Indira Gandhi National Open University

presents

A Video Lecture

Course:

Computer Platforms

BACKING STORAGE

AND

INTERFACING

Objectives

Define some backing storage technologies

Discuss about the storage formatsDiscuss about the head mechanism of

hard disksDefine some of the interfacing

mechanisms for backing storage

What is Magnetic Disk?

A circular metal or plastic coated with magnetisable material

A mechanism using conducting coil which reads/write on disk: called head

Concentric tracks with width same as the head size

Disk-data layout

Why does Gap exist between adjacent tracks?

• To minimise errors due to Misalignments

• To minimise interference of magnetic fields

The track on outermost side has same number of bits as the innermost track.

Density per linear inch increase as we move from outer to inner track

REASON: SIMPLE ELECTRONICS

Data are stored in block-size region called SECTORS

– Typically 10-100 sectors/track

– To avoid unnecessary precision requirements on system, adjacent sectors are separated by intra track gaps

How are sector position within a track identified?

• Control data for recording start and end of each sector and starting position on a track

• Example, 600 bytes are used for storing 512K data

Head Motion:Fixed head - one/track.

Moveable head (one per surface)

Disk portability : Non removable/ Removable

Sides : Single/Double

Platter : Single/ Multiple

Some Characteristics of Disks

Head Mechanism

• Contact (Floppy)

• Fixed gap

• Aerodynamic gap (Winchester)

R A I D (Redundant Array of Independent Disks)

• A set of physical disk drives viewed as a single logical drive

• Data is distributed across the physical drives of an array

• Redundant disk capacity is used to guarantee data recoverability in case of a disk failure

Optical Memory

• CD-ROM

• WORMs

• Erasable Optical Disks

CD-ROMs

CD-ROMs

• Appropriate for distribution of large amount of data to a large number of users

• High information storage capacity

• Mass replication is inexpensive

• Removable disks thus may be used for archival storages

Disadvantages:

• Once fabricated cannot be changed

• Higher access time than disk

WORMs

• Applicable for applications when small number of copies of a set of data is needed

• Disk can be written by a special writer once only

• WORMs are useful for archival storage of documents

Erasable Optical Disk

• Can be read or written just like magnetic disk

• Technology is magnetic-optical system

• High Capacity

• Portable disks

• Reliable disks

• Use formats same as that of Hard disk

Some Newer Storage Technologies

ZIP Drives• A high capacity floppy disk drive

developed by Image Corporation

• Hold about 100 MB of data

• Inexpensive and Durable

• Useful as a popular backing storage media

• Useful for transporting large files

Digital Video Disks

• A new type of CD-ROMs

• Have a capacity of 4.7 GB to 17 GB

• Store movie date in MPEG-2 format

• Smaller and easier to store and durable

• Better picture quality and alternate sound Tracks

Interfacing storage devices to The Computer

Why I/O Interfaces needed?

• For coordinating the flow of data between Internal resources and external devices

• Data buffering : To cater for the speed differences between the storage devices & CPU

• Error detection

Communicates with System

through System or I/O bus

Communicates with devices

The Interface

What is ESDI?

Enhanced Small Device Interface (ESDI) an interface standard for connecting disk drives to PCs.

ESDI controllers are needed to use ESDI disks

What is IDE and Enhanced IDE?

• An interface for mass storage devices

• The controller is integrated into the disk or the drive

Intelligent Electronics or

Integrated Device Electronics

• EIDE supports data rate between 4 Mbps to 16.6 Mbps three to four time faster then IDE

• EIDE can support mass storage up to 8.4 GB whereas IDE was up to 528 MB

What is UDMA?

• Supports data transfer rates of 33.3 Mbps

• Twice as fast as previous disk standards for PCs

SUM UP:

• The secondary storage technologies such as disks, CD-ROMs

• The data format of disks & CD-ROMs

• Head mechanisms for disks• The interface standards for storage

devices