Computer Component :

Storage DeviceStorage Device(External Memory, Secondary Memory, Secondary Storage)

• Storage Types• Magnetic Types• Optical Types

Storage Types

๏ Technology Types•Magnetic (Disk & Tape)•Optical (Disk)

๏ Access Method Types•Sequential Access (Magnetic Type)•Direct Access (Magnetic & Optical

Disk)

๏ Shape Types• Disk (Fixed, Removable)• Tape (Cartridges, Cassette)

Magnetic Types

๏ Magnetic Disk•Definition•Data Organization, Formatting and Disk

Layout Method•Physical Characteristic •Disk Performance Parameters

๏ Magnetic Tape•Definition•Data Organization and Formatting

Magnetic Disk Definition

• A Disk– Circular platter constructed of metal or of plastic

coated with a magnetizable material. – Data are recorded on and retrieved from the disk

via a conducting coil, head. – To write, electricity flows through a coil

produces a magnetic field then magnetic patterns (positive or negative) are recorded.

– To read, magnetic field moves relative to a coil produces an electrical current of the same polarity as it recorded in the coil

Magnetic Disk Data Organization

• Tracks - the organization of data on the platter in a concentric set of rings, each track is the same width as the head.

• Data are transferred to and from a disk in blocks• Sectors - data are stored in these block-size

regions that maybe either fixed or variable length.• Adjacent tracks or sectors are separated by gaps• Density - bits per inch, increases from outer track

to inner track• Clusters - groups of sectors that use to store a file• Cylinders - tracks in the same position of each

side in multiple platter

Magnetic Disk Formatting

• A process that prepare a disk for use• Create tracks and sectors on disk• Record information of disk

•number of byte/sector and sector/track•File Allocation Table (FAT)

•Root folder (directory) • A bit near the center of a rotating disk

travels past a fixed point (e.g.. read-write head) slower than a bit on the outside.

Magnetic Disk Disk Layout Methods

Multiple Zone Recording• devide disk into a number of

zones (~1 6 )• withina zone, t he number of bi t s

per t r ack i s const ant• zonesfartherfromt he cent er cont ai n mor e b

its (more sectors) than the closer.

• the time to move a head for r eads and writes vary from on

e zone to another.• Pro: i ncr easi ng capaci t y

Constant Angular Velocity (CAV)

• increasing space between bits• bits can be scanned at the

same rate• devide disk into a number of

pie-shaped sectors and a series of concentric tracks

• Pro : block can be directly address by track and sector

• Con : long outer track can store data in the same amount as short inner tracks

Magnetic Disk Physical Characteristics

• Head Motion– Fixed head (one per

track)– Movable head (one

per surface)

• Disk Portability– Nonremovable disk– Removable

• Sides– Single sided– Double sided

• Platters– Single platter– Multiple platters

• Head Mechanisms– Contact (Floppy)– Fixed gap– Aerodynamic gap

(Winchester)

Magnetic Disk Disk Performance Parameters

• Average Access Time ( Ta )– Seek time ( Ts )– Latency time ( r - Rotational delay

)– Transfer time ( T )

Ta = Ts + 1 + T 2r

Magnetic Disk Disk Performance Parameters

• Seek Time ( Ts )– The time required to move the head to the requ

ired track– - A movable head system moves the head– - A fixed head system electrically selects one he

ad

m = constant depends on the disk driven = number of tracks traverseds = startup time

Ts = m x n + s

Magnetic Disk Disk Performance Parameters

• Latency Time ( r - Rotational delay )– The time required to move the beginning o

f the sector to the head– - Floppy disk rotates 300 600 rpm,

- average delay will be 100 200 ms– Other disks rotate 3600-15000 rpm

(1 revolution per 16.7 ms to 1 revolution per 4 ms ), average delay will be 8.3 ms

Magnetic Disk Disk Performance Parameters

•Transfer Time ( T )– The time to transfer data to or from the disk– Depend on the rotation speed of the disk

b = number of bytes to be transferredN = number of bytes on a trackr = rotation speed, in revolutions per second

T = b rN

20 1Ts = ms, transfer rate = MB/s, 512 bytes/sector,

32 sectors/track, a file consists of 256 sectors for 128 KB

Magnetic Disk A Timing Comparison

A sequential organization 8 32( adjacent tracks x sector)

• mm20 .0• mm8. 3• 32Read sectors 1

m m6 .7 m m45

• 1Read track 8.3+16.7= 25 ms

• Ta = 45 + 7 x 25 = 220 ms

A random organization

• 2Average seek m m0 .0

• mm8. 3• 1Read sector

05. ms 288. ms

• Ta = 256 x 28.8 = 7373 ms

Magnetic Tape Definition

• The medium is flexible polyester tape coated with magnetizable material.

