computer memory (2)

8/8/2019 Computer Memory (2)

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Computer data storage, often called storage or memory, refers to computer components andrecording media that retain digital data used for computing for some interval of time. Computer datastorage provides one of the core functions of the

modern computer, that of information retention. It isone of the fundamental components of all modern

computers, and coupled with a central processingunit (CPU, a processor), implements the basiccomputer model used since the 1940s.

AB24008 Aref Kashani AB24009 Arpit Solanki 2


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Memory consists of a number of

storage locations, each ofwhich is identified by a uniqueaddress.

Each location stores a word i.e.the number of bits that can be

processed by the CPU in asingle operation. Word lengthmay be typically 16, 24, 32 or asmany as 64 bits.

A large word length improves

system performance, thoughmay be less efficient onoccasions when the full wordlength is not used


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The amount of information that can be

stored on a medium.


Unit Description ApproximateSize

1 bit 1 binary digit

1 nibble 4 bits

1 byte 8 bits 1 character

1 kilobyte 1,024 bytes page, double

spaced

1 megabyte 1,024 kilobytes 500,000 pages

1 gigabyte 1,024 megabytes 5 million pages

1 terabyte 1,024 gigabytes 5 billion pages


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Computer MemorySystem

Location

Internal

External

PhysicalCharacteristic

Volatile/ NonVolatile

Erasable/ NonErasable

Physical Type

Magnetic Type

Semiconductor

Optical

Method ofAccess

Sequential

Random

Direct

Performance

Access Time

Transfer Rate

AB24008 Aref Kashani AB24009 Arpit Solanki

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Location refers to whether the memory is internal orexternal to the computer. Internal memory is oftenequated with the main memory. External Memory

consists of peripheral storage devices such as disk ortape, that are accessible to processor via I/o

controllers


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Volatile /Non Volatile- Volatile Memory is the memorythat requires power to maintain the storedinformation. As soon as the power is switched off, it

loses all the information stored in it. Examples areRAM. Non Volatile memory retains the stored

information when the power is not supplied. Exampleare ROM, flash memory ,optical disks.


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Magnetic core memory is an early form of randomaccess computer memory. It uses small magneticrings, the cores, through which wires are threaded to

store information via the polarity of the magnetic fieldthey contain. Such memory is often just called core

memory

Semiconductor memory-In this memory

semiconductor chips are used to make mainmemory.


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Sequential

Start at the beginning and read through in order

Access time depends on location of data andprevious location

e.g. tape

Direct

Individual blocks have unique address

Access is by jumping to vicinity plus sequentialsearch

Access time depends on location and previouslocation

e.g. disk


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Random

Individual addresses identify locations exactly

Access time is independent of location orprevious access

e.g. RAM Associative

Data is located by a comparison with contentsof a portion of the store

Access time is independent of location or

previous access e.g. cache



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Access time-Access time is the time taken to performa read or write operation.

Memory cycle time-this concept is primarily added tothe random access memory and consists of the

access time plus any additional time required beforea second access can commence.

Transfer Rate-Transfer rate is the rate at which the

data can be transferred into or out of a memory unit.


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In general

Slow memory is cheap and has more storagecapacity

Fast memory is expensive and has less storage

capacity

Users want

Faster memory, which will ensure faster data fetching,less wait for the processor, in turn shortens responsetime. Never the less we want to have more storage atan affordable price


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The cache is a smaller, faster memory which storescopies of the data from the most frequently used mainmemory locations. Sits between normal main memory

and CPU. May be located on CPU chip or module


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1. Cache fetches data

from next to current

addresses in main

memory

2. CPU checks to see

whether the next

instruction it requires is in

cache

3. If it is, then the

instruction is fetched from

the cache a very fast

position

4. If not, the CPU has to

fetch next instructionfrom main memory - a

much slower process

Main

Memory

(DRAM)

CPU

Cache

Memory

(SRAM)

= Bus connections



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Random access memory or RAM most commonly refers to

computer chips that temporarily store dynamic data to

enhance computer performance. By storing frequently used

or active files in random access memory, the computer can

access the data faster than if it to retrieve it from the far-larger hard drive. Random access memory is also used inprinters and other devices.

The original name of RAM comes from the fact that anyportion on the memory can be accessed in a constant

time. (Randomly)

Today it used to refer to main memory.


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RAM

SRAM

DRAM

Fast PageMode DRAM

SDRAM

DDR SDRAM RDRAM

EDO RAM



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Bits stored as charge in capacitors

Charges leak

Need refreshing even when continuouslypowered

Simpler construction & Smaller per bit

Less expensive Slower



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Bits stored as on/off switches

No charges to leak

No refreshing needed when powered

More complex construction

Larger per bit

More expensive

Faster



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o It is non volatile in nature.

o It is created like an integrated chip, with the dataactually wired in chip as a part of fabrication process.

o Used to store system programs, library routines forfrequently used functions.

o Contains a permanent pattern of data that can notbe changed. This is in the strict sense Masked ROM.(MROM)

ROM

MROM PROM EPROM EEPROMFlash

Memory


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It is a type of ROM that can be programmed.

It comes without instructions and the user program

it according to his requirements.

Once PROM programmed, it is not possible tomake any changes or reprogram it.

There are other types of ROM memories such asEPROM (can be erased and programmed again)



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Each bit consists of a floating FET.

Have clear window in centre of top of the chip

Chips erased by focusing high-intensity UV light

through window, so care has to be used

UV radiation can erase entire contents of a chip



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Contains an array of FETs.

Use higher than normal voltage to erase contents

Dont need to be removed from socket to

reprogram

Can be manipulated at byte level, so individual

bytes can be erased without erasing entire chip

Common for storing setup parameters for printers

and other peripherals



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Essentially same as EEPROM except voltage required are at

levels normally found inside PC

Flash ROM bulk-erase typically, but newer ones have multiple

independently erasable blocks (size 4K to 128K) Two styles

Sectored-erase flash

Boot block

Usually one or more blocks are specially protected from normal

erase operations (firmware).

Support random reading and writing

Current generations have on-board logic to handle operations



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Secondary storage (also known as external memory orauxiliary storage), differs from primary storage in that it isnot directly accessible by the CPU. The computerusually uses its input/output channels to accesssecondary storage and transfers the desired data usingintermediate area in primary storage. Secondarystorage does not lose the data when the device ispowered downit is non-volatile.

Types:

Magnetic Storage Devices

Optical Storage Devices


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Magnetic disks: A form of secondary storage on amagnetized disk divided into tracks and sectorsthat provide addresses for various pieces of data.

Examples include Hard Disk Drives, Floppy Disks.

Each disk consists of a number of invisibleconcentric circles called tracks. Information isrecorded on tracks of a disk surface in the formof tiny magnetic spots.

The presence of a magnetic spot representsone bitand its absence represents zero bit. Theinformation stored in a disk can be read manytimes without affecting the stored data.



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Hard disk: A form of secondary storage that

stores data on platters divided into concentric

tracks and sectors, which can be read by a

read/write head that pivots across the rotating

disks.

Floppy disk: A form of easily portable secondarystorage on flexible disks; also called floppy disks.



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Optical storage devices: A form of secondary storagein which a laser reads the surface of a reflectiveplastic platter.

Compact disk, read-only memory (CD-ROM): A form

of secondary storage that can be only read and notwritten on.

Digital versatile disk (DVD): An optical storage deviceused to store digital video or computer data.

Other types include: HD-DVDs, Blu-Ray Disks, etc.


computer memory (2)

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