Lecture 04Memory Management Consise

8/8/2019 Lecture 04Memory Management Consise

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Memory Management

By

Varsha Kapoor


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Objectives Basic fundamentals related to primary memory andsecondary memory.

Storage locations and addresses , How to determine the capacity of the storage unit Fixed and variable word length storage organizations

Primary Memory RAM ROM Registers

Secondary Memory Access Method Types of Secondary Memory


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Units of MeasurementSpatial units:o byte: 8 bitso kilobyte (KB): 1024 or 2 10 byteso megabyte (MB): 1024 kilobytes or 2 20 byteso gigabyte (GB): 1024 megabytes or 2 30 bytes

Time units:o nanosecond (ns) one- billionth (10 -9 ) of a secondo microsecond ( s) one- millionth (10 -6 ) of a secondo millisecond (ms) one- thousandth (10 -3 ) of a second


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Storage unit Criteria

Access Time (fast) Storage Capacity (large)

Cost per bit of storage (low)


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Types of Memory

Primary Memory (RAM) Holds data and programs used by a

process that is executing Only type of memory that a CPU deals

with Secondary Memory (i.e. hard disk)

Non-volatile memory used to store datawhen a process is not executing


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1. Primary Storage

Primary Storage of a computer , also calledas its main memory, is used to hold theprogram instructions & data, intermediateresult of processing , & recently produced

results on which the computer is currentlyworking. These piece of information are represented

electronically in the main memory chips &

while it remains in the main memory , cpu canaccess it directly at a very fast speed.However , primary storage can holdinformation only while computer system is on.


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As soon as the computer system switchesoff or resets, the information held in primary

memory is erased. Moreover , primary storage normally haslimited storage capacity because it is veryexpensive.


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Storage Location andAddress

The Primary or internal storage section is made up of several small storage locations (ICs) called cells . Each of these cells can store a fixed number of bits called word length .

Each cell has a unique number assigned to it called theaddress of the cell and it is used to identify the cells.

A given memory is divided into N words , where N generallyis some power of 2. Each word has a number or address assign to itself. Each word can store either a data item or aninstruction

The address starts at 0 and goes up to (N-1). the memory islike a large cabinet containing as many drawers as there areaddresses on memory. Each drawer contains a word and theaddress is written on outside of the drawer.


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orage oca on an ress(Contt.)

The Primary or internal storage section is made up of

several small storage locations (ICs) called cells .Each of these cells can store a fixed number of bitscalled word length .

Each cell has a unique number assigned to it calledthe address of the cell and it is used to identify thecells.

A given memory is divided into N words , where Ngenerally is some power of 2. Each word has anumber or address assign to itself. Each word can

store either a data item or an instruction The address starts at 0 and goes up to (N-1). the

memory is like a large cabinet containing as manydrawers as there are addresses on memory. Each

drawer contains a word and the address is written onoutside of the drawer.


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Organization of a Primary StorageUnit having N- words

0

1

2

3

4

5

N-2

N-1 =====

Bit1 Bit 2

A d d r e s s e s

The words of a

primary StorageUnit

Each wordcontains the samenumber of bits = word lengt

}


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Storage Capacity In Bytes Words :We need to know

the total number of bits per word Total number of Words A memory with 4096 locations each with

different address and with each locationstoring 16 bits is called a 16 bit 4096 wordmemory or 4K 16 bit memory.

If the word size is 8 bits then it becomesimmaterial whether the memory capacity isexpressed in bytes or words.

More bits per word more rapid flow of elctronicsignals and faster computers


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Fixed and Variable word lengthStorage

0501 B O M B A Y

0502 D E L H I

0503

1024

0025B

0026O

0027M

0028B

0029A

0030Y

0031

||

||||

||

||

4096

0051D

0052E

0053L

0054H

0055I

0056

4096

AddressNumber


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Difference Between variable andFixed Word Length

Fixed storage space may be

wasted computers have faster

calculating speeds . Theycan add two data words ina single operation

Used in larger mainframecomputers , super-computers like the CRAY-1

Variable make the most efficient

use of available storagespace

Slower . only one digit(byte) in each number canbe added during a singlemachine operation. Thus,eight steps are required tocomplete the calculation

Used in microcomputersMost of todays business and scientific processing is handled byMost of todays business and scientific processing is handled byflexible computers (in terms of the instruction set) which canflexible computers (in terms of the instruction set) which canemploy either fixed or variable word length.employ either fixed or variable word length.