• The tape and tape drive are analogous to a home tape recorder system.

• Use the same reading and recording techniques as disk system.

• Tapes used to be packaged as open reels that have to be threaded through a second spindle for use.

• Today all tapes are housed in cartridge

Magnetic Tape Data Organization & Formatting

• Structured as a number of parallel tracks running le-ngth wise.

• Serial recording - data are laid out as a sequence o f bits along each track.

• D ata are read and written in contiguous blocks, phy sical records.

• Block are separated by interrecord gaps.• The tape is formatted to assist in locating physical re

cords.• Data are recorded serially along individual tracks.• Blocks in sequence are stored on adjacent tracks.

Magnetic Tape Characteristics

• A tape drive is a sequential-access device. • Read all of the front records sequentially

to get the desired one.• Tape is in motion only during read or write

operation.• It is the first kind of secondary memory. • Usually use for backup data from the

system.• Lowest-cost, slowest-speed memory

Optical Types

๏ Compact Disk (CD)•CD-ROM•CD Recordable•CD Rewritable

๏ Digital Versatile Disk (DVD)

Compact Disk CD-ROM

• 1983CDdigitalaudiosystemwasintroducedin as a noner asabl e di sk t hat can st or e m 6 0 .ore than mins of audio information

• mm mmm mm -mm m mmmmm m mmm mmmm mmmmmmmmmmm.• mm -mm m mmmmmm mmm m mmm mmmmmm mmm mmmm mmmmm m

mmmmmmm.• mmmm mm mmmm mm mmmm m mmmmmm .• Digitally recorded information is imprinted as a se

ries of microscopic pits on the surface and then co m mmm m mmmmmm mmmmmmmmmm mmmmmmm mmmmm mmmm mm ( mmmm mmmmmm m mmm mmmmmmmm) .

Compact Disk CD-ROM Layout Method

• CD and CD-ROM contain a single spiral track, begin near the center and spiraling out to the outer edge of the disk.

• Sectors near the outside are the same length as those near the inside.

• The information is packed in segments of the same size and these are scanned at the same rate by rotating disk at a variable speed, known as Constant Linear Velocity (CLV).

• The capacity of a track and the rotational delay increase for positions nearer the outer edge of disk.

Compact Disk CD-ROM data Organization

• Data are organized as a sequence of block• A typical block consists of

– Sync : identifies the beginning of a block (12 bytes).

– Header : contain block address and mode byte (4 bytes). • Mode 0 - a blank data field• Mode 1 - the use of an error-correcting code and 2048

byte of data• Mode 2 - 2336 bytes of data with no error-correcting code

– Data : User data– Auxiliary : Additional user data in mode 2. In mode

1, 288-byte error-correcting code (ECC).

Compact Disk CD-ROM Pros & Cons

Pros:• The optical disk can be mass replicated

inexpensively.• The optical disk is removable, allowing the

disk itself to be used for archival storage.Cons:• It is read-only and cannot be updated.• It has an access time much longer than

that of a magnetic disk.

Compact Disk CD Recordable (CD-R)

• A disk is prepared in the way that it can be subsequently written once with a laser beam with a more expensive disk controller than for CD-ROM

• The medium is similar to (not identical) that of CD or CD-ROM.

• Information is recorded by the the pitting of the surface, which changes reflectivity.

• The disk can be read on a CD-R or CD-ROM drive.• It provides a permanent record of large volumes

of user data.

Compact Disk CD Rewritable (CD-RW)

• The disk can be repeatedly written and overwritten.

• Current materials can be used for 500,000-1,000,000 erase cycles.

• It can be rewritten and used as a true secondary storage.

• It is higher reliability and longer life than magnetic disks.

Digital Versatile Disk

• The industry has a found an acceptable replaceme nt for the analog VHS video tape and replace the C- D ROM in PCs and servers.

• Itcan be r andoml y accessed l i ke audi o CDs.• It can store data with vivid quality, currently 7 time

-s of CD ROM.• The greater capacity is due to :

– 47Bits are packed more closely on a DVD ( . GB).– - mmmm(8.5), ( 17).

• DVDs come in recordable, rewritable as well as rea- d only version.