MCS Lecture # 1

8/3/2019 MCS Lecture # 1

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Memory Management Sadaqat Ali Khan Bangash 1

Memory ManagementLecture

Sadaqat Ali Khan BangashInstitute of Information Technology,University of Science & Technology Bannu


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Memory Hierarchies

Like everything else in computers, Computer Memory is alsoorganized into a hierarchy. This is as follows:

At 1st Level (closest to Processor) are the registers At 2nd Level, one or two Levels of Cache Memory

At 3rd Level, Main Memory (also known as primary memory)


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Memory Hierarchies .

All above are considered part of InternalMemory (i.e. Internal to Computer System).Then the Hierarchy continues onto External

Memory

At 4th Level, Secondary Storages (Hard disks,tapes etc)

At 5th Level, other forms of memory comessuch as Zip Disks, Floppy Drives, etc


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Register,Cache,

Main Memory

Hard disk.

CD-Rom,DVD,

Internal Storage

Internal

Memory

ExternalMemory


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Registers

These temporary locations within the CPU areextremely fast, very small, very expensive & volatile.

Cache

Buffer memory regions between CPU & Main

Memory.


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Main Memory Main Memory also called Primary Memory, Random

Access Memory (RAM). Hundreds of Megabytes of

Medium Speed & Volatile

Secondary Memory

Hundreds of Gigabytes of memory with Slow speed

We may recall the factors of Cost, Size, and Speed ofaccess that are the orders in which Memory

components are divided into this hierarchy.


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Purpose of Memory Management

To ensure fair use of memory for processes

To ensure secure access, i.e., space allocatedfor process is not allocated for another process,

or a process does not have access to anythingbeyond its "process address space

To ensure orderly memory access, i.e., whenmemory is to be allocated, when de-allocated,etc. etc.

To ensure efficient use of memory. Sincememory is expensive, it should be usedefficiently


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Uni-programming environment

In a Uni-programming environment, main

memory is divided into two parts:

1. One part for the Operating System

2. One part for the Program currently beingexecuted. (or the User Part)


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Multi-programming environment

In a multi-programming environment, the user part of thememory is further divided to accommodate multipleprocesses. This task of dividing and allocation is carried outdynamically by the Operating System and is known as

memory management. Memory Manager has the following basic responsibilities:

1. Keep track of used & free memory spaces (Also knownas Free Space Management)

2. When, where and how much memory to allocate or de-

allocate

3. Swapping processes in and out of main memory tosecondary memory.


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Logical & Physical Addresses

Logical Address

An address generated by the CPU

This normally refers to an instruction or datawithin the process address space

Physical Address

An address for a main memory location where instructionor data resides


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Since the Process is already loaded in memory, the logicaladdresses of the process need to be translated to itscorresponding physical addresses.

The CPU generates all logical Addresses for a particular process.A set of these logical addresses constitute the "Logical AddressSpace". Correspondingly, all the physical addresses thattranslate back to the logical addresses are known as the"Physical Address Space" for that process.

This will be explained in detail in paging & segmentation. Thelogical address is also known as the Offset Address. So in

simpler words,


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Logical Address Space

The set of all logical addresses generated by a process

Physical Address SpaceThe set of all physical addresses corresponding to those

logical addresses

This translation or mapping from Logical toPhysical address is done by a piece of CPUhardware known as the Memory ManagementUnit.


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CPU

MMU

Base orRelocation

Register Value:14000

LogicalAddress

346

PhysicalAddress

14346

Process

14000

The MMU will check if the logical address is within the limitspecified by the limit register. If yes, it will add the value of therelocation register. If no, it will give an addressing error or atrap.


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Using Memory Efficiently

Dynamic Loading

With dynamic loading, a routine is not loaded into main memoryuntil it is called. All these routines are kept on the disk in a re-locatable format. The main program is loaded into memory &executed.

The best example for explaining this would be MS-Word. Wordhad thousands of features. At any given time, a user will only beusing a couple of these features. The routines for thoseadditional features are kept on disk whereas only the mainprogram for MS-Word is loaded.

