Virtual Memory

Virtual Memory

Virtual memory is a technique which gives an application program the impression that it has contiguous working memory, while in fact it may be physically fragmented and may overflow on to disk storage.

Course Name: Operating Systems Level(UG/PG): UG Author(s) : Phani Swathi ChittaMentor: Aruna Adil

*The contents in this ppt are licensed under Creative Commons Attribution-NonCommercial-ShareAlike 2.5 India license

Learning ObjectivesAfter interacting with this Learning Object, the learner will be able to:• Explain the concept of virtual memory

Definitions of the components/Keywords:

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1 Glossary:

• Virtual Memory: It is a technique which gives an application program the impression that it has contiguous address space but physically it is fragmented in the main memory and secondary storage devices.

• Physical Memory: It refers to the real storage and describes the total amount of memory installed in the computer. Usually consists of Random Access Memory (RAM) and hard drives.

• Page Table: A page table is the data structure used by a virtual memory system in a computer operating system to store the mapping between virtual addresses and physical addresses. It has the ability to mark an entry invalid through valid-invalid bit.

• TLB: TLB [Translation Lookaside Buffer] is a table in the processor’s memory that contains information about the pages in memory the processor has accessed recently. The table cross-references a program’s virtual addresses with the corresponding absolute addresses in physical memory that the program has most recently used.


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1 Glossary:• Paging: In the paging memory-management scheme, the operating system retrieves data from secondary storage in same-size blocks called pages.

•Memory Management Unit: A memory management unit (MMU), sometimes called paged memory management unit (PMMU), is a computer hardware component responsible for handling accesses to memory requested by the CPU. Its functions include translation of virtual addresses to physical addresses (i.e., virtual memory management), memory protection, cache control, bus arbitration and in simpler computer architectures (especially 8-bit systems), bank switching.

• Demand Paging: A refinement of paging in which it is not necessary to load in the entire contents of the text and data segments of a program from disk when the process is started. Instead, pages are brought into memory as they are accessed.

• Lazy Swapper: A lazy swapper never swaps a page into memory unless that page will be needed.


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1 Glossary:

• Virtual Address Space: The virtual address space of a process refers to the logical view of how a process is stored in memory.

• Thrashing: It is a degenerate situation where large amounts of computer resources are used to do a minimal amount of work.

• Segmentation: Segmentation is a memory management technique in which memory is divided into variable sized chunks which can be allocated to processes. Each chunk is called a segment.


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1Virtual Memory is a technique which gives an application

program the impression that it has contiguous working memory, while in fact it may be physically fragmented and may overflow on to disk storage.

Programs use virtual addresses than real addresses to atore data. When the program is actually executed, the virtual addresses are converted into real memory addresses.

When a process requests access to its memory, it is the responsibility of the operating system to map the virtual address to the physical address.

A page table is used to store mapping between virtual addresses and physical addresses.

Analogy

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1 Person

House

Bank Need Fulfilled

No

Looks for money

Got enough money?

Yes

Goes to the Bank

Here HOUSE acts as the physical memory and BANK acts as the backing store

Analogy

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1 • If a person needs money, first he will check in his house.

• If he cannot find out enough money there then he will go to the bank and withdraw the money that he needs.

• Here bank is analogous to virtual storage space and house or wallet is analogous to the physical space.

Master Layout 1

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1

• Give a PLAY button• Give PAUSE and RESTART/CLEAR buttons• Give two radio buttons FAST and SLOW

Step 1: 1

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4Instruction for the animator Text to be displayed in the working area (DT)

• When the user clicks PLAY, show fig in the master layout

• The text in DT should appear in parallel to the figure

• The process of finding the page is shown in the next slides

• Follow the figures and the flow of finding the page should be accordingly

• Programs use virtual address to store data • When a program is executed, virtual address is converted to real logical

address• A page table is used to store mapping between virtual address and logical

address

Step 2: 1

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Page 0

Instruction for the animator Text to be displayed in the working area (DT)• The action showed in the figure must be

performed • The text in DT should appear in parallel to

the figure

• Memory Access of Page 0• Here Application program tries to access page 0• So first the TLB is checked for the virtual address 000v

Step 3: 1

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• The action showed in the figure must be performed


• It exists in TLB and it means a TLB Hit which says page 0 exists in physical memory itself

