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Chapter 7Memory Management

Operating Systems:Internals and Design Principles, 6/E

William Stallings

Dave Bremer Otago Polytechnic, N.Z.

2009, Prentice Hall


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R oadmap

Basic requirements of MemoryManagement

Memory PartitioningBasic blocks of memory management Paging

Segmentation


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T he need for memory

managementMemory is cheap today, and gettingcheaper

But applications are demanding more andmore memory, there is never enough!

Memory Management, involves swappingblocks of data from secondary storage.Memory I/O is slow compared to a CPU T he OS must cleverly time the swapping to

maximise the CPUs efficiency


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Memory Management

M emory needs to be allocated to ensure areasonable supply of ready processes toconsume available processor time


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Memory ManagementR

equirementsR elocationProtection

SharingLogical organisationPhysical organisation


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R equirements: R elocation

T he programmer does not know where theprogram will be placed in memory when it

is executed, it may be swapped to disk and return to mainmemory at a different location (relocated)

Memory references must be translated tothe actual physical memory address


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Memory ManagementT

ermsTerm DescriptionFrame Fixed - length block of main

memory.Page Fixed - length block of data in

secondary memory (e.g. on disk).Segment Variable-length block of data that

resides in secondary memory.

Table 7.1 Memory Management Terms


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A ddressing


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R equirements: Protection

Processes should not be able to referencememory locations in another processwithout permissionImpossible to check absolute addresses atcompile timeMust be checked at run time


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R equirements: Sharing

A llow several processes to access thesame portion of memory

Better to allow each process access to thesame copy of the program rather thanhave their own separate copy


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R equirements: Logical

OrganizationMemory is organized linearly (usually)Programs are written in modules

Modules can be written and compiledindependently

Different degrees of protection given tomodules (read -only, execute -only)Share modules among processesSegmentation helps here


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R equirements: Physical

OrganizationCannot leave the programmer with theresponsibility to manage memory

Memory available for a program plus itsdata may be insufficient Overlaying allows various modules to be

assigned the same region of memory but istime consuming to program

Programmer does not know how muchspace will be available


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T ypes of Partitioning

Fixed PartitioningDynamic Partitioning

Simple PagingSimple SegmentationVirtual Memory Paging

Virtual Memory Segmentation


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Fixed Partitioning

Equal -size partitions (see fig 7.3a) A ny process whose size is less than

or equal to the partition size can beloaded into an available partition

T he operating system can swap aprocess out of a partition If none are in a ready or running

state


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Fixed Partitioning Problems

A program may not fit in a partition. T he programmer must design the program

with overlays

Main memory use is inefficient. A ny program, no matter how small, occupies

an entire partition.

T his is results in internal fragmentation.


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Solution Unequal Size

PartitionsLessens both problems but doesnt solve completely

In Fig 7.3b, Programs up to 16M can be

accommodated without overlay Smaller programs can be placed in

smaller partitions, reducing internalfragmentation


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Placement A lgorithm

Equal -size Placement is trivial (no options)

Unequal -size Can assign each process to the smallest

partition within which it will fit Queue for each partition Processes are assigned in such a way as to

minimize wasted memory within a partition


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Fixed Partitioning


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R emaining Problems with

Fixed PartitionsT he number of active processes is limitedby the system I.E limited by the pre -determined number of

partitionsA large number of very small process willnot use the space efficiently In either fixed or variable length partition

methods


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Dynamic Partitioning

Partitions are of variable length andnumber

Process is allocated exactly as muchmemory as required


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ExampleE xternal FragmentationMemory external to allprocesses is fragmentedCan resolve usingcompaction OS moves processes so

that they are contiguous T ime consuming and

wastes CPU time

OS (8M)

P1(20M)

P2(14M)

P3(18M)

Empty(56M)

Empty (4M)

P4(8M)

Empty (6M)

P2(14M)

Empty (6M)

R efer to Figure 7.4


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Operating system must decide which freeblock to allocate to a process

Best-fit algorithm Chooses the block that is closest in size to the

request Worst performer overall Since smallest block is found for process, the

smallest amount of fragmentation is left Memory compaction must be done more often


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First- fit algorithm Scans memory form the beginning and

chooses the first available block that is largeenough

Fastest May have many process loaded in the front

end of memory that must be searched over when trying to find a free block


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Next- fit Scans memory from the location of the last

placement More often allocate a block of memory at the

end of memory where the largest block isfound

T he largest block of memory is broken up intosmaller blocks Compaction is required to obtain a large block

at the end of memory


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A llocation


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Buddy System

Entire space available is treated as asingle block of 2 U

If a request of size s where 2U -1

< s


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Example of Buddy System


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T ree R epresentation of

Buddy System


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R elocation

When program loaded into memory theactual (absolute) memory locations aredeterminedA process may occupy different partitionswhich means different absolute memorylocations during execution Swapping Compaction


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A ddresses

Logical R eference to a memory location independent

of the current assignment of data to memory.R elative A ddress expressed as a location relative to

some known point.

Physical or A bsolute T he absolute address or actual location in

main memory.


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R elocation


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R egisters Used during

ExecutionBase register Starting address for the process

Bounds register Ending location of the processT hese values are set when the process isloaded or when the process is swapped in


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R egisters Used during

ExecutionT he value of the base register is added toa relative address to produce an absoluteaddressT he resulting address is compared withthe value in the bounds register If the address is not within bounds, aninterrupt is generated to the operatingsystem


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Paging

Partition memory into small equal fixed -size chunks and divide each process intothe same size chunksT he chunks of a process are called pagesT he chunks of memory are called frames


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Paging

Operating system maintains a page tablefor each process Contains the frame location for each page in

the process Memory address consist of a page number

and offset within the page


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Processes and Frames

A .0 A .1 A .2 A

.3B.0B.1B.2C.0C.1C.2C.3

D.0D.1D.2

D.3D.4


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Page Table


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Segmentation

A program can be subdivided intosegments Segments may vary in length T here is a maximum segment lengthA ddressing consist of two parts a segment number and an offset

Segmentation is similar to dynamicpartitioning


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Paging


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Segmentation

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