Basics of Computer Memory

Basis of Computer MemoryBasis of Computer Memory

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Computer MemoryComputer Memory

• Internal Storage area Internal Storage area • Stores data or instructions Stores data or instructions –Temporarily orTemporarily or– Permanently Permanently

• Memory normally refers to Temporary Memory normally refers to Temporary MemoryMemory

• Storage refers to permanent dataStorage refers to permanent data

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Basic Types of Computer MemoryBasic Types of Computer Memory

• Main Memory

• Cache Memory

• Virtual Memory

• Hard Drives

Main MemoryMain Memory

• Every Computer has Main Memory

• Array of Boxes holding single byte

information

• Two Types of Main Memory

–RAM – Random Access Memory

– ROM – Read Only Memory

Random Access MemoryRandom Access Memory

• Temporarily holds programs & data for Temporarily holds programs & data for

processingprocessing

• Modern Computers – holds OSModern Computers – holds OS

• Every file or application opened is placed in Every file or application opened is placed in

RAMRAM

Random Access MemoryRandom Access MemoryCPU – takes data from RAM & after processes CPU – takes data from RAM & after processes

sends back to RAM sends back to RAM

This process repeats millions of times in a secondThis process repeats millions of times in a second

RAM is volatile in natureRAM is volatile in nature

RAM is on MotherboardRAM is on Motherboard

Also known as SIMMs Also known as SIMMs

(Single-in-line-memory modules) (Single-in-line-memory modules)

Types of RAMTypes of RAM

• SRAM – Static RAMSRAM – Static RAM–Uses Multiple transistors (4 to 6)Uses Multiple transistors (4 to 6)– Expensive, faster & less power consumingExpensive, faster & less power consuming– Used in routers, printers, LCD screen & Used in routers, printers, LCD screen &

digital camerasdigital cameras– Used primarily for cacheUsed primarily for cache


• DRAM – Dynamic RAMDRAM – Dynamic RAM– Memory Cells with paired transistors & Memory Cells with paired transistors &

capacitorscapacitors– Main memory in PC, laptops & Main memory in PC, laptops &

workstationsworkstations


• FRM DRAM – FRM DRAM – Fast Page Mode Dynamic Fast Page Mode Dynamic RAMRAM– Improvement over DRAM– Uses one row address for multiple accesses– Slowest type of memory – Max bus speed 66 MHz– Used in early 486 computers


• EDO DRAM – Extended Data-out EDO DRAM – Extended Data-out Dynamic Dynamic RAMRAM– 5 % faster than FPM DRAM– Allows memory access to begin before last

one is finished– Overlapping processes to retrieve data – Max bus speed 66 MHz– Cheaper than FPM DRAM


• SDRAM – SDRAM – Synchronous dynamic RAMSynchronous dynamic RAM– Burst mode concept– 5 % faster than EDO RAM– Common in most Desktop PC


• DDR SDRAM – DDR SDRAM – Double Data Rate Double Data Rate Synchronous dynamic RAMSynchronous dynamic RAM– Similar to SDRAM– Higher bandwidth, greater speed– Max transfer rate to L2 cache – 1064 MBps– DDR3 is the latest used in PCs & Laptops


• RDRAM – RDRAM – Rambus Dynamic RAMRambus Dynamic RAM– Uses Rambus in-line memory module

(RIMM) – Work in parallel to achieve data rate of 800

MHz, or 1600 MBps– Uses 16 bit DRAM chips at 400-800 MHz– Used only in Video games consoles &

Graphics Accerelator Boards


• VRAM – VRAM – Video RAMVideo RAM– Used for Video adapters or 3-D accelerators– Resolution & colour depth of display

depends on VRAM– True Multiport VRAM is expensive – Cheaper SGRAM (Synchronous graphic RAM)

is used as substitue

Random Access MemoryRandom Access Memory

• Temporarily holds programs & data for Temporarily holds programs & data for

processingprocessing

• Modern Computers – holds OSModern Computers – holds OS

• Every file or application opened is placed in Every file or application opened is placed in

RAMRAM

Read Only Memory (ROM)Read Only Memory (ROM)

