8.1 Hardware devices - Storage

Ranjit Bassi

Hard disk• The hard disk uses magnetic disk drives which allows rapid random access

to large amount of data• All disk drives use a thin circular platter made of non-ferrous metal or

plastic and rotate it at up to 10,000 revolutions per minute beneath a read-write head that moves radially across the surface of the platter

• The platter is coated with an emulsion of iron or cobalt oxide particles that act as tiny magnets

• Binary data is recorded by aligning these tiny magnets in one direction to represent a binary 0 and in the opposite direction to represent a binary 1.

• To read data stored on the hard disk, the read write head is moved to the desired track and waits for the relevant sector to pass beneath it.

• In hard disk’s the greater amount of platters, the greater amount of storage capacity

Magnetic tape

• This is a cheap alternative to store large quantities of data• Magnetic tape consists of a thin ribbon of plastic, the tape is

coated on one side with a material that can be magnetised to record the bit patterns that represent the data

• The cartridge which contains the tapes is completely sealed apart from an access door to put the tape into contact with the recording heads in the tape drive

• A mechanism moves the tape between the supply and take-up reels

• Storage capacities range from 10GB to 800GB and data transfer speeds range from 200KB to 20MB per second

Optical media

• Optical media disks (LaserVision) records the video image as a frequency modulated (FM) signal on a tight spiral track that makes 54,000 turns in covering a disc surface.

• The turns of the spiral are 1.6 micrometres apart, corresponding to a track density of almost 16,000 tracks per inch(630 tracks per millimetre)

• The FM signal is recorded by making pits or tranches in the disc’s surface; a pit is a physical depression

• These discs have a very high storage capacity• Compact discs (CDs) store up to 74 minutes of very high

quality audio on one surface of a 120mm plastic discs

CD-ROM• Success of CDs led to CD-ROM (Compact disks read only

memory)• CD-ROMs had a high storage capacity of 600-700mb• They offer fast data access and portable and they are read only• Data is written onto the discs by using disc-mastering machinery

that impresses pits into a continuous spiral track• The silvery data surface contains pits in a single track 3.5 miles

long• The disc spins at 200-500 revolutions per minute depending on

which radius is being read• A data bit is read by focusing a laser beam onto a point in the

reflective metal layer where the pits are impressed

CD-R

• CD-recordable discs that support WORM operation are available in the standard in the standard size of 120 mm

• Also known as writable CDs• They can record about 600-700 MB of data• Any region of the disc can be written on but

then it cannot be written on it again or altered• Data bits are recorded by burning a pit in a thin

film of metal using a high-power laser

CD-RW

• A CD-rewritable can be read and written on over and over again

• CD-RW drives have a magnet and a laser• The write operation uses the magnet and laser

but the read operation uses only the laser• To record or change data, the laser heats a

precise spot on the disc to a temperature of 200 degrees

• Capacity is similar to the CD-R

DVD-ROM

• Digital versatile disc or digital video disc is an optical standard that offers much greater storage capacity than CDs

• On a DVD the minimum length of the pit is 0.4 micrometres• This lets the disc-mastering machinery squeeze the pits

more closely together.• On a DVD the data track’s spiral spacing is 0.74 micrometres • DVD drives uses lasers with short wavelengths to read to

very small pits• Reducing pit sizes increases the disc storage capacity to

about 4.7 GB

DVD-R

• DVD-R is a WORM format similar to CD-R• It replaces the layer of read only

polycarbonate with an organic dye• DVD-R records data by burning spots in the

dye to alter the amount of laser light reflected • The discs have a capacity of 4.7 GB• The life of these discs vary from 70 to 100

years

DVD-RW

• The DVD-RW format provides a rewritable optical disc with a typical capacity of 4.7 GB

• These discs are made to be re-written up to 1000 times before a replacement is needed

• A benefit of these discs compared to others are that when a writing error is made the whole disc is not ruined , it can still store data by erasing the faulty data

• The recording layer in these discs are not organic dye but a special phase-change metal alloy

• Data can be written, erased and rewritten

DVD-RAM

• DVD-RAM is a rewritable format that has built in error control and a defect management system

• DVD-RAM is can be written and erased very easily• These discs store data in one spiral track and

require special software for reading and writing• Storage capacities range from 2.58GB to 9.4GB

Blu-Ray Disc

• This is a high density optical disc that stores digital information, including high definition video

• Its name derives from the blue-violet laser used for reading and writing

• It uses a wavelength of 405nm, this allows it to store more data

• A single layer blue ray disc can store 25 GB and a dual layer Blu-ray disc can store up to 50GB

• This disc offers very high transport speeds

HD DVD

• This type of DVD can store up to 30GB• This disc rivalled the Blu-ray disc but lost the

race due to the backing of Hollywood studios causing HD DVD’s demise

• They are written and erased like a normal DVD using a red laser as normal

Solid state Memory

• NAND-type flash memory is a type of EEPROM which supports erasure of an individual block of memory cells

• To alter the contents of a particular memory location, it copies the entire block into an off-chip buffer memory, erases the block, then rewrites the data back into the same block, making the necessary alteration to the relevant memory location

• It requires a dedicated microprocessor with RAM buffer and this is included in a USB flash memory drive.

• It can also be used as a music coded to turn a flash drive into an MP3 player

USB Flash Drive• A USB flash drive or memory stick is a NAND-type flash-memory device plus a

USB interface.• A flash drive consists of a small printed circuit board typically in a plastic or

metal casing, making the drive sturdy enough to be carried about in a pocket• Only the USB connector protrudes from this from this protection and it is

usually covered by a removable cap• A NAND-type flash memory chip is mounted on the printed circuit board• Flash drives are relatively dense form of storage; even the cheapest flash

drive can store all the data on dozens of floppy discs• Storage capacities range from 64MB to 32GB• Manufacturers are starting to ship laptops with flash memory secondary

storage instead of magnetic hard disk storage• This is due to low power consumption and faster booting of the operating

system

Memory Card

• A memory card is a solid state flash memory device used for data-storage in digital camera

• These cards offer high capability, power-free storage, a small form factor and rugged environmental specifications

• Storage capacities range from 128MB to 4GB

Storage medium Capacity Transfer speed Access time (ms)

Magnetic Hard disk 19.3GB to 1.2 TB 5-100MB/s Under 10

Magnetic floppy disk 737,280 to 1,474560 bytes 250-500kbit/s 94

Magnetic tape cartridge 10-800GB 200 KB/s to 20MB/s Long

CD-ROM,CD-R,CD-RW 600-700GB 153,600 to 7,372,800 B/s, 1x to 48x ~100

DVD-ROM,DVD-R, DVD-RW, DVD-RAM 2.8-17.1GB 1,385,000 to 22,160,000 B/s,1X to

16X ~100

Blu-ray 25-50GB 36-288 Mbit/s 1X to 8X 110

PDD 25-50GB 10MB/s 100

HD DVD 15-51GB 36 Mbit/s 500

USB flash drive 32MB to 5GB 1-60 MB/s 0.8 to 10

Memory Card 128MB to 4GB 900 KB/s to 22.5 MB/s 0.8 to 10