“Memory Management Function”

Presented By

Lect. Rimple Bala

GPC,Amritsar

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Contents

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Basic memory management Swapping Virtual memory Paging Page replacement algorithms Modeling page replacement algorithms Design issues for paging systems Implementation issues Segmentation

Memory Management Memory – a linear array of bytes

Holds O.S. and programs (processes)Each cell (byte) is named by a unique memory address

Recall, processes are defined by an address space, consisting of text, data, and stack regions

Process executionCPU fetches instructions from the text region

according to the value of the program counter (PC)Each instruction may request additional operands from

the data or stack region

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Memory Management Ideally programmers want memory that is

largefastnon volatile

Memory hierarchy small amount of fast, expensive memory – cache some medium-speed, medium price main memorygigabytes of slow, cheap disk storage

Memory manager handles the memory hierarchy

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Multiprogramming with Fixed Partitions

Fixed memory partitionsseparate input queues for each partitionsingle input queue

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Modeling Multiprogramming

CPU utilization as a function of number of processes in memory

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Degree of multiprogramming

Names and Binding

Symbolic names Logical names Physical names○ Symbolic Names: known in a context or path

file names, program names, printer/device names, user names

○ Logical Names: used to label a specific entity job number, major/minor device numbers, process id (pid), uid.

○ Physical Names: address of entityinode address on disk or memoryentry point or variable addressPCB address

Address Binding Address binding

fixing a physical address to the logical address of a process’ address space

Compile time bindingif program location is fixed and known ahead of time

Load time bindingif program location in memory is unknown until run-time

AND location is fixed

Execution time bindingif processes can be moved in memory during executionRequires hardware support

P: : push ... jmp 175 :

foo: ...

0

100

175

LibraryRoutines

P: : push ... jmp 1175 :

foo: ...

1000

1100

1175

LibraryRoutines

P: : push ... jmp 1175 :

foo: ...

1000

1100

1175

LibraryRoutines

P: : push ... jmp 175 :

foo: ...

0

100

175

LibraryRoutines

1000

Base register

Execution Time Address Binding

Load Time Address Binding

Compile Time Address Binding

Dynamic relocation with a base register

Memory Management Unit (MMU) - dynamically converts logical addresses into physical address

MMU contains base address register for running process

process i

Operatingsystem

Max addr

0

Max Mem

0

Physical memory address

Relocation register for process i

1000

+

MMU

Program generated address

Protection using base & limit registers

Memory protectionBase register gives starting address for processLimit register limits the offset accessible from the

relocation register

base

+

Physicaladdress memory

register

<

limitregister

yes

no

addressing error

logicaladdress

Logical vs. Physical Address Space

The concept of a logical address space that is bound to a separate physical address space is central to proper memory management.○ Logical Address: or virtual address - generated by CPU○ Physical Address: address seen by memory unit.

Logical and physical addresses are the same in compile time and load-time binding schemes

Logical and physical addresses differ in execution-time address-binding scheme.

04/20/23 12Edusat lect. Rimple GPC,Amritsar

Features of Memory Management Relocation

1. Static

2. Dynamic

Protection Sharing Logical organization Physical organization Memory Compaction

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Various Methods used by OS

Swapping

Virtual Memory

Paging

Segmentation

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Swapping

Memory allocation changes as processes come into memoryleave memory

Shaded regions are unused memory15

Swapping

Allocating space for growing data segment Allocating space for growing stack & data segment

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Memory Management with Bit Maps

Part of memory with 5 processes, 3 holestick marks show allocation unitsshaded regions are free

Corresponding bit map Same information as a list

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Memory Management with Linked Lists

Four neighbor combinations for the terminating process X

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Managing memory with linked lists Searching the list for space for a new process

First FitNext Fit

○ Start from current location in the listBest Fit

○ Find the smallest hole that will work○ Tends to create lots of really small holes

Worst Fit○ Find the largest hole○ Remainder will be big

Quick Fit○ Keep separate lists for common sizes

Virtual Memory Virtual Memory

○ Separation of user logical memory from physical memory.

○ Only PART of the program needs to be in memory for execution.

○ Logical address space can be much larger than physical address space.

○ Need to allow pages to be swapped in and out.