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Memory Hierarchy

Magnetic Tape

Magnetic Disk

Main Memory (RAM)

CacheFast, Small,

Experience

Slow,

Large, Cheap


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Memory Hierarchy At the primary storage level , the memory

hierarchy giabytes of data in DRA, in somecases, entire databases can be kept in themain memory (with a backup copy onmagnetic disk), leading to main memory

databases . includes, at the most expensiveend cache memory , which is a static RAM (Random Access Memory).

The next level of primary storage is DRAM

(Dynamic RAM), The advantage of DRAM isits low cost, lower speed compared withstatic RAM.

Programs normally reside and execute in

DRAM.


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Primary Memory

RAM Large memory, which is fast butnot as fast as internal processorregister.

Processor directly accesses thismemory.

It is mainly based on integratedcircuit.

Read/Write Memory


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Types of RAM1. Static RAM (SRAM):

Doesnt need refreshing

Retains contents as long as power applied to the

chip Access time around 10 nanoseconds

Used for cache memory

Also for date and time settings as powered by smallbattery


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Types of RAM

2. Dynamic Random Access Memory (DRAM)

Contents are constantly refreshed 1000 times per second

Access time 60 70 nanoseconds

Note : a nanosecond is one billionth of a second!

3. Synchronous Dynamic Random Access Memory (SDRAM)

Quicker than DRAM

Access time less than 60 nanoseconds

4. Direct Rambus Dynamic Random Access Memory (DRDRAM)

New type of RAM architecture Access time 20 times faster than DRAM

More expensive


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5. Cache memory

Small amount of memory typically 256 or 512 kilobytes

Fast small Memory between CPU and main memory

Temporary store for often used instructions

Also called as small high speed buffer

Level 1 cache is built within the CPU (internal)

Level 2 cache may be on chip or nearby (external)

Faster for CPU to access than main memory Temporary data frequently needed in present calculations/ store segments

of programs currently being executed

Types of RAM


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The operation of cache memory

1. Cache fetches datafrom next to currentaddresses in mainmemory

2. CPU checks to seewhether the nextinstruction it requires is incache

3. If it is, then theinstruction is fetchedfrom the cache a veryfast position

4. If not, the CPU has to

fetch next instructionfrom main memory - amuch slower process

Main

Memory(DRAM)

CPU

Cache

Memory

(SRAM)

= Bus connections


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Types of RAM

6. Video Random Access memory

Holds data to be displayed on computer screen

Has two data paths allowing READ and WRITE to occur at the same time

A systems amount of VRAM relates to the number of colours and resolution

A graphics card may have its own VRAM chip on board

7. Virtual memory

Uses backing storage e.g. hard disk as a temporary location for programsand data where insufficient RAM available

Swaps programs and data between the hard-disk and RAM as the CPU

requires them for processing A cheap method of running large or many programs on a computer system

Cost is speed: the CPU can access RAM in nanoseconds but hard-disk inmilliseconds (Note: a millisecond is a thousandth of a second)

Virtual memory is much slower than RAM


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Microprograms Special programs written to perform thecomplicated electronic circuits 196 nanoseconds Deal with low level machine functions and

are substitutes for additional hardware. Are written to aid the control unit in

directing all operations of the computersystems

ROMs are used by computer manufacturersfor storing Microprograms so that theycould be modified by the users


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ROM

essential component of the memory unit This memory is permanent and is not erased

when the system is switched off

can be read only and not be written by user/programmer.

ROM varies from 64 KB to 256 KB Permanent Stores or Dead stores or field

stores

f


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Types of ROMROM available for microcomputers

Mask ROM : Mask ROM is the basic ROM chip. information is stored at the time of its

manufacturing.

cannot be altered or erased later on. PROM:

PROM stands for Programmable Read OnlyMemory.

the information is stored by programmersafter its manufacturing.

cannot be altered or erased later on.

f


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EPROM: EPROM stands for Erasable ProgrammableRead Only Memory.

It is similar to PROM, but its information canbe erased later on by ultra violet light

it can be reprogrammed. EEPROM:

EEPROM stands for Electrically Erasable

Programmable Read Only Memory. It is similar to EPROM, but its information can

be erased by using a high voltage current.

