Lecture 11 Virtual Memory by Rab Nawaz Jadoon4

8/12/2019 Lecture 11 Virtual Memory by Rab Nawaz Jadoon4

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Department of Computer Science

DCS

COMSATS Institute ofInformation Technology

Virtual MemoryRab Nawaz Khan adoonAssistant Professor

COMSATS Lahore

Pakistan

Operating System Concepts


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Virtual Memory

In computing, virtu l memory is a memorymanagement technique developed formultitasking kernels.

Virtual memory system provides processes with the

illusion that they have more memory than is builtinto the computer.

There are two types of addresses in virtual memorysystems.

Those referenced by processes and, Those available in main memory

The addresses that processes references are calledvirtual addresses.

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Virtual Memory

The addresses that processes references arecalled virtual addresses.

Those available in the main memory are called,Real addresses or physical addresses. Whenever a process accesses a virtual address, they system

must translate in to real address.

Virtual memory systems contain special purpose hardwarecalled the Memory Management Unit (MMU), that quickly maps

virtual addresses to real addresses.

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Virtual memory

4

Chip

Address Space Virtual Memory

Disk Drive

PhysicalMemory


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How to increase Virtual Memory

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Virtual memory

The purpose of virtual memory is to enlarge theaddress space, the set of addresses a programcan utilize.

virtual memory might contain twice as many

addresses as main memory.

A program using all of virtual memory, therefore,would not be able to fit in main memory all at once.

Nevertheless, the computer could execute such a

program by copying into main memory thoseportions of the program needed at any given pointduring execution.

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Virtual Memory

To facilitate copying virtual memory into realmemory, the operating system divides virtualmemory into pages, each of which contains afixed number of addresses.

Each page is stored on a disk until it is needed.

When the page is needed, the operating systemcopies it from disk to main memory, translating thevirtual addresses into real addresses.

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Virtual Memory

The process of translating virtual addresses intoreal addresses is called mapping.

The copying of virtual pages from disk to mainmemory is known as pagingor swapping.

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Virtual Memory

Application programs and data are broken upinto small segments, called pages.

Only a small number of these pages are actuallyresident in RAM; the rest are stored in a temporary

area on the hard disk known as a swap file. When the application refers to data that is not

currently in a page in RAM, a page fault occurs.

The operating system then decides which pages in

RAM are least likely to be needed soon, and removesthese pages.

The required data is then moved from the swap file into thepage frames left empty from the previous pages.

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Virtual Memory

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RAM


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Virtual Memory

When a program is executing, the CPU is

continuously fetching and decoding millions ofinstructions every second. To obtain the nextinstruction or piece of data, the CPU uses thefollowing search path.

Look for the instruction in the L1 cache (built into the CPU). If not found in L1 cache, look in the L2 cache (found on

SRAM chips).

If not found in L2 cache, look for the instruction in RAM(disk cache).

If not found in the disk cache, look for the instruction inRAM (where the program is resident).

If not found in resident RAM ("page fault"), go to the HDDand get the appropriate page(s) from the swap file.

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Virtual Address Space

A process virtual address space is V is the rangeof virtual addresses that a process mayreference.

The range of the real addresses called, real or

physical address space R.

The number of addresses in V is represented |V| andthe number of addresses in R is denoted |R|.

In virtual memory systems, it is normally the case

that |V| >> |R|, i.e. the virtual address space ismuch larger than the real address space.

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Two level storage

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Other types of

Auxiliary storage

Processor (s) Main memory andCaches

Secondary or auxiliary orBacking storage


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Implementation detail

A key to implementing virtual memory systemsis how to map virtual addresses to physicaladdresses.

The system must map or translate the virtual

address into real address as process execute.

The system must perform the translation veryquickly, otherwise the performance degradedgracefully.

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Dynamic Address Translation (DAT)

To convert virtual addresses to physicaladdresses during execution.

The system that exhibit the property of DAT, thecontiguous addresses in a processs virtual address

space need not to be contiguous in physical memory,this is called artificial contiguity.