Virtual Memory can be implemented viaPagingSegmentation

Virtual Memory

The position and function of the MMU

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Paging

The relation betweenvirtual addresses

and physical memory addresses given

bypage table

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Page Tables

Internal operation of MMU with 16 4 KB pages23

Page Tables

32 bit address with 2 page table fields Two-level page tables

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Top-level page table

Page Tables

Typical page table entry

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TLBs – Translation Look aside Buffers

A TLB to speed up paging

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Inverted Page Tables

Comparison of a traditional page table with an inverted page table

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Page Replacement Algorithms Page fault forces choice

which page must be removedmake room for incoming page

Modified page must first be savedunmodified just overwritten

Better not to choose an often used pagewill probably need to be brought back in soon

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Page Replacement Algorithms

Optimal Page Replacement

Not Recently Used Page Replacement Algorithms.

FIFO Page Replacement Algorithms

Second Chance Algorithms

Clock Page Replacement Algorithms

Least Recently used

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Optimal Page Replacement Algorithm

Replace page needed at the farthest point in futureOptimal but unrealizable

Estimate by …logging page use on previous runs of processalthough this is impractical

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Not Recently Used Page Replacement Algorithm

Each page has Reference bit, Modified bit bits are set when page is referenced, modified

Pages are classified1. not referenced, not modified

2. not referenced, modified

3. referenced, not modified

4. referenced, modified

NRU removes page at random from lowest numbered non empty class

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FIFO Page Replacement Algorithm

Maintain a linked list of all pages in order they came into memory

Page at beginning of list replaced

Disadvantagepage in memory the longest may be often used

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Second Chance Page Replacement Algorithm

Operation of a second chancepages sorted in FIFO orderPage list if fault occurs at time 20, A has R bit set

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The Clock Page Replacement Algorithm

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Least Recently Used (LRU) Assume pages used recently will used again soon

throw out page that has been unused for longest time

Must keep a linked list of pagesmost recently used at front, least at rearupdate this list every memory reference !!

Alternatively keep counter in each page table entrychoose page with lowest value counterperiodically zero the counter

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Review of Page Replacement Algorithms

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Modeling Page Replacement Algorithms Belady's Anomaly

FIFO with 3 page frames FIFO with 4 page frames P's show which page references show page faults

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Stack Algorithms

State of memory array, M, after each item in reference string is processed

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7 4 6 5

Page Size

Small page size Advantages

less internal fragmentation better fit for various data structures, code sectionsless unused program in memory

Disadvantagesprograms need many pages, larger page tables

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Page Size

Overhead due to page table and internal fragmentation

Where s = average process size in bytes p = page size in bytes e = page entry

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2

s e poverhead

p

page table space

internal fragmentatio

n

Optimized when

2p se

Shared Pages

Two processes sharing same program sharing its page table41

Implementation Issues

Four times when OS involved with paging1. Process creation

determine program size create page table

2. Process execution MMU reset for new process TLB flushed

3. Page fault time determine virtual address causing fault swap target page out, needed page in

4. Process termination time release page table, pages

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Backing Store

(a) Paging to static swap area(b) Backing up pages dynamically

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Separation of Policy and Mechanism

Page fault handling with an external pager

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Segmentation

One-dimensional address space with growing tables One table may bump into another

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Segmentation

Allows each table to grow or shrink, independently46

Implementation of Pure Segmentation

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Segmentation with Paging: MULTICS

A 34-bit MULTICS virtual address

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Segmentation with Paging: MULTICS

Conversion of a 2-part MULTICS address into a main memory address

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Segmentation with Paging: MULTICS

Disadvantages of Segmentation 1. Segmentation mapping requires two memory references

per logical address which slows down the computer system . Caching is the method used to solve this problem

2. Where there is more number of segments, segment table size will grow. so it cannot be accommodated in any of the registers and has to be kept in memory

3.Segmentation is prone to external fragmentation. This may occur when all the blocks in memory are too small to accommodate a segment

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Advantages of Segmentation Protection bits are associated with segments, which

check that attempt to write a read only segment should fail.

As with paging sharing of code or data is possible even with the segmentation technique. Here entries in two different segment tables can be made to point to a common physical location.

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Comparison of paging and segmentation

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Summary The memory management allows us how to manage the memory. The virtual memory is the memory which does not existence. The paging process which allows us to change the pages from

memory to the secondary storage devices such processes are known as swap in and swap out.

The segmentation allows us to make the division of the memory into the various segments

The fragmentation occurs due to internally or externally wasteage of memory.

The page replacement is the concept of changing the page from the various frames in the memory whenever required.the page replacement is needed due to the page fault.

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