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EC1003 UNIT III

UNIT III STORAGE DEVICES 9

The floppy drive – magnetic storage – magnetic recording principles – data and disk organization – floppy drive – hard drive – data organization and hard drive – sector layout – IDE drive standard and features – Hard drive electronics – CD-ROM drive – construction – CDROM electronics – DVD-ROM – DVD media – DVD drive and decoder.

Medium

The technology or product type that holds the data

Access time

The time to get to the data

Specified as an average in seconds (e.g., s, ms, µs, ns, etc.)

Throughput

The rate of transfer for consecutive bytes of data

Specified in bytes/s (e.g., Kbytes/s, Mbytes/s)

Online storage

Memory that is accessible to programs without human intervention

Primary storage and secondary storage are “online”

Primary storage

Semiconductor technology (e.g., RAM)

Volatile (contents are loss when powered off)

Secondary storage

Magnetic technology (e.g., disk drives)

Non-volatile (contents are retained in the absence of power)

Offline storage

Memory that requires human intervention in order for it to be accessed by a program (e.g., loading a tape)

Sometimes called “archival storage”

Direct Access Storage Device (DASD)

Pronounced “dazz-dee”

Term coinded by IBM

Distinguishes disks (disk head moves “directly” to the data) from tapes (see below)

Sequential access storage devices

Tape drives

Tape reel must wind forward or backward to the data

Magnetic storage

A magnetic substance is coated on a round surface

The magnetic substance can be polarized in one of two directions with an electromagnet (“writing data”)

The electromagnet can also sense the direction of magnetic polarization (“reading data”)

Similar to a read/write head on a tape recorder (except the information is digital rather than analogue)

Floppy drive

First PC floppy drive could hold only 160K.

PC/XT came with a 5.25-inch 360K.

Largest capacity for a 5.25-inch is 1.2MB.

Do not attempt to format a 360K diskette to 1.2MB—you will have multiple bad sectors, and if you write data to the disk, you run the risk of losing the data.

Also called “flexible disks” or “diskettes”

The platter is “floppy”, or flexible (e.g., mylar)

Most floppy disk drives can hold one diskette (two surfaces)

The diskette is removable

Typical rpm: 300, 360

Capacities: 700 KB to 1.4 MB (& up to 100 MB “zip” disks)

  • Organization of tracks and sectors
  • Density at which data can be stored
  • Intensity of magnetic spots on magnetized plastic surface of the disk

l Cluster

u Smallest logical unit of space allocated to a file

u On a 3½-inch high-density floppy disk, one cluster = one sector (512 bytes)

Potential Problems

  • Application points to a different drive
  • Unrelated error locked up the system
  • System BIO or CMOS setup is not correctly configured
  • Disk in drive is not formatted
  • Floppy drive is bad

Hard Disk Components

w Disk

n Assembly of disk platters

w Disk Drive

n Electromechanical system

l Spins disk

l Moves read/write heads

Hard Disk Organization

w Disk Controller

n Electronic circuitry

w Organization of data

n Concentric tracks

n Divided into sectors

n Divided into logical partitions

w Formatting

n Divides disk into tracks and sectors

n Excludes defective sectors

n About 15% overhead

l Sector headers

l Error-correction codes

l Intersector gaps

w Primary computer storage device

n Secondary Memory

w Spins, reads and writes one or more fixed disk platters

w Storage medium in desktop and laptop computers

w The term “hard” differentiates high-capacity rigid disks made aluminum or glass from low-capacity floppy disks made of plastic

1) First Microcomputer Hard Disk Seagate introduced the first hard disk for personal computers in 1979. At 5MB, the ST506 held 10 times as much as the RAMAC at a fraction of its size.

A sealed metal housing.

Protection against dust particles

An electrical motor connected to a spindle

Spends as many as 8 magnetically coated platters

Today’s platters are coated with an alloy about three millionths of an inch thick

Several thousand revolutions/minute

How a Hard Disk Drive Works

Logic board receives commands from the drive’s controller.

Managed by the operating system and BIOS

Translates commands into voltage fluctuations

Forces the head actuator to move read/write heads

Makes sure the spindle turning the platters is at a constant speed

Tells the drive head to read or write

A head actuator pushes and pulls the read/write heads across the surfaces of the platters with critical precision.

Aligns heads with the tracks

Read/write heads slide in unison across both the top and bottom surfaces of the platters.

Write the data coming from the disk controller by aligning the magnetic fields of particles

Read the data by detecting the polarities of particles that have already been aligned

How Disk Space is Organized

When a disk is manufactured,

Its surface is on a large area.

An organizational structure must be imposed that uniquely names each physical location on the disk

Drive controller can specify the exact physical spot where a given bit of data should be written or retrieved

Cylinders, Heads, Sectors , and Tracks

CHS

Each side or surface of one hard drive platter is called a head.

The number of heads in the same as the number of disk platter surfaces available for writing data

Almost all magnetic disks are double-sided

There is a separated read/write head for each side.

The numbering starts at the bottom side of the bottom platter with 0.

An average number of heads for a hard disk today is 16

A track is a combination of the cylinder and head location over the writeable portion of the hard disk.

In a multiplatter or multiside disk, each side has its own separate tracks

A 1.44MB floppy disk has 40 tracks per side

A large hard disk can have tens of thousands

A sector is the smallest unit that can be read from or written to a disk.

Pie slices made by lines that cross over the track lines.

Each sector holds exactly 512 bytes of data

Modern 1.44MB floppy disks use 36 sectors/track

A typical IDE hard drive usually has 63 sectors/track

A SCSI hard drive can have 600 or more sectors per track

Cylinders are the concentric writeable tracks found on the surface of the platters that make up the hard drive.