• Non- volatile Memory ChipsNon- volatile Memory Chips• Data burnt into chipsData burnt into chips• Low Level machine functions Low Level machine functions

are written on ROMare written on ROM• Eg – System boot ProgramEg – System boot Program

Types of ROMTypes of ROM

• Manufacturer Programmed Manufacturer Programmed (cannot be re-programmed)(cannot be re-programmed)• User Programmed (Programmable)User Programmed (Programmable)


• PROM – PROM – programmable ROMprogrammable ROM– Customized by userCustomized by user– Record information using PROM Record information using PROM

programmerprogrammer– Can be programmed only once Can be programmed only once – Blank PROM are very cheapBlank PROM are very cheap– Used in video games consoles, Used in video games consoles,

HDMI, mobile phonesHDMI, mobile phones


• EPROM – EPROM – Erasable Programmable Erasable Programmable ROMROM– Reprogrammable ROMReprogrammable ROM– Can be erased by exposing to Can be erased by exposing to

ultraviolet light ultraviolet light – Used by R & D personnelUsed by R & D personnel

Types of ROMTypes of ROM• EEPROM – EEPROM – Electrically Erasable Electrically Erasable

Programmable ROMProgrammable ROM– Stored data erased using high voltage Stored data erased using high voltage

electric pulse electric pulse – Used in BIOS chips for saving system Used in BIOS chips for saving system

settingssettings

Types of ROMTypes of ROM• Flash Memory – Flash Memory –

Special type Special type EEPROM EEPROM – Erased & Rewritten Erased & Rewritten

fasterfaster– Also called as Flash Also called as Flash

ROM or flash ROM or flash EEPROMEEPROM– Used in memory Used in memory

cards, USB flash cards, USB flash drives, MP3 players drives, MP3 players & Digital Cameras& Digital Cameras

CACHE MEMORYCACHE MEMORYIntermediary between CPU & RAMIntermediary between CPU & RAM

CACHE MEMORYCACHE MEMORY• 2 Levels2 Levels• L1 level (on CPU)L1 level (on CPU)– 2 to 128 kB2 to 128 kB

• L2 LevelL2 Level– 256 kb to 2 mb256 kb to 2 mb

• Uses SRAMUses SRAM

VIRTUAL MEMORYVIRTUAL MEMORY• Comes in place when applications Comes in place when applications

are too large for RAMare too large for RAM

• OS uses hard disk to store data OS uses hard disk to store data temporarilytemporarily

• Swap file is used to store data on HDD Swap file is used to store data on HDD for faster access speed for faster access speed

HARD DISK DRIVEHARD DISK DRIVE• TO STORE DATA TO STORE DATA

PERMANENTLYPERMANENTLY

• USUALLY C DRIVE HAS OS USUALLY C DRIVE HAS OS (MS WINDOWS) (MS WINDOWS)

• CAPACITY - GIGABYTESCAPACITY - GIGABYTES

Memory HierarchyMemory Hierarchy

Units of MemoryUnits of Memory• Memory Capacity measured in byte multipleMemory Capacity measured in byte multiple

• Memory capacity also started in terms of Memory capacity also started in terms of number of wordsnumber of words– Eg 4096 storing 16 bits or 4k , 16 bit memoryEg 4096 storing 16 bits or 4k , 16 bit memory

• If word size of memory is 8 bits (equal to a If word size of memory is 8 bits (equal to a byte) – 2byte) – 2 16 16 with each word of 8 bits – 64k with each word of 8 bits – 64k memorymemory

Units of MemoryUnits of Memory1 Byte1 Byte = 8 bits= 8 bits

1 Kilo Byte (KB)1 Kilo Byte (KB) = 1024 byte or 10= 1024 byte or 103 3 byte (approx)byte (approx)

1 Mega Byte 1 Mega Byte (MB)(MB)

= 1024 KB or 10= 1024 KB or 106 6 byte (approx)byte (approx)