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Artificial contiguity

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Virtual Memory

Physical Memory

Address MappingMechanism

Co

ntiguousVirtual

memory

Locations


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Terminology

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Virtual address Space Real Address Space

A processs virtual address spacerefers to the set of addresses a

process may reference to accessmemory when running on avirtual memory system.

Process do not see the physicaladdresses, which locate physical

locations in main memory.


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Block Mapping

DAT mechanisms must maintain addresstranslation maps indicating which regions of aprocesss virtual address space V are currentlyin main memory and where there are located.

If this mapping had to contain entries for everyaddress in V, then this required more space inavailable memory, so this is infeasible.

For this purpose the information is grouped in blocks.

The system then keep track where in main memoryeach virtual memory block has been placed.

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Mapping

Either the block size should be all the same orof different sizes.

When blocks are of fixed size, they are called pagesand the associated virtual memory organization is

called paging. When blocks may of different sizes, they are called

segments and the associated virtual memoryorganization is called segmentation.

Some system combine the two techniques,implementing segments as variable size blockscomposed of fixed size pages.

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M i i l dd l


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Mapping virtual address to realaddress

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Virtual MemoryPhysical Memory

Address mappingmechanism


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Block Mapping

In the block mapping the system representsaddresses as ordered pairs.

To refer to a particular item in the processs virtualaddress space, a process specifies the block in which

the item resides and the displacement/offset of theitem from the start of the block.

A virtual address vis denoted by an ordered pairv(b,d), where b is the block number and d is the

displacement from the start of the block.

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Block mapping

Virtual address format in a block mappingsystem

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Block Number

B

Displacement

d

Virtual Address

V=(b,d)

Vi t l Add T l ti (Bl k


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Virtual Address Translation (BlockMapping)

The translation of V=(b,d) to real memoryaddress rproceed as follows,

The system maintains in memory a block map tablefor each process and entries are kept in sequential

order.At context switch time the system determines the

real address, a, that corresponds to the address inmain memory of the new process, block map table.

The system load this address into a high speedspecial purpose register called block map table originregister (BMTOR).

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During execution the process reference v=(b,d).

The system add the block number bto the baseaddress aof the processs block map table to formthe real address of the entry for block b in the blockmap table.

This entry contains the address b, for the start ofthe block bin main memory.

The system then adds the displacement d to theblock start address, b, to form the desired real

address r. Thus the system compute the real address r from the virtual

address v through the equation r = b+ d where b is storedin the block map table cell located at the real memoryaddress a+b.

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Issues

Two addition should be computed very fast, notordinary machine language instruction addition.

Accessing the information from the table forentries should be so fast.

If not, then the systems performance will bedegraded gracefully.

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Paging

A virtual address in a paging system is v=(p,d),where pis the number of the page in virtualmemory on which the referenced item residesand d is the displacement within page pat

which the referenced item is located.

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Page Number

p

Displacement

d

Virtual AddressV=(p,d)


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Paging

A process may run if the page it is currentlyreferencing is in main memory.

When the system transfers a page from secondarystorage to RAM, it places the page in the RAM block,

called a page frame, that has the same size as theincoming page.

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Dynamic Address Translation


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Dynamic Address Translation(Paging)

DAT under paging proceeds as follows,A running process references a virtual memory

address v=(p,d).

A page mapping mechanism, looks up page pin the

processs page map table and determines that pagepis in page frame p.

P is a page frame number, not a physical memoryaddress.

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Page fault

Whenever a process reference a page, that isnot in the main memory, the processorgenerates a page fault, which invokes theoperating system to load the missing page into

memory from secondary storage.

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Page table entry (PTE)

A page table is the data structure used by avirtual memory system in a computer operatingsystem to store the mapping between virtualaddresses and physical addresses.

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Page resident bit

r

Secondary storage address

(if page is not in RAM), s

Page frame number

If page is in RAM, p


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Page table entry

A resident bit ris set to 0 if the page is not inthe main memory and 1 if it is.

If the page is not in the RAM, then sis itssecondary storage address.

If the page is in main memory, then pis itsframe number.

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Page address translation by direct


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Page address translation by directmapping

A process references virtual address v=(p,d)

At context switch time the OS loads the mainmemory address of a processs page table into thepage table origin register (PTOR).