The stack of tracks accessible at a given position constitutes a cylinder.

The number of cylinders a drive has in the same as the number of tracks on a single disk side

All areas of a disk at a certain in/out head position on all disk sides combined

Platter

A round surface – the disk – containing a magnetic coating

Track

A circle on the disk surface on which data are contained

Head

A transducer attached to an arm for writing/reading data to/from the disk surface

Head assembly

A mechanical unit holding the heads and arms

All the head/arm units move together, via the head assembly

Cylinder

A set of tracks simultaneously accessible from the heads on the head assembly

Calculating the capacity of the drive

Cylinders x heads x sectors/track x 512 bytes/sector

Divide by

1024 = KB

1048576 = MB

1073741824 = GB (Typical size)

Three possible settings in most BIOS Setup programs (translations)

  1. CHS (Cylinder, Head and Sector) Mode or Normal Mode
  2. Extended CHS Mode (ECHS) or Large Mode
  3. Logical Block Addressing (LBA)

Types of Hard Disks

ž IDE: Integrated Drive Electronics (IDE) is the least expensive way to connect a hard drive to a computer IDE can support two hard drives. Each drive cannot have a storage capacity of more than 528 MB

ž EIDE: Most new computers come with Enchanced IDE (EIDE). EIDE is faster and can connect more devices to a computer than IDE. EIDE can support up to four devices. These devices can be hard drives with storage capacities over 528 MB, or other devices such as CD-ROM and tape drives.

ž SCSI: Small Computer System Interface (SCSI) is the fastest, most flexible, most reliable, but most expensive way to connect a hard drive and other devices to a computer. SCSI is pronounced “scuzzy.” SCSI can connect up to seven devices. These devices can include removable hard drives, CD-ROM drives, tape drives, scanners and printers. These are best used when the system will act as a sever or process large quantities of data.

Hardware Problems

ž Hard drive not found

ž Invalid drive or drive specification

ž Damaged boot record

ž Damaged FAT or root directory or bad sectors

ž Cannot boot from the hard drive

ž Drive retrieves and saves data slowly

File Systems

l FAT16

u Supported by all Windows systems

l FAT32 (and VFAT)

u Supported by Windows 95 Second Edition, Windows 98, Windows 2000, Windows XP

l NTFS

u Supported by Windows NT, Windows 2000, Windows XP

l Each logical drive has its own file system

Optical Disks

Data are stored as “pits” and “lands”

These are burned into a master disk by a high powered laser

Master disk is reproduced mechanically by a stamping process

Data surface is protected by a clear coating

Data are read by sensing the reflection of laser light

A pit scatters the light

A land reflects the light

CD vs. DVD

  • Compact Disk (CD)

n CD-ROM

n CD-R

n CD-RW

  • Digital Versatile Disk (DVD)

n Shorter wavelength laser

l Smaller focus, smaller pits, closer tracks

  • Capacity

n CD = 800 MB

n DVD = 4.7 GB

CD-ROM Data Organization

Uses light generated by lasers to record and retrieve information

Information is stored by varying the light reflectance characteristics of the medium

Available in read-only (CD-ROM) and read/write formats

270,000 blocks of 2048 bytes each (typically)

270,000 ´ 2048 = 552,960,000 bytes

Extensive error checking and correction (e.g., bad regions of the disk flagged)

Substantial overhead for error correction and identifying blocks

Capacity can be as high as 630 MB

CD-ROM

Magnetic Disk

One spiral track (3 miles long!)

Multiple tracks of concentric circles

Constant bit density

Variable bit density

Disk speed varies (CLV, constant linear velocity)

Disk speed constant (CAV, constant angular velocity)

Constant transfer rate

Constant transfer rate

Capacity: 550 MB

Capacity: varies

Magnetic Tape Systems

  • Off-line storage of large amounts of data
  • Back-up and archival storage
  • Data organized into records and files
  • Highest capacity
  • Slowest
  • Cheapest

CD’s

  • Read-only; data physically embedded into disc surface
  • Store data as pits and lands
  • Use constant linear velocity (CLV) and constant angular velocity (CAV)
  • Look for multisession feature
  • Use precautions when handling

DVD

  • Has large storage capacity (8.5 GB one side; 17 GB both sides)
  • Uses UDF file system
  • Uses MPEG-2 video compression; requires MPEG-2 controller to decode compressed data
  • Stores audio in Dolby AC-2 compression
  • Recently: HD-DVD and read-writable DVDs

A DVD-ROM drive is a device that reads information stored on DVD-ROM discs. DVD-ROM stands for Digital Versatile Disc-Read-Only Memory (DVD-R). Read-only means you cannot change the information stored on a disc. DVD-ROM disc is similar in size and shape to a CD-ROM disc, but can store a lot more information. DVD-R or DVD-RW are for computers. Usually DVD-RAM are for video players.

DVD-ROM drives can play DVD-Video discs, which hold full-length, full-screen movies with much better quality than videocassettes. Many DVD-Video discs allow you to change the way you view the movie, such as displaying subtitles. You may need special hardware, such as an MPEG-2 video decoder card, for the best playback of DVD-Video discs.

A single DVD disc can store at least 4.7 GB of data, which equals over seven CD-ROM discs. Unlike a CD-ROM disc, a DVD disc can be single-sided or double-sided. Each side can store one or two layers of data. The speed of a DVD-ROM drive determines how quickly data can transfer from a disc to the computer. Current DVD-ROM drives commonly have a speed of 6X.

Installing a DVD Drive

Installing a DVD Drive

Categories: EC1003 UNIT V
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