Dynamic Loading is also used in handling exceptions or errors.Routines for handling exceptions or errors are only loaded whenthey occur, otherwise they will remain on disk.


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Using Memory Efficiently

Advantages:

Less time needed to load a program

Less memory space needed for program

Disadvantages:

More run-time activity. Lot of I/O requests are made

which is time consuming.


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Dynamic Linking

Advantages:

Less time needed to load a program

Less memory space needed Less disk space needed to store binaries

Updated libraries are used without

recompiling a program

Disadvantages: More run-time activity resulting in slower

program execution


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Overlays

Overlays allow a process to be larger than theamount of memory allocated to it. It works bykeeping only those instructions in memorythat are needed at any given time. I.e., lets

suppose our Main Memory is 100 MB, will anexecutable of 110 MB will be able to run? Theanswer is yes if the underlying OperatingSystems employs Virtual Memory techniques.This is the concept behind Virtual Memory.

It is essential to understand Overlays becauseit gives a flavor of Virtual Memory which wewould be cover later on.


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Overlays

When other instructions & data are needed,they are loaded into the space occupiedpreviously by instructions that are no longer

needed. Overlays are implemented solely bythe programmer. If the program does nothave support for Virtual Memory or Overlays,then the Operating System cannot do anything

about it.


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Swapping

A process can be swapped temporarily out of memory to abacking store, and then brought back into memory forcontinued execution.

Backing store fast disk large enough to accommodatecopies of all memory images for all users; must providedirect access to these memory images

Roll out, roll in swapping variant used for priority-basedscheduling algorithms; lower-priority process is swapped out

so higher-priority process can be loaded and executed

Major part of swap time is transfer time; total transfer time isdirectly proportional to the amount of memory swapped


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Swapping

Main Memory

Secondary Memory

The area where swapped processes are kept in secondary

memory is known as Swap Space

Memory Management Techniques Multi-Programming with Fixed Number of Tasks (MFT)

Multi-Programming with Variable Number of Tasks (MVT) Paging

Segmentation

Virtual Memory


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SwappingModified versions of swapping are found on many systems (i.e.,UNIX, Linux, and Windows)

System maintains a ready queue of ready-to-run processes whichhave memory images on disk


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Contiguous Allocation Main memory usually divided into two partitions:

Resident operating system, usually held in lowmemory with interrupt vector

User processes then held in high memory

Relocation registers used to protect user processesfrom each other, and from changing operating-system code and data

Base register contains value of smallest physical

address Limit register contains range of logical addresses

each logical address must be less than the limit register

MMU maps logical address dynamically


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HW address protection with base and limit registers


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Contiguous Allocation (Cont.) Multiple-partition allocation

Hole block of available memory; holes of various sizeare scattered throughout memory

When a process arrives, it is allocated memory from ahole large enough to accommodate it

Operating system maintains information about:a) allocated partitions b) free partitions (hole)

OS

process 5

process 8

process 2

OS

process 5

process 2

OS

process 5

process 2

OS

process 5

process 9

process 2

process 9

process 10


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Dynamic Storage-Allocation Problem

First-fit: Allocate thefirsthole that is big enough

Best-fit: Allocate the smallesthole that is big enough;

must search entire list, unless ordered by size

Produces the smallest leftover hole

Worst-fit: Allocate the largest hole; must also search

entire list

Produces the largest leftover hole

How to satisfy a request of size n from a list of free holes

First-fit and best-fit better than worst-fit in terms of speed andstorage utilization


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Fragmentation External Fragmentation total memory space exists

to satisfy a request, but it is not contiguous Internal Fragmentation allocated memory may be

slightly larger than requested memory; this sizedifference is memory internal to a partition, but notbeing used

Reduce external fragmentation by compaction

Shuffle memory contents to place all free memorytogether in one large block

Compaction is possible only if relocation is

dynamic, and is done at execution time I/O problem

Latch job in memory while it is involved in I/O

Do I/O only into OS buffers


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End of Lecture

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MCS Lecture # 1