Chap 01 02

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Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 1-1

Mass Storage (Secondary Storage)

• On-line versus off-line• Typically much larger than main memory• Less volatile than main memory• Access much slower than main memory

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Mass Storage Systems

• Magnetic Systems– Disk– Tape

• Optical Systems– CD– DVD

• Flash Drives

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Figure 1.9 A magnetic disk storage system

magnetic coating

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• each track is split into sectors/arcs, each sector can be accessed as an independent string of bits

• a sector could contain upto a few KB• tracks and sectors are not a permanent part of a

disk's physical structure. They are marked magnetically through formatting the disk.

• hard disks v floppy disks• capacity varies - number of disks, density of

sectors and tracks• for speed, heads might float off the disk and not

touch it. one particle of dust could jam between head and disk - head crash.

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Disk performance

• Seek time – moving heads from one track to another

• Rotation delay/latency time – half the time a disk takes to rotate (the average time it takes for the desired data to rotate round to a head). Eg several thousand revs per minute

• Access time – seek time + rotation delay• Transfer rate – rate data transferred to/from disk

eg MB a second

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Disk Fragmentation

• Data can become fragmented over time. • De-fragmentation: the process of re ordering

data to make storage more efficient.

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Figure 1.10 Magnetic tape storage

Optical SystemsCD has reflective material covered with a clear

protective coating (600-700MB)• Data recorded by creating variations in the

reflective surfaces• Data retrieved by laser beam that monitors

irregularities on the reflective surface as it spins• A single track spirals from the middle outwards• Track divided into sectors (2KB)• Data stored at a uniform linear density over entire

track –more data on outer part of disk than innerDVDs –have multiple semi transparent layers which

are distinct surfaces when viewed by a precisely focused laser. More storage (several GB).

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Figure 1.11 CD storage

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Copyright © 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

Flash Drives

• No heads or spinning or moving lasers, just electronic circuitry –fast! Not sensitive to physical shock

• Bits are stored by sending electronic signals directly to the storage medium where they cause electrons to be trapped in tiny chambers of silicon dioxide

• Repeated erasing damages the silicon dioxide chambers. So not used for main memory, used for digital cameras, PDAs

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Files

• File: A unit of data stored in mass storage system– Fields and keyfields

• Physical record (eg all the data on a sector) versus Logical record (eg a staff member details)– One physical record could hold many logical– One logical record could spread over many

physical• Buffer: A memory area used for the temporary

storage of data (usually as a step in transferring the data)

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Figure 1.12 Logical records versus physical records on a disk

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Representing Information:Representing Text

• Each character (letter, punctuation, etc.) is assigned a unique bit pattern.– ASCII: Uses patterns of 7-bits to represent

most symbols used in written English text– Unicode: Uses patterns of 16-bits to represent

the major symbols used in languages world side

– ISO standard: Uses patterns of 32-bits to represent most symbols used in languages world wide

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Figure 1.13 The message “Hello.” in ASCII

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Representing Numeric Values

• Using ASCII, to store 25 need 16 bits, largest number is 99

• Binary notation: Uses bits to represent a number in base two; 16 bits could represent 0 to 65535

• Limitations of computer representations of numeric values– Overflow – occurs when a value is too big to

be represented– Truncation – occurs when a value cannot be

represented accurately

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Representing Images

• Bit map techniques– Pixel: short for “picture element”– Black & white picture: 1 for black, 0 for white or 8

bits to record shades of greyness– Colour picture, two approaches:

• RGB – each pixel is 3 colours, record intensity of each colour, need 3 bytes

• Luminance (brightness- the sum of RGB components), red chrominance (difference between luminance and amount of red light in the pixel) and blue chrominance

– can’t rescale• Vector techniques – scalable egTrueType, PostScript

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Representing Sound• Sampling techniques

– Eg 8000 samples/sec for long distance phone– Used for high quality recordings eg 44,100 samples/sec– Need 16 bits a sample, so each second of music is more

than a million bits– Records actual audio

• MIDI– Used in music synthesizers, video game sound,– Records “musical score” – what instrument is to play

which note and for how long. So a clarinet playing D for 2 seconds can be encoded in 3 bytes rather than over 2 million

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Figure 1.14 The sound wave represented by the sequence 0, 1.5, 2.0, 1.5, 2.0, 3.0, 4.0, 3.0, 0

Chap 01 02

Spiritual

disk fragmentation data

temporary storage of

mb data

ordering data

unit of data

desired data

cd storage

mass storage secondary