01/04/2014IS50004A1 Chapter 3 Data Storage

01/04/2014 IS50004A 2 Learning outcomes By the end of this Chapter you will know the difference between Electronic memory Magnetic memory Optical memory

01/04/2014 IS50004A 3 Additional Reading Essential Reading Stalling (2003): Chapters 5 and 6 Further Reading Burrell (2004): Chapters 3 and 7 Schneider and Gersting (2004): Chapters 4 and 5 White (2002): Parts 3 and 4.

01/04/2014 IS50004A 4 Hardware Components

01/04/2014 IS50004A 5 Introduction Information can be stored in different ways: Books, Films Information is stored to be re-used. Information in computers must be able to able to be processed by computers: Information must be stored in appropriate places

01/04/2014 IS50004A 6 Introduction (Cont) Computers use the binary system to store and process information. Different type of media storage Electronic memory (main memory) Magnetic memory (hard disc) optical memory (CD-ROM)

01/04/2014 IS50004A 7 CPU Main memory Add. bus Data bus Control bus

01/04/2014 IS50004A 8 Main memory

01/04/2014 IS50004A 9 Main Memory (Electronic Memory) What is the role of the main memory? Main memory stores data which are currently been processed or ready to be processed Electronic memory (fast) Based on electronic principles. Formed with logic gates Main Memory is volatile It is divided into cells Each cell is a sequence of one-bit memories Each cell has a unique address

01/04/2014 IS50004A 10 Arrangement of Memory Cells value = Each cell has a unique address Longer strings stored by using consecutive cells RAM (random access memory)

01/04/2014 IS50004A 11 Media storage Magnetic Storage Tapes Hard drives (not always removable) Floppy Zips Jaz,Optical Storage CDs DVDs Solid State Storage Compact flash Smart Media

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01/04/2014 IS50004A 13 What is Magnetic Media? The most common and enduring form of removable- storage technology is magnetic storage. The magnetic medium can be easily erased and rewritten, and it will "remember" things stored onto the medium for many years.

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01/04/2014 IS50004A 15 Magnetic Memory

01/04/2014 IS50004A 16 Magnetic tape (2) Serial access (slow) Good choice for off-line data storage (archives)

01/04/2014 IS50004A 17 Magnetic Tape (1) Serial access Slow Very cheap High capacity Backup

01/04/2014 IS50004A 18 Magnetic Memory

01/04/2014 IS50004A 19 Magnetic disc Each track contains same number of sectors Each sector contains a number of magnetized and demagnetized spots spots sector

01/04/2014 IS50004A Stalling (2003) pages: Magnetic Disk Terminology Platter: rigid metal or glass platter Coated with magnetic material. rotating at constant angular velocity Arm: With movable magnetic read/write heads Track: A complete ring of data The disk surface is divided into tracks Sectors: Each track is subdivided into sectors Cylinder: A vertical collection of tracks at the same radial position

01/04/2014 IS50004A 21 Magnetic Disks

01/04/2014 IS50004A 22 Magnetic disc density Same number of bits per track Tracks near the centre are more dense The further is the tracks the less dense it is. Constant Angular Velocity (CAV) The disc rotation speed is constant to maintain a constant transfer. Stalling (2003) pages:

01/04/2014 IS50004A 23 Thus as the platter rotates under the head, a stream of bits can be written and later read back. Magnetic Disks

01/04/2014 IS50004A 24 Read and Write Mechanism (1) Recording and retrieval via conductive coil called a head May be single read/write head or separate ones During read/write, head is stationary, platter rotates Write Electric Current through coil of wire produces magnetic field Magnetic Pulses sent to the head Magnetic pattern recorded on surface below Read Magnetised bit pattern Magnetic field induces an electrical current in the coil The bit pattern contains 1 Demagnetised bit pattern No Magnetic field induced, hence, no electrical current in the coil The bit pattern contains 0

01/04/2014 IS50004A 25 Fixed/Movable Head Disk Fixed head One read/write head per track Heads mounted on fixed ridged arm Movable head One read/write head per side Mounted on a movable arm

01/04/2014 IS50004A 26 Access Information on a Floppy disk To access information on a floppy: Track number, and Sector number. Head moves to the target track. waits for the desired sector to spin underneath it read/write begins.

01/04/2014 IS50004A 27 Maximum data transfer rate It is the rate at which data passes under the read/write head (bytes/sec). Number of bytes / track * Number of rev / sec

01/04/2014 IS50004A 28 Multiple Platter (hard disk) Permanent storage that is inside of the computer, and NOT portable. Consists of several platters which spin very fast Heads are joined and aligned Aligned tracks on each platter form cylinders Data is striped by cylinder reduces head movement Increases speed (transfer rate)

01/04/2014 IS50004A 29 Multiple Platters (2) Disk platters speed (3600 to rpm (rev/min). floppy (360rpm). The read data we need to specify cylinder, head, and sector numbers. Each cylinder represents a track number.

01/04/2014 IS50004A 30 Cylinders

01/04/2014 IS50004A 31 Magnetic Tape (1) Serial access Slow Very cheap High capacity Backup

01/04/2014 IS50004A 32 Features of Magnetic Memory Memory capacity: Floppy can hold 700KB – 120MB. Hard disk can hold dozen of GB, 10, 20,.. Tapes can hold 100MB- 80GB. Access method Floppy and hard disks is random as the main memory Tape is serial Access time: It is the average time taken to position the R/W head over the data to be read For disk drives is about sec when in MM sec. Transfer rate: is slower. It is the transfer of data between MM and Mag/M. Floppy (500kB-2MB) and hard disc (4-12MB).

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01/04/2014 IS50004A 36 Optical Storage – CD-ROM Is a disc with highly reflective surface. Tiny areas flat and depressed: Flat (land) strong reflection. Depressed (pits) low reflection. Laser land strong reflection photo-sensor generates electrical voltage store 1s. Laser: (light Amplification stimulated emission of radiation). Light pits low reflection no electrical voltage stores 0s.

01/04/2014 IS50004A 37 CD-ROM Operation Data stored by creating variations in the reflective surface Data retrieved by means of a laser beam Data stored uniformly (so CD rotation speed varies) Random access much slower than for magnetic disks

01/04/2014 IS50004A 38 The pits and lands are written in a single continuous spiral starting near the hole and working out a distance of 32 mm toward the edge. The spiral makes 22,188 revolutions around the disk (about 600 per mm). If unwound, it would be 5.6 km long.

01/04/2014 IS50004A 39 Optical disc– Random access Difficult Move head to the right position Set correct speed Read address Adjust to required location

01/04/2014 IS50004A 40 Constant linear velocity centre edges rev/m sector Constante density

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01/04/2014 IS50004A 51 Flash memory A very popular type of removable storage for small devices, such as digital cameras and PDAs, is Flash memory. Flash memory stores information in an array of memory cells made from floating-gate transistors. In traditional single-level cell (SLC) devices, each cell stores only one bit of information. Some newer flash memory, known as multi-level cell (MLC) devices, can store more than one bit per cell by choosingfloating-gate transistorssingle-level cellmulti-level cell

01/04/2014 IS50004A 52 Summary Main memory RAM Low storage capacity Fast (electrical signals) Volatile. Magnetic memory Floppy disk Hard disk Magnetic tape Optical memory CD_ROM disk DVD Solid state storage Flash memory