1 Giga Byte 1 Giga Byte = 1024 MB or 10= 1024 MB or 109 9 byte (approx)byte (approx)

1 Tera Byte (TB)1 Tera Byte (TB) = 1024 GB or 10 = 1024 GB or 10 1212 byte (approx) byte (approx)

1 Peta Byte (PB)1 Peta Byte (PB) = 1024 TB or 10= 1024 TB or 101515 byte (approx) byte (approx)

Secondary /Auxiliary MemorySecondary /Auxiliary Memory

[Live Storage / Back Up Storage][Live Storage / Back Up Storage]

Hard DiskHard Disk• Magnetic Storage Device– Member of metal plates coated with ferromagnetic

substance mounted on a central shaft ½ inch apart– Shaft’s rotational speed - 7200 RPM

• Tracks– The surface of the disk is divided into a number of invisible

concentric circles called tracks. Seek time is the time that elapses to place the R/W head over the specific track

• Sectors– A disk surface is also divided into invisible pie-shaped

segments called sectors.. These are often 8 or more sectors per track. Latent time is the time that elapses to place the R/W head over the specific sector. Nowadays 2 tera byte hard-disks are also available

Hard DiskHard Disk

Hard DiskHard Disk• TypesTypes– Desktop HDD – 3.5”, IDE and SATA interfaceDesktop HDD – 3.5”, IDE and SATA interface– Laptop HDD – 2.5”,1.8”, IDE and SATA Laptop HDD – 2.5”,1.8”, IDE and SATA – Server HDD – 3.5” SATA, SCSIServer HDD – 3.5” SATA, SCSI

Floppy DiskFloppy Disk• Magnetic Storage Device• Plastic disk coated with ferromagnetic

substance sealed in a square jacket.• 3 ½” floppy disk– Storage – 1.4 MB– Tracks – 40-80– Sectors – 18-36 sectors per track– 300 – 400 RPM– Data tfr rate – 10 – 30 KB/se

• Almost Obsolete as on date.

• Magnetic Storage Device• Plastic disk coated with ferromagnetic

substance sealed in a square jacket.• 3 ½” floppy disk– Storage – 1.4 MB– Tracks – 40-80– Sectors – 18-36 sectors per track– 300 – 400 RPM– Data tfr rate – 10 – 30 KB/se

• Almost Obsolete as on date.

Compact Disk• Optical Storage Device• Made up of polycarbonate substance and

coated with a reflective substance • Unlike magnetic disks having several

tracks, an optical disk has one long track starting at the outer edge and spiraling inward to the centre

• The spiral track is ideal for reading large blocks of sequential data, such as audio or video

Compact Disk• The spiral track is split up into equal length

sectors regardless of the position of the sector from the center

• Dia – 12 cm • Weight – 16 gms • Storage – 650 MB• Data tfr rate – 150 KB/sec– 4x drivers tfrs at the rate of 4*150 = 600 KBPS

Layers of a CD• Writing Surface(WS)Writing Surface(WS)– For labeling the media after data is burnt on it for For labeling the media after data is burnt on it for

easy identification. It is the uppermost layer.easy identification. It is the uppermost layer.• Protective LayerProtective Layer– It lies underneath the writing layer. It is meant to It lies underneath the writing layer. It is meant to

protect the CD from UV radiations, heat, scratches protect the CD from UV radiations, heat, scratches etcetc

– Amkette CD-R uses Dura shield protective layerAmkette CD-R uses Dura shield protective layer• Reflective LayerReflective Layer– Gold – Least reactive metal so more stable. Used for Gold – Least reactive metal so more stable. Used for

archiving, data backup.archiving, data backup.– Silver – More reflective than gold. May be easily read Silver – More reflective than gold. May be easily read

by most CD-ROM drives.by most CD-ROM drives.

