Chap 01 02

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

1- Mass Storage Systems Magnetic Systems Disk Tape Optical Systems CD DVD Flash Drives

1- Figure 1.9 A magnetic disk storage system magnetic coating

1- 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.

1- 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

1- Disk Fragmentation Data can become fragmented over time. De-fragmentation: the process of re ordering data to make storage more efficient.

1- Figure 1.10 Magnetic tape storage

Optical Systems CD 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 inner DVDs –have multiple semi transparent layers which are distinct surfaces when viewed by a precisely focused laser. More storage (several GB).

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 1-

1- 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)

1- Figure 1.12 Logical records versus physical records on a disk

1- 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

1- Figure 1.13 The message “Hello.” in ASCII

1- 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

1- 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

1- 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

1- 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

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