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Operating SystemConcepts

Ku-Yaw Chang

[email protected]

Assistant Professor, Department ofComputer Science and Information Engineering

Da-Yeh University
mailto:[email protected]:[email protected]


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2Ku-Yaw Chang Chapter 9 Memory Management

Chapter 9 Memory Management

Keep several processes in memory to increase

CPU utilization

Memory management algorithms

Require hardware support

Common strategies

Paging

Segmentation


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9.1 Background

Program must be brought (loaded) into memory and

placed within a process for it to be run.

Address binding

A mapping from one address space to anotherA typical instruction-execution cycle

Fetchan instruction from memory

Decodethe instruction

May cause operands to be fetched from memory Executethe instruction

Storeresults back into memory


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9.1.1 Address Binding

Input queue

A collection of processes on the disk that are waiting

to be brought into memory to run the program.

A user program will go through several stepsbefore being executed

Addresses in source program are symbolic

A compiler binds these symbolic addresses to

relocatable addresses

A loader binds these relocatable addresses to

absolute addresses


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Compile time

Absolute code can

be generated

Know at compiletime where the

process will reside

in memory

MS-DOS .COM-

format programsare absolute code


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Load time

Relocatable code

can be generated

Not known atcompile time where

the process will

reside in memory

Final binding is

delayed until loadtime


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Execution time

The process can be

moved from one

memory segmentto another

Binding must be

delayed until run

time

Special hardwaremust be available


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9.1.2 Logical- Versus Physical-

Address SpaceLogicaladdress An address generated by the CPU

Compile-time and load-time

Also called virtual address

Logical-address spaceThe set of all logical addresses

Physicaladdress An address seen by the memory unit

The one loaded into the memory-address unit Execution time

Logical and physical address spaces differ

Physical-address space


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9.1.2 Logical- Versus Physical-

Address Space

Memory-Management Unit (MMU)

A hardware device

Run-time mapping from virtual to physical addresses

Different methods to accomplish such a mappingLogical addresses

0to max

Physical addresses

R+ 0 to R+ max


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9.1.3 Dynamic Loading

The entire program and data must be in memoryfor its execution The size of a process is limited to the size pf physical

memory.

Dynamic Loading All routines are kept on disk in a relocatable load

formatThe main program is loaded into memory and is executed

A routine is not loaded until it is called

Advantage An unused routine is never loaded


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9.1.4 Dynamic Linking and

Shared Libraries

Dynamic Linking

Linking is postponed until execution time

Small piece of code, called stub, used to locate the

appropriate memory-resident library routine

OS checks if routine is in processes memory address

If yes, load the routine into memory

Stub replaces itself with the address of the routine, and

executes the routine.

Dynamic linking is particularly useful for libraries.


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9.1.5 Overlays

Keep in memory only those instructions and data

that are needed at any given time

Needed when process is larger than amount of

memory allocated to itFeatures

Implemented by user

No special support needed from operating system

Programming design is complex


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9.1.5 Overlays


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1. Background

2. Swapping

3. Contiguous Memory

Allocation

4. Paging

5. Segmentation

6. Segmentation with

Paging

7. Summary

8. Exercises


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Swapping

A process can be

Swapped temporarily out of memory to a backing

store

Commonly a fast disk

Brought back into memory for continued execution

A process swapped back into

The same memory space

Binding is done at assembly or load time

A different memory space

Execution-time binding


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Swapping of two processes


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Swapping

Major part of the swap time is transfer time

Directly proportional to the amount of memory

swapped

Factors

How much memory is actually used

To reduce swap time

Be sure the process is completely idlePending I/O


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1. Background

2. Swapping


Allocation

4. Paging

5. Segmentation


Paging

7. Summary

8. Exercises


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Contiguous Memory Allocation

Memory

One for the resident operating system

In either low or high memory

Location of the interrupt vector

One for the user processes

Contiguous memory allocation

Each process is contained in a single contiguous

section of memory


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Memory Protection

A relocation register with a limit register


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Memory Allocation

Fixed-sized partitions Simplest

Each partition contain exactly one processDegree of multiprogramming is bounded

Strategies

First fitBest fit

Worst fit

Problem External fragmentation

Internal fragmentation50-percent rule Given N allocated blocks

Another 0.5 N blocks will be lost


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1. Background

2. Swapping


Allocation

4. Paging

5. Segmentation


Paging

7. Summary

8. Exercises


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Paging hardware


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Paging Model


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Paging Example


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Free Frames

Before Allocation After Allocation


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1. Background

2. Swapping


Allocation

4. Paging

5. Segmentation


Paging

7. Summary

8. Exercises


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1. Background

2. Swapping


Allocation

4. Paging

5. Segmentation


Paging

7. Summary

8. Exercises


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1. Background

2. Swapping


Allocation

4. Paging

5. Segmentation


Paging

7. Summary

8. Exercises


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35Ku Yaw Chang Chapter 9 Memory Management


1. Background

2. Swapping


Allocation

4. Paging

5. Segmentation


Paging

7. Summary

8. Exercises

[09]memory management 01

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