Layers of a CD• Recording LayerRecording Layer– Layer where recording takes placeLayer where recording takes place– Made up of a photo-reactive dyeMade up of a photo-reactive dye– In order to burn dataIn order to burn data• CD writer fires a laser beam on this layerCD writer fires a laser beam on this layer• The dye turns into a non-reflective pit The dye turns into a non-reflective pit

– It is this layer, with the reflective layer that gives It is this layer, with the reflective layer that gives the CD-R its distinct colourthe CD-R its distinct colour

– Diff manufacturers use different dyesDiff manufacturers use different dyes• Acer – Phthalocyamic dyeAcer – Phthalocyamic dye• VERBATIM – AZO – recording dyeVERBATIM – AZO – recording dye

Layers of a CDLayers of a CD• Polycarbonate SubstratePolycarbonate Substrate– It is a transparent layer used to give protection to It is a transparent layer used to give protection to

the layers that lie over above it.the layers that lie over above it.– This is the shiny side of the CD-RThis is the shiny side of the CD-R

• Commonly used optical diskCommonly used optical disk– CD-ROM – Compact Disk Read OnlyCD-ROM – Compact Disk Read Only– WORM – Write once Read manyWORM – Write once Read many– CD-RW – Many times re-writable (upto 100 min)CD-RW – Many times re-writable (upto 100 min)– DVD – Digital Versatile DiscDVD – Digital Versatile Disc– BD – Blue Ray DiscBD – Blue Ray Disc

DVD• Optical Disc– Stores Data and Video Content

• Engineered for all storage capacities– Single sided – 4.7 GB– Single sided double layered – 8.54 GB– Double sided single layer – 9.40 GB– Double sided, one side single layer, one side –

13.24GB– Double sided double layer – 17.08 GB

Blue Ray Disc

• Optical disc– Storing data and high definition videos

• Stores aprox 50 GB of data– 10 times of that of a DVD– Roughly 20 hrs of audio and video

• Data can read, recorded or re-written– BD-ROM– BD-R– BD-RE

Optical Disk vs magnetic Disk

Optical Disk Magnetic Disk

Single long track starts from outer edge & spirals inward to the centre of the disk

Concentric tracks

Slow reading – 180-300 KBPS Faster reading time in hard disks – 1000-2100 KBPS

More densely packed

No R/W head

Access Mechanism of Optical DiscsAccess Mechanism of Optical Discs• Two laser beams of different intensities

are used• Greater intensity layer – Write–By turning ON & OFF at varying rate to

burn pits

• Spin Rate– It is the rotation Speed of the disk.–Octal speed drive fastest due to less access

time

Memory Storage DevicesMemory Storage Devices

Flash MemoryFlash Memory• Non volatile storage chip that can be electrically

erased and reprogrammed

• Primarily used in Memory cards, USB flash drives, MP3 players and solid state drives

• Used for general storage and transfer of data between computers and other digital products

• It’s a specific type of EEPROM.

Flash Drive• Small size, light weight, low cost flash memory

chip ideal for mobile people to carry or transfer data from one PC to other

• Highly durable and retention capacity of more than 10 yrs.

• Versatile use– Read, write, copy, delete and move data.– Run applications, view videos or play MP3 directly

• Available in various capacities– 128, 256, 512 MB– 2,4,8,16,32,64,128 and 256 GB

Memory Card (SD/MMC)• Flash memory based Flash memory based

storage devicesstorage devices• Popular typesPopular types– SD – Secure DigitalSD – Secure Digital– MMC – Multimedia MMC – Multimedia

cardcard

• Used primarily on Used primarily on Cameras and Cell Cameras and Cell phonesphones

Memory Card (SD/MMC)• Facilitates easy transfer of data from devices Facilitates easy transfer of data from devices

to Computer for safe storage and further to Computer for safe storage and further processingprocessing– Eg : Photos from cameras downloaded and Eg : Photos from cameras downloaded and

printedprinted

• Each card has it own interface and specific Each card has it own interface and specific design features for use with certain types of design features for use with certain types of devices.devices.

Magneto Optical Storage DevicesMagneto Optical Storage Devices• Writes Magnetically – Reads Optically– Plastic disk coated with ternary alloy of Terbim Ferric

Cobalt (TbFeCo)

Magneto Optical Storage DevicesMagneto Optical Storage Devices• Reading from MO– Uses polarized light to read data from the disk– Change in light polarization occurs due to presence of a

magnetic field on the surface of the disk. – Known as Kerr Effect, where the polarity of the

reflected light is allotted depending on the orientation of the magnetic particles

Magneto Optical Storage Devices• Recording in MO– Based on effects of heat on magnetic property.– The recording layer is heated by a laser to a point

where the magnetic orientation is dissipated. – As this spot cools the new magnetic orientation –

corresponding to the new information is set by the magnetic head to correspond to ‘0’ and ‘1’ of digital signals.

