Operating System

The Least Recently Used (LRU) Page Replacement Algorithm
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The Least Recently Used (LRU) Page Replacement Algorithm

A good estimation to the best algorithm is based on the observation that pages that have been heavily used in the last few instructions will possibly be heavily used again in the next few. On the other hand, pages that have not been used for ages will possibly remain unused for a

Page Size
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Page Size

The page size is sometimes a parameter that can be chosen by the operating system. Even if the hardware has been designed with, for instance, 512-byte pages, the operating system can easily regard page pairs 0 and 1, 2 and 3, 4 and 5, and so on, as 1-KB pages by always

Shared Libraries / Mapped Files
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Shared Libraries / Mapped Files

Sharing can be executed at other granularities than individual pages. If a program is started up twice, most operating systems will automatically share all the text pages so that only one copy is in memory. Text pages are always read only, so there is no problem here.

IMPLEMENTATION ISSUES
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IMPLEMENTATION ISSUES

Implementers of virtual memory systems have to make selections among the major theoretical algorithms, such as second chance versus aging, local versus global page allocation, and demand paging versus prepaging. But they also have to be aware of a number of practical

An Example Program Using File System Calls
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An Example Program Using File System Calls

In this section we will consider a simple UNIX program that copies one file from its source file to a destination file. It is listed in Figure 1. The program has minimal functionality and even worse error reporting, but it gives a reasonable idea of how some of the system calls related to

Implementing Directories
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Implementing Directories

Before a file can be read, it must be opened. When a file is opened, the operating system uses the path name supplied by the user to locate the directory entry. The directory entry provides the information required to find the disk blocks. Depending on the system, this information

File System Backups
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File System Backups

Destruction of a file system is sometimes a far greater tragedy than destruction of a computer. If a computer is destroyed by fire, lightning surges, or a cup of coffee poured onto the keyboard, it is annoying and will cost money, but usually a replacement can be purchased with a

Defragmenting Disks
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Defragmenting Disks

When the operating system is at first installed, the programs and files it requires are installed consecutively starting at the beginning of the disk, each one directly following the previous one. All free disk space is in a single contiguous unit following the installed files. On the other

EXAMPLE FILE SYSTEMS
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EXAMPLE FILE SYSTEMS

In the next sections we will discuss numerous example file systems, ranging from quite simple to more sophisticated. Since modern UNIX file systems and Windows Vista's native file system are covered in "UNIX" and "Windows Vista" we will not cover those systems here. We will,

Memory-Mapped I/O
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Memory-Mapped I/O

Each controller has a few registers that are used for communicating with the CPU. By writing into these registers, the operating system can command the device to deliver data, accept data, switch itself on or off, or otherwise perform some action. By reading from these registers,

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