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• Logical vs Physical address

• MMU, Swapping

Storage Management

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Recap

In the last class, you have learnt

• Dynamic linking

-advantages and disadvantages

• Overlays

-advantages and disadvantages

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Objectives

On completion of this class, you will be able to

know

• Logical and Physical addresses

• MMU

• Swapping

Logical Vs Physical addressLogical address• Address generated by the CPU

Physical address• Address seen by the memory unit i.e. the one

loaded into memory- address register

Compile –time and Load time address binding methods generate same logical address and physical address

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Logical Vs Physical address

• Execution time address binding scheme results

in different logical and physical addresses

• Address generated by CPU at execution is

referred as Virtual address

• Logical address space is the set of all logical

addresses generated by program is a logical

address space

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Memory Map Unit (MMU)

• MMU is a hardware device

• Maps virtual address to physical address at run

time

• A simple MMU scheme consists relocation register

• A relocation register is a base register

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Relocation Register

Memory

Base Register

CPU Instruction

Address

+

BA

MA MA+BA

PhysicalAddress

LogicalAddress

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Memory Map Unit (MMU)

• Address generated by the CPU are added to

relocation register as shown in previous slide

• Physical address = logical address + contents

of relocation register

• This process is called Dynamic relocation

• The user program deals with logical addresses;

– it never sees the real physical addresses

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Memory Map Unit

• Example 1:

Logical address = 346

Relocation register = 14,000

Physical address = 346+14000

= 14346

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Dynamic relocation ( MMU )Example 1

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Logical address

1800MemoryCPU

14000

Relocation register

Physical address

15800

Dynamic relocation ( MMU )

MMU

+

Example 2

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Swapping

• A process needs to be in memory to be

executed

• A process can be swapped temporarily out of

memory to a backing store

• Then brought back into memory for continued

execution as shown in next slide

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Schematic View of Swapping

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Swapping

Backing store

– Fast disk large enough to accommodate copies of all

memory images for all users

– Must provide direct access to these memory images

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Swapping

• In Multiprogramming environment and round-

robin CPU scheduling

• When time quantum expires

– the memory manager will swap-out the process to

backing store

– and swap-in another process to the memory space

occupied by the swapped-out process

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• This process continues until all the processes

are executed

• The memory manager swaps the processes

fast enough

• All ways one process is present in memory

Swapping

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Priority based scheduling

• Another variant of swapping policy

• Higher priority process is swapped-in and lower

priority swapped-out

• Here the variant used is

– roll-in and roll-out

• Example for Priority based scheduling in next

slides

Swapping

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Swapping

Monitor

Disk

UserPartition

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Monitor

Disk

User 1

UserPartition

Swapping (contd.)

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Monitor

User 1

Disk

User 1

UserPartition

Swapping (contd.)

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Monitor

User 2

User 1

Disk

User 1

UserPartition

Swapping (contd.)

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Monitor

Disk

User 2

User 2

UserPartition

User 1

Swapping (contd.)

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Monitor

Disk

User 2

User 2

UserPartition

User 1

Swapping (contd.)

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Monitor

Disk

User 1

User 2

UserPartition

User 1

Swapping (contd.)

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• Major part of swap time is transfer time

• Total transfer time is directly proportional to the

amount of memory swapped

• Modified versions of swapping are found on

many systems, i.e., UNIX, Linux, and Windows

Swapping

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Summary

In this class, you have learnt

• Logical and physical addresses

• MMU

• Swapping

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Frequently Asked Questions

• Write the differences between logical and physical address space

• Define logical, virtual and physical addresses

• Draw and explain the basic dynamic relocation unit (MMU)

• Explain the concept of swapping

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Quiz

1. Address seen by the memory unit

a) Logical

b) Physical

c) Virtual

d) None

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2. Address generated by the CPU at compile

and load time

a) Logical

b) Physical

c) Virtual

d) None

Quiz

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3. Address generated by the CPU at execution time

a) Logical

b) Physical

c) Virtual

d) None

Quiz

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4. A relocation register is a ______register

a) Base

b) Limit

c) Offset

d) None

Quiz

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Quiz

5. For ___________scheduling when time quantum

expires the memory manager will swap-out the

process to backing store

a) Round robin

b) Priority

c) FIFO

d) None

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• Web designing

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• java

home

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6. Another variant of swapping policy-

a) Priority

b) Round robin

c) FIFO

d) None

Quiz

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7. Total transfer time is directly proportional to the

amount of memory swapped

a) True

b) False

Quiz