T f ROM


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Types of ROMROM available for microcomputers EAPROM:

EAPROM stands for Electrically Alterable Read OnlyMemory. As compared to EPROM and EEPROM, the information

stored in EAPROM can be altered later.

Flash ROM Similar to EEPROM However, can be reprogrammed while still in the

computer Easier to upgrade programs stored in Flash ROM Used to store programs in devices e.g. modems

Access time is around 45 90 nanoseconds


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Memory Hierarchy-flash memory Flash memory, since 1988 it has become

common, particularly because it isnonvolatile, using EEPROM (ElectricallyErasable Programmable Read-OnlyMemory) technology. Its life is 10,000-1,000,000 times erase Read/write isfast, but erase is slow

Therefore special arrangements aremade for the file system, regarding filedelete or update.

Capacities up to 128 GB has beenrealized todate.

T f ROM


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ROM cartridges Similar to EEPROM Commonly used in games machines

Prevents software from being easily copied


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Memory configurations for Intel CPUs


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Registers

Internal Processor memories Not a part of RAM

High speed temporary memorylocations Length of the register denotes the

number of bits it can store


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Types Of Registers

MAR: Active Memory Location MBR: Holds the contents of memory word read

from or written in memory

PC :address of next instruction to be executed ACR: Initial data / immediate results / final

result of the operation IR : current instruction being executed I/O R:


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Secondary Storage. Secondary storage of a computer also know as

auxiliary storage or Backup storage, is used totake care of the limitation of Primary Storage.

That is , it supplements the limited storagecapacity, Cost per bit of storage & the volatilecharacteristic of primary storage.

This is because secondary storage is muchcheaper than primary storage & it can retain theinformation even when a computer systemswitches off.

Magnetic disk is the most commonly usedsecondary storage medium.


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Access MethodsTwo major types of secondary storage devices

Sequential Magnetic tapes (very fast sequential access) Monthly pay slips Electricity Bills Magnetic tapes (very fast sequential access)

Getting the Bank account statement Magnetic Discs

Hard disks (high capacity, low cost, fast)Floppy disks (low capacity, lower cost, slow)

Optical DisksCD-ROM = (Compact disc, read-only memory


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Categories Of Storage Punched paper tapes Magnetic

Floppy disks Zip disks Hard drives

Optical CD-ROM DVD

Solid state storage devices USB Key (a very common form of solid state

storage)


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Punched Paper Tape

Disadvantages:No of characters stored in the given space is lessPaper tape media is easily torn and mutilatedCost per bit stored is very low


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Magnetic Tape Sequential Access

Medium for Large Data Same as tape used on tape recorder except that it is of

high quality and more durable Can be erased and used indefinitely Frames (Vertical columns) and Channels/Tracks

(horizontal rows ) Older tapes has 7 tracks and they used the 6 bit BCD

code format for data recording


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Magnetic Tape Parity Bit or Check Bit Most modern magnetic tape have 9

tracks ad they use 8 bit EBCDIC code Magnetic tape drive is a machine that can

either read data from a tape into the CPUor it can write the information beingproduced by computer into a tape


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Quality measurement of Magnetic Tape

Data Transfer Rate : Tape Density * Tape Speed Tape Density : no of frames that can be

compressed into 1 inch tape. (CPI)

556 BPI, 800 BPI, 1600 BPI, 3250 BPI , 6250 BPI Tape Speed: inches per second Data Transfer rate : 1 million bytes per second


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Parity or Check bit A parity bit is a bit that is added toensure that the number of bits with

the value one in a set of bits is even

or odd. Parity bits are used as the simplestform of error detecting code.7 bits of data 8 bitsincluding parity

(number of 1s)

Even Odd

0 0000000 0 0000000 1 0000000

1010001 (3) 11010001 0 1010001

1101001 (4) 0 1101001 11101001 1111111 (7) 11111111 0 1111111


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Advantages of Magnetic Tapes

Unlimited Storage High Data Density

Low Cost Rapid Transfer Rate Ease of handling

Portability


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Limitations of Magnetic Tapes

No Direct access Indirect Interpretation

Environment Problems


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Magnetic Storage Exploits duality of magnetism andelectricity Converts electrical signals into magnetic

charges Captures magnetic charge on a storage

medium

Later regenerates electrical current fromstored magnetic charge

Polarity of magnetic charge representsbit values zero and one


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1. Magnetic Drives


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Magnetic Disk Flat, circular platter with metallic coating that is rotated

beneath read/write heads Direct Access Secondary Storage A disk pack consist of number of these disks three or more

mounted about half and inch apart from each other on acentral shaft which rotates at speed of 2400 or more rpm

