Memory

The Model of Run Time
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The Model of Run Time

Once the operating system binary has been linked, the computer can be rebooted and the new operating system started. Once running, it may dynamically load pieces that were not

Process Creation
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Process Creation

Operating systems require some way to make processes. In very simple systems, or in systems designed for running only a single application (e.g., the controller in a microwave oven), it may be possible to have all the processes that will ever be needed be present when

Process Termination
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Process Termination

After a process has been created, it starts running and performs whatever its job is. However, nothing lasts forever, not even processes. Sooner or later the new process will end, generally due to one of the following conditions:

Modeling Multiprogramming
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Modeling Multiprogramming

When multiprogramming is used, the CPU utilization can be improved. Crudely put, if the average process computes only 20% of the time it is sitting in memory, with five processes in memory at once, the CPU should be busy all the time. This model is unrealistically hopeful,

Managing Free Memory
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Managing Free Memory

When memory is allocated dynamically, the operating system must manage it. Generally, there are two methods to keep track of memory usage: bitmaps and free lists. In this section and the next one we will study these two methods.

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

Simulating LRU in Software
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Simulating LRU in Software

Although both of the previous LRU algorithms are (in principle) realizable, few, if any, machines have the required hardware. Instead, a solution that can be implemented in software is required One possibility is called the NFU (Not Frequently Used) algorithm. It requires a

Summary of Page Replacement Algorithms
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Summary of Page Replacement Algorithms

We have now considered a variety of page replacement algorithms. In this section we will briefly summarize them. The list of algorithms discussed is given in Figure 1.

DESIGN ISSUES FOR PAGING SYSTEMS
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DESIGN ISSUES FOR PAGING SYSTEMS

In the preceding sections we have described in detail how paging works and have given a few of the fundamental page replacement algorithms and shown how to model them. But knowing the bare mechanics is not sufficient To design a system, you have to know a lot more

Load Control
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Load Control

Even with the best page replacement algorithm and optimal global allocation of page frames to processes, it can come about that the system thrashes. Actually, whenever the combined working sets of all processes exceed the capacity of memory, thrashing can be expected. One

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