chapter 4 memory management-part1

8/6/2019 Chapter 4 Memory Management-part1

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Operating Systems:

,William Stallings

ap er

M m r M n m n

Dave Bremer

Otago Polytechnic, N.Z.2009, Prentice Hall


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The need for memorymanagement

,

cheaper

more memory, there is never enough!

,blocks of data from secondary storage.

emory s s ow compare o a The OS must cleverly time the swapping to

max m se e s e c ency


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emory nee s to e a ocate to ensure a

reasonable supply of ready processes toconsume ava a e processor me


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Memory ManagementRequirements

Protection

ar ng

Logical organisation Physical organisation


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program will be placed in memory when it,

it may be swapped to disk and return to main

Memory references must be translated to


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memory locations in another process

Impossible to check absolute addresses at

Must be checked at run time


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Requirements: LogicalOrganization

Programs are written in modules

o u es can e wr en an comp eindependently

eren egrees o pro ec on g ven omodules (read-only, execute-only)

Share modules among processes Se mentation hel s here


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Requirements: PhysicalOrganization

responsibility to manage memory

data may be insufficient ver ay ng a ows var ous mo u es o eassigned the same region of memory but is

Programmer does not know how much


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Dynamic Partitioning

mp e ag ng

Simple Segmentation Virtual Memory Paging


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.

The programmer must design the program

Main memory use is inefficient.

, ,an entire partition.

.


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Solution Unequal SizePartitions

but doesnt solve completely

. ,

Programs up to 16M can be

Smaller programs can be placed in

,fragmentation


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-

Placement is trivial (no options)

-

Can assign each process to the smallest

Queue for each partition

rocesses are ass gne n suc a way as ominimize wasted memory within a partition


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Remaining Problems withFixed Partitions

by the system . -

partitions

not use the space efficiently

methods


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number

memory as required


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Dynamic PartitioningExample

Ex rn l Fr m n i n

Memory external to allP2

Can resolve using

(20M)

Empty (6M)

OS moves processes so(14M)

(56M)Empty (6M)

Time consuming and

P3(18M)

Empty (4M)

Refer to Figure 7.4


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block to allocate to a process-

Chooses the block that is closest in size to the

Worst performer overall

nce sma es oc s oun or process, esmallest amount of fragmentation is left


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-

Scans memory form the beginning and

enough

May have many process loaded in the frontend of memor that must be searched over

when trying to find a free block


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actual (absolute) memory locations are

A process may occupy different partitions

locations during execution

wapp ng Compaction


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Reference to a memory location independent.

Relative

some known point.

The absolute address or actual location in

.


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Registers Used during

Execution

Starting address for the process

Ending location of the process These values are set when the process is

loaded or when the process is swapped in


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Registers Used during

Execution

a relative address to produce an absolute

The resulting address is compared with

If the address is not within bounds, an

n errup s genera e o e opera ngsystem

chapter 4 memory management-part1

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