To determine the main memory address that

corresponds to the referenced virtual address, thesystem first adds the processs page table baseaddress, b, to the reference page number, p.

Thus b+pis the main memory address of the PTE for

page p. This PTE indicates that virtual page pcorresponds to

page frame p. The system then concatenates pwiththe displacement dto form the real address.

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Paging address translation by direct


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Paging address translation by directmapping

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Issues

In direct mapping RAM is to be accessed twice. Once for accessing the page table and then to access

the real address from the frame, actually slows theprocessor speed.

As there is a Page table for each process is kept inmain memory, so in this method the RAM is occupiedby these Page tables of the processes instead ofprocesses.

Solution: To keep these table into the content addressed memory i.e.

associative memory.

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Paging address translation by


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Paging address translation byassociative memory

One way to increase the performance of theDAT is to place the entire page table into acontent addressed associative memory.

Which has a cycle time greater than main memory.

A process refer to the virtual address v=(p,d).

Every entry in the associative memory is searchedsimultaneously for page p.

The search returns pas the page frame

corresponding to page p.

Pis then concatenated with d to form the realaddress.

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Paging address translation by


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Paging address translation byassociative memory

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Issues

This mapping has only one drawback that it isvery expansive.

Only small amount of memory can be offered by thecomputer system, so we cant keep large table on

associative memory.

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Paging address translation with


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Paging address translation withcombined associative/direct mapping

So in the direct mapping all the page tables arein RAM, so many process cant reside inmemory.

It also slows down the processor speed. Then

associative mapping was developed, which was fastbut very expansive.

If a process is very large and contains many pagesthen all the page entries cant be included in the

associative memory. Due to this combined paging address translation was

an option.

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Paging address translation with


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Paging address translation withcombined associative/direct mapping

It is often called TLB (translation look aside buffer).

In combined approach the page map tables aredivided into two parts.

One is in RAM contain those pages which are not

frequently used, and another is in Associativememory, the pages that has cluster of statements.

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Steps

Running process references virtual addressv=(p,d).

The address translation mechanism first sendscontrol to the associative page map.

If pis found then the frame number of thecorresponding page found simultaneously and thisframe number is concatenated with displacement dto form real address r.

If there is TLB miss, the processor must access mainmemory to obtain the page frame number.

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Paging address translation with combined


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Paging address translation with combinedassociative/direct mapping

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PTORVirtual AddressV=(p,d)

Performedo

nly

Ifnomatch

in

associativem

ap

Direct Map(All pages)

Translation lookaside Buffer (TLB), AssociativeMemory

Attempt

associative

SearFirs

t


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Sharing in paging systems

In multiprogramming/time sharing systems, it iscommon for the user to execute the sameprogram concurrently.

If the system allocated the individual copies, then

much of the memory would be wasted. The obvious solution is to share those pages

common to individual processes.

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Sharing in paging systems

The system must carefully control sharing toprevent one process from modifying data thatanother process is accessing.

Mostly today system that implements sharing,

programs are divided into separate procedure anddata areas.

Non modifiable procedures / pure procedures / reentrantprocedures

These can be shared.

Modifiable procedures

Cant shared without careful concurrency control

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Segmentation

Dividing virtual address spaces into variable-length segments.

A virtual address here consists of a segment numberand an offset within the segment.

Each segment consists of contiguous locations, However the segments need not to be the same size nor

must they be placed adjacent to one another in mainmemory.

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Virtual memory

Paged Virtual Memory

Memory divided into fixed sized pages

Each page has a base physical address

Segmented Virtual Memory

Memory is divided into variable length segments Each segment has a base physical address + length

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Segmentation

In a VM segmentation system, we will have tomaintain only those segments in main memorythat a program requires to execute at a certaintime.

The remainder of the segments reside on secondarystorage.

Under pure segmentation, the system transfers asegment from secondary storage as a complete unit

and places all the locations within that segment incontiguous location in main memory.

Incoming segment can be placed in any availablearea in RAM that is large enough to hold it.

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Segmentation

The placement for segmentation is identical tothose used in variable partitionmultiprogramming.

