as it enters primary storage. When a page needs to be replaced, we choose the one that has been in storage the longest. That intuitive appeal of this strategy seems reasonable namely that this page has had its change and it’s time to give another page a chance, for example on large timesharing systems it is common for many users to share a copy of a text editor as they enter and correct programs. First in first out page replacement or such a system might choose heavily used editor page to replace. The page would be recalled implemented with a simple FIFO queue as each page arrives, it is placed at the tail of the queue and pages are replaced from the head of the queue. They also include FIFO Anomaly it would seem reasonable that the more page frames allocated to a process, the fewer page faults the process would experience discovered under FIFO page replacement, certain page reference patterns actually cause more page faults when the number of page frames allocated to a process is increased. This phenomenon is called the FIFO Anomaly or Be lady’s Anomaly.
Least-Frequently -Used Page replacement
One apporoximation to LFU is the least-frequently-used (LFU) strategy. Hare that are concerned with how intensive the used of each page has been. The page to replace is that page that is least frequently used or least intensively referenced. Here again the intuitive appeal is real.the page has been used once were as all other pages in main storage could have been used more than once. Now page replacement mechanism replaces this page when in fact it would be highly likely to be used immediately. This true simply because we cannot accurately predicy the future. Thus a page replacement stratery that make reasonable decisions most of the time and has low overhead is desired.
Modifications to FIFO, clock Page Replacement and Chance Page Replacement
The clear weakness of the FIFO strategy is that it may choose to replace a heavily used page that has been in memory for a long time. This possibility can be avoided by replacing only those pages whose referenced bits are off.
The second change variation of FIFO examines the referenced bit of the oldest page, if this bit is off, the page immediately selected for replacement. If the referenced bit is on,it is set off and the page is moved to the tail of the FIFO list and treated essentially as a new arrival,this page gradually moves to the head of the list from which it will be selected for replacement only if its referenced bit is still off. This essentially gives the pages a second change to remain in primary storage if indeed its referenced bit is turned on before the page reaches the head of the list. Active pages will repeatedly have their refenced bits set on, move to the head of the list, have their referenced bits set off, and move to the tail of the list, tus remaining in primary storage. Obviously, a modified page must be copied to secondary storage before it can be replaced so when its referenced bit is set off, the page remains temporarily unreplaceable until the push is completed.
Answer question 2
The security measures that can be taken to protect data of security requirements for a given system define what it means for that system to be secure. These requirements serve as the basis for determining if the implemented system is secure, without a precise set of security requirement it makes little sense to ask if the system is secure. Similarly, if the requirements are weakly stated it does not say much about the true security of the system.
A Total Approach Security
A total approach security is essential, that also discuss some of areas, of computer other than operating system security. External security is concerned with securing the computer facility from intruders and from disasters such as fire and floods. once a user is allowed physical access to a computer facility, the user identification must be established by the operating system before access is allowed to the program and data stored on the system this might be called user interface security. Internal security deals with the various controls built into the hardware and the operating system to ensure the reliable and uncorrupted operating of the computer system and the integrity of program and data.
External security
External security consists of physical security and operational security. Physical securities include protection against disasters and protection against intruders. Detection mechanisms are important to physical security, smoke detectors and heat sensors can provide early warning in case of fire the motion detectors can determine if an intruder has entered a computing facility. The protection against disaster can be expensive to implement and is often treated too lightly, the need for such protection depends on the consequences of loss. Most organizations simply cannot afford the expense of a thorough disaster protection scheme. The major physical security efforts are directed toward keeping out the intruders.
Amplification
The amplification it is necessary to give a surveillance program greater access right so that it may effectively execute certain user requests. Thus to compute the average income of all taxpayer. This is called amplification. Clever users might still be able to infer information about individuals by making several different requests and examining the overlap in the results. Surveillance program can be designed to detect such attempts and notify a system administrator.
Password protection
The password protection has many weaknesses. User tends to choose passwords that are easy to remember, such as the name of a friend or relative. Someone who knows the user might try to login in several times using the names of people this person knows this might result in a security breach by use of repeated trials. Some early system used short passwords these systems were easily compromised by simply attempting all possible passwords. Most systems today use longer passwords to thwart such penetration attempts. Using long passwords is not necessarily wise either if password are difficult to remember, then users will be more inclined to make a note of them this of course make it easier for someone to obtain a password.
A simple defence against repeated trials is to limit the number of login attempts that may be initiated in any period of time from a single terminal or workstation. Certainly, people make typing errors when attempting to log in. if another series of futile login attempts occurs, the operating system might issue a warning at the terminal that the police are being called or that an alarm is about to go off.
Hardware security
As hardware costs decline, it becomes increasingly desirable to incorporate operating system functions into hardware. These functions are then more secure than when they are accessible as easily modified software instructions. Functions incorporated into hardware can run much faster than in software various monitoring function may be performed more frequently
Conclusion
Operating system a software that that control the hardware by everything operating system is perform many functions such as implementing the user interface, sharing hardware among users and allowing user to share data among themselves, preventing and rapid access and handling network communications.