• SO Page 0 can be accessed directly from physical memory

Step 4: 1

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• Page 0 can be accessed directly from physical memory since it is found in physical memory

Step 5: 1

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Page 1



the figure


Step 6: 1

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• Page 1 does not exist in TLB and it means a TLB miss• Now page table is to be checked

Step 7: 1

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• Page table is checked for 111v and is not present which results in a page fault

• Now secondary memory is to checked for 111v

Step 8: 1

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• Now secondary memory is checked for 111v• It exists there• Now page 1 will be shifted from secondary memory to physical memory• Appropriate updating is done in TLB and page table

Step 9: 1

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Page 2



the figure


Step 10: 1

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Step 11: 1

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Step 12: 1

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Page 3



the figure


Step 13: 1

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Step 14: 1

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• Page table is checked for 333v and is not present which results in a page fault

• Now secondary memory is to checked for 333v

Step 15: 1

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• Now secondary memory is checked for 333v• It exists there• Now page 3 will be shifted from secondary memory to physical memory• Appropriate updating is done in TLB and page table

Step 16: 1

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Page 4



the figure


Step 17: 1

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4 Instruction for the animator Text to be displayed in the working area (DT)• The action showed in the figure must be


the figure



Step 18: 1

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Step 19: 1

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Page 5



the figure


Step 20: 1

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Step 21: 1

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• Page 5 is found in page table

Step 22: 1

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• Now page 5 is present in page table but not in TLB• So an entry should be done in TLB• Now TLB is updated

Step 23: 1

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• Now page 5 exists in both TLB and page table• The logical address will be got from TLB and can access page 5 from

physical memory

Step 24: 1

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• Page 5 is found in physical memory

Step 25: 1

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Page 6



the figure


Step 26: 1

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Step 27: 1

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• Page 6 is not present in page table which results in a page fault. Now secondary memory is to be checked for 666v

Step 28: 1

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• 666v is present in secondary memory. Now it has to be moved to physical memory

• But physical memory is full. So we have to make use of some replacement algorithm to make room available for page 6

Step 29: 1

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• Here LRU [Least Recently Used] algorithm is used• Then page 0 which is least recently used, is moved to secondary memory

and page 6 is now present in physical memory• Appropriate updates are done in TLB, page table and secondary memory

Step 30: 1

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Page 7



the figure


Step 31: 1

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• Page 7 does not exist there and it means a TLB miss• So page table is to be checked

Step 32: 1

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• Page 7 is not present in page table also and which results in a page fault• Now secondary memory is to be checked for 777v

Step 33: 1

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• Page 7 is not present in secondary memory too.• Now 777v exists in neither page table nor in secondary memory• It means no translation is available for 777v – that is page 7 is not present

anywhere• It may be due to some program error. So either system will provide some

error message or OS will use some techniques to handle the ________

Introduction

Credits

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Definitions Test your understanding (questionnaire) Lets Sum up (summary) Want to know more…

(Further Reading)

Try it yourself

Interactivity:

Analogy

Slide 1

Slide 3

Slide 44-46

Slide 47

Electrical Engineering

• Give page numbers in the application program as radio buttons• Give two radio buttons as FAST and SLOW

•The same algorithm as in demo is followed• When a page is selected, show the process how the page is found• The layout must be the same as in slide 10

Questionnaire1. Which is bigger, virtual memory or physical memory?

Answers: a) Virtual Memory b) Physical Memory c) Any of them

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Questionnaire

2. How are the pages actually kept in the physical memory which are referenced in the virtual memory?

Answers: a) Contiguously Pages are kept non-contiguously in physical memory b) Non-contiguously c) Can’t say Pages are kept non-contiguously in physical memory

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Questionnaire3. The advantages of virtual memory are:

i) Programmer don’t need to bother about the size of the programii) Number of processes that can reside in main memory is less

iii) Less I/O would be needed to load or swap a programAnswers: a) i & iicomment - Number of processes that can reside in main

memory is moreb) i & iiic) ii & iii d) None

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Links for further readingReference websites:http://en.wikipedia.org/wiki/Virtual_memoryhttp://en.wikipedia.org/wiki/Page_table

Books:

Virtual Memory

Documents

physical memory

virtual memory management

memory protection

processors memory

main memory

virtual memory system

virtual memoryvirtual

memory management technique