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Segmentation

A virtual memory segmentation address is anordered pair v=(s,d), where sis the segmentnumber in which the referenced item residesand dis the displacement within segment sat

which the reference item is located.

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Segment Number

s

Displacement

d

Virtual AddressV=(s,d)


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Quiz

Segmented virtual memory systems do notincur fragmentation??? (True/False)

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Answer:False: Segmented virtual memory systems can incur external fragmentationexactly as in variable partition multiprogramming systems.

Segmentation address translation


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Seg e tat o add ess t a s at oby direct mapping

A process references a virtual address v=(s,d)to determine where in main memory thereferenced segment resides.

The system adds the segment number sto the

SMTOR. The resulting value b+s is the location of the

segments table map entry.

Each entry contains several pieces of information

about the segment, which the mapping mechanismuses to translate the Virtual address to physicaladdress.

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Segmentation address translation


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gby direct mapping

If the segment currently resides in mainmemory, the entry contains the segments mainmemory starting address s.

The system adds the displacement dto this address

to form the referenced locations real memoryaddress r=s+d.

We cannot simply concatenate dto s, becausesegments are of variable size.

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Direct Mapping

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Segment Number

s

Displacement

d


Base address, b, of

Segment table

SMTOR

+

+

Segment Number

S

Displacement

d

SegmentMap Table

b + s

sb

s + d

s

d

s

Real Addressr=(s,d)


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Segment map table entry (SMTE)

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r a l s

Segmentresident bit

SecondaryStorage addressIf not present inRAM

SegmentLength

Protectionbits

Base addressof Segment


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Segmentation

Resident bit

It shows whether or not the segment is currently inmain memory, if it is then sis the main memoryaddress at which the segment begins.

Secondary storage address

If segment is not present in RAM, then ais thesecondary address from which the segment must beretrieved before the process may proceed.

Segment length It specifies the length of the segment referenced. If

it is out of the range then an overflow exception isgenerated.

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Segmentation

Protection bits These are checked to ensure that the operation

being attempted is allowed. If it is then the baseaddress, s, of the segment in main memory is added

to the displacement, d, to form the real addressr=s+d.

If the operation is not allowed, then a segmentprotection exception is generated, causing the OS togain control of the system and terminate theprocess.

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Segmented Address Translation


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gwith Associate Mapping

Running process references virtual addressv=(s,d).

The system sends the control to the associativememory and every entry in the associative memoryis searched simultaneously for segment s.

If r=1, then displacement dis checked against thelength of the segment.

If d


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gwith Associate Mapping

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Segment Number

s

Displacement

d


s+

Segment Number

S

Displacement

d

Associative Memory

s

Real Addressr=(s,d)

s + d


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Segmentation/paging Systems

In this technique the program is first dividedinto segment of variable length logically.

Segments are usually arranged across multiplepages.

All the pages of the segment need not be in mainmemory at once, and virtual memory pages that arecontiguous in virtual memory need not to becontiguous in main memory.

Under this scheme the virtual address is

implemented as an ordered pair, v = (s,p,d), where sis the segment#, pis the page within the segmentand dis the displacement within the page at whichthe desired item is located.

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DAT in a Segmentation/Paging


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g g gSystem

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System

A process references virtual address v=(s,p,d). The most recently referenced pages have entries in

the associative memory map i.e. TLB.

The translation mechanism performs an associative

search to locate (s,p). If found then it returns p, thepage frame in which page presides.

This value is concatenated with dto form the realaddress r.

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System

If the TLB does not contain an entry for (s,p),the processor must perform a complete directmapping as follow,

The base address bof SMT (in RAM) is added tosegment s, to form the address b+s.

This address corresponds to the physical memorylocation of the segments entry in the SMT.

The SMT entry indicates the base address sof thepage table (in RAM) for segment s.

The processor adds the page pto the base addresss, to form the address of the page table entry forpage pof segment s.

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System

This table entry indicates that pis the pageframe number corresponding to virtual page p.

This frame number pis concatenated with thedisplacement dto form the real address r.

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Documents

Lecture 11 Virtual Memory by Rab Nawaz Jadoon4