Virtual Memory

Disks
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Disks

Next in the hierarchy is magnetic disk (hard disk). Disk storage is two orders of magnitude cheaper than RAM per bit and often two orders of magnitude larger as well. The only problem is that the time to randomly

Virtual Memory
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Virtual Memory

Virtual memory (discussed in "MEMORY MANAGEMENT"), gives the ability to run programs larger than the machine's physical memory by moving pieces back and forth between RAM

OUTLINE OF THE REST OF THIS BLOG
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OUTLINE OF THE REST OF THIS BLOG

We have now completed our introduction and bird's-eye view of the operating system. It is time to get down to the details. As mentioned already, from the programmer's point of view,

Swapping
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Swapping

If the physical memory of the computer is large enough to hold all the processes, the schemes explained so far will more or less do. But in practice, the total amount of RAM required by all the processes is often much more than can fit in memory. On a typical Windows or Linux

VIRTUAL MEMORY
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VIRTUAL MEMORY

As base and limit registers can be used to create the abstraction of address spaces, there is another problem that has to be solved: managing bloatware. While memory sizes are increasing rapidly, software sizes are increasing much faster. In the 1980s, many universities

Paging
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Paging

Most virtual memory systems use a technique called paging, which we will now explain. On any computer, programs reference a set of memory addresses. When a program implements an instruction like

Page Tables
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Page Tables

In a simple execution, the mapping of virtual addresses onto physical addresses can be summarized as follows: the virtual address is split into a virtual page number (high-order bits) and an offset (low-order bits). For instance, with a 16-bit address and a 4-KB page size, the

Speeding Up Paging
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Speeding Up Paging

As we have just studied the basics of virtual memory and paging, it is now time to go into more detail about possible implementations. In any paging system, two main issues must be faced:

PAGE REPLACEMENT ALGORITHMS
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PAGE REPLACEMENT ALGORITHMS

When a page fault takes place, the operating system has to choose a page to remove (remove from memory) to make room for the incoming page. If the page to be removed has been customized while in memory, it must be rewritten to the disk to bring the disk copy up to date. If,

The Not Recently Used Page Replacement Algorithm
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The Not Recently Used Page Replacement Algorithm

In order to allow the operating system to collect useful page usage statistics, most computers with virtual memory have two status bits associated with each page. R is set whenever the page is referenced (read or written). M is set when the page is written to (i.e., modified).

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