Random access device; read/write head can be moved toany location on the platter

Hard disks and floppy disks Cost performance leader forgeneral-purpose on-line secondary storage


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Magnetic Disk Storage Of Information Storage Capacity

Tracks per inch of surface and bits per inch of track

Diameter of standard disk size is 14 inch Some disk are quite large upto 4 feet in diameter Tot no. of bytes stored in disk pack = number of

cylinders* tracks per cylinders * sectors per track*

bytes per sector Accessing Time Accessing Data


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Magnetic Disk Accessing Data: Data is stored on tracks and read from surface byone or more read/write head Two types of disk system moving head and fixed

head

Moving Head : One read/write head for eachdisk surface mounted on an access arm whichcan be moved in and out. The head moveshorizontally across the surface of the disk

Fixed head: The access arm is non movable. Alarge number of read/write heads are distributedover the disk surfaces one head for each track.No head movement is required and so access

arms are non-movable

erence etween


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erence etweenMagnetic Tape and

Magnetic DiskMagnetic Disk Sequential and Direct

access storage

Less Vulnerable todamage from dust orcareless handling

Access is fast (few

msec) Data Transfer Rate ishigh

Magnetic Tape Sequential

Less Vulnerable todamage from dust orcareless handling

Access is slow

Data Transfer Rate islow


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50

sectorseach track isdivided into pie-shaped wedges

clustertwo or moresectorscombined

tracksdata is recorded inconcentric circularbands


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Structure Of Floppy Disks Initially Floppy disks were 8-inches wide, they then

shrank to 5.25 inches, and today the most widelyused folly disks are 3.5 inches wide and can typicallystore 1.44 megabytes of data.

A folly disk is a magnetic disk, which means that it

used magnetic patterns to store data. Data in floppy disks can be read from and written to. Formatting is the process of preparing a disk for

reading and writing.

A track is a narrow recording band that forms a fullcircle on the surface of the disk. Capacity of Diskette depends on size (diameter) and

type (mode of data recording)


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Magnetic Drums

They were used to storemathematical tables , data orprogram segments that werefrequently needed during theprocessing operations

Permanently mounted, cannot bereplaced, low storage capacity areused seldomly


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Mass Storage

Advantage of both tape and disk technologies E is IBM 3850 Mass Storage System that can

store 472 billion characters of data

Access time is measured in seconds insteadof milliseconds Archival Storage Cost effective alternative to online magnetic

tape or disk storage


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2.Optical DrivesCD's (Compact Disk)

~ 700 MB storage CD-ROM (read only) CD-R: ( r ecord) to a CD CD-RW: can write and erase CD to reuse it

(r e- w ritable)

DVD(Digital Video Disk)


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Compact Discs (CD) A compact disk (CD), also called an optical disc, is a

flat round, portable storage medium that is usually4.75 inch in diameter.

A CD-ROM (read only memory), is a compact discthat used the same laser technology as audio CDsfor recording music. In addition it can contain othertypes of data such as text, graphics, and video.

The capacity of a CD-ROM is 650 MB of data.


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DVD-ROM Over 4 GB storage (varies with

format) DVD- ROM (read only) Many recordable formats (e.g.,

DVD-R, DVD-RW; ..) Are more highly compact than

a CD. Special laser is needed to read

them

DVD (Digital Video Disk)

M ti B bbl M


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Magnetic Bubble MemoryMBM

Electronic Storage made with solid state electronic chips

uses a thin film of a magnetic material to hold smallmagnetized areas, known as bubbles or domaiins eachof which stores one bit of data

no moving part Used as main memory in microprocessor applications ,

telephone system that redirects improperly dialednumbers, machine tools , robots and military computers

St g i M d


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Storage in ModernComputers

Alternative Nomenclature Primary Storage Secondary Storage Tertiary StoragePrimary Storage

Cache, Main MemorySecondary Storage

Flash Memory, Magnetic Disk Tertiary Storage Optical Disks, CDs, DVDs, Tape


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Memory Hierarchy

CACHE

MAIN MEMORY

MAGNETIC DISK

TAPE

OPTICAL DISK

Price &

Speedincreases

Reliabilityincreases

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Lecture 04Memory Management Consise

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