• To rewrite data it is necessary to erase all previous signals– Use two lasers (one to erase and one to record)– Single laser ( First erase and then record)– The later requires a longer recording time

Mass Storage DevicesMass Storage Devices• Storage capacity of several trillions of bytes• RAID, RAID 1, RAID 3, RAID 4, RAID 5 based on

disk fault tolerance• Multiple units of storage media • Commonly used types – Disk array using a set of magnetic disks– Automated type library using a set of magnetic

tapes– CD-ROM juke box using a set of CD-ROMs

Mass Storage DevicesMass Storage Devices• Slow access time – measured in seconds

instead of milliseconds• Very cost effective• Used for archival storage

and data backup

Disk ArrayDisk Array• Set of hard disk drives in a single box• Commonly known as RAID (Redundant Array

of Inexpensive Disks)• Developed for large secondary storage • Enhanced reliability because of multiple disks• Safer way to store

large data

Automated Tape LibraryAutomated Tape Library• Set of magnetic tapes,

magnetic tape drives

• Unit has 1 or more tape drives to read & write data on tapes

• Multiple drives lead to improved reliability

Automated Tape LibraryAutomated Tape Library• Uses robotic arms to retrieve the

appropriate tape from tape library

• Storage capacity – several Terabytes

• Used for data archiving & online data backup

CD-ROM JukeboxCD-ROM Jukebox• Set of CD-ROM disks • Read only possible – cannot write data• Unit has 1 or more CD-ROM drives to read data

CD-ROM JukeboxCD-ROM Jukebox• Faster data transfer rates• Improved reliability • Uses robotic arms to retrieve the appropriate

CD-ROM & mount it on one of the CD-ROM drives for processing

• Storage capacity – several hundred CD-ROM disks

• Used for on-line museums, digital libraries, encyclopedia etc

Firewire Drives (BS)Firewire Drives (BS)• Used for high speed data

communication & large bandwidth application

• Video editing & video conferencing

• Data transfer speed upto 1.6 gbps

– Eg : Apple products like Iomega’s Peerless drives

DAS (Direct Attached Storage)(BS)DAS (Direct Attached Storage)(BS)• Storage devices like Hard Disc connected

directly to the network server• Can have single or multiple independent

hard-disk called JBOD (just a bunch of disks)

• Array of hard disks can also be configured

DAS (Direct Attached Storage)(BS)DAS (Direct Attached Storage)(BS)

DAS (Direct Attached Storage)(BS)DAS (Direct Attached Storage)(BS)• Merits :– Easy to implement & reasonably good

performance– Lower administrative cost

• Demerits :– If server down data cannot be accessed– Cannot be connected over long distances– Pose management problem as the data has to

be managed server-by server basis

SAN (Storage Area Network)(BS)SAN (Storage Area Network)(BS)• A separate network linked to company’s

main network via high speed interface using fibre channel

• Fibre channel have data transfer rates of 1 GB/sec

• Useful for companies having high transaction volumes like banks, customer-services which need quick access of data

SERVERLESS BACKUPSERVERLESS BACKUP• Traditional backup requires the server to

initiate, control & complete backup

• Serverless backup system – server initiates the backup with a command to a bridging device

• Serverless back device takes over & completes backup process leaving the server through SAN leaving server and network free to run routine tasks

SERVERLESS BACKUPSERVERLESS BACKUP

POINT IN TIME BACKUPPOINT IN TIME BACKUP• Used in the context of disaster – recovery

planning & not for archiving

• Point in time takes regular snapshots of data & not continuous mirroring of data

• Positioned at an alternate location

• heaper to set up & run compared to continuous – mirroring systems

Basics of Computer Memory

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