Computer Systems and Elements of Programming

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Computer Systems and Elements of Programming

• Lecturer Steve Maybank
• School of Computer Science and Information
  Systems
• sjmaybank_at_dcs.bbk.ac.uk
• Autumn 2009
• Week 6a Data storage

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Hard Disk
http//www.storagereview.com/ guide2000/ref/hdd/op
/index.html
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Stack of Magnetic Disks
http//www.ntfs.com/hard-disk-basics.htmHard
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Physical Basis for MD

• Each bit is stored by magnetising a small region
  of the disk surface.
• Floppy disk plastic, 1.44 MB, 300 rev/min,
  transfer rate KB/sec.
• Hard disk aluminium, 2 TB/disk, 10,000 rev/min,
  transfer rate 125 MB/sec.
• Seek time, latency time, access time, transfer
  rate.

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Organisation of an MD

• Data is stored in concentric circular tracks on
  the disk.
• Each track is divided into sectors.
• Each track has the same number of sectors. Each
  sector has the same size, e.g. 512 Bytes or 1024
  Bytes.
• Formatting creation of tracks and sectors on a
  disk. Formatting makes the previous disk contents
  inaccessible.

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Compact Disk
Spiral track spacing between turns 1.6
microns width 0.5 microns total length 5 km!
Disk read from below using near infra red
laser, wavelength 780 nm.

• http//entertainment.howstuffworks.com/cd1.htm

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Cross Section of CD-R Disk
Bumps. Width 0.5 micron Length 0.83 micron Ht
0.126 micron Fabrication injection moulding.
CD-RW no bumps, but same spiral track.

• http//entertainment.howstuffworks.com/cd1.htm

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Organisation of a CD-R

• Single spiral track read from the centre
  outwards.
• Bit density along track is constant.
• Track divided into sectors 2 KB. Total capacity
  0.8 GB.
• Each bit stored as a mark or bump on the
  surface, and read using laser light.

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Organisation of Magnetic Tape

• Each bit is stored by magnetising a small region
  of the tape surface.
• Reliable, cheap. Formerly the only way to store
  GBs of data
• Large access times, as tape is read sequentially.

Compact audio cassette http//en.wikipedia.org/wik
i/Magnetic_tape
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Flash Memory

• Bits stored by accumulating charge in small
  chambers.
• Shock resistant.
• Cost/bit gt Hard drive cost/bit.
• Eventually damaged by repeated erasure -gt not
  suitable for very long term use.

USB flash memory device http//en.wikipedia.org/wi
ki/Flash_memory
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Random Access Memory (RAM)

• Fast electronic memory.
• Used for computer main memory (primary storage).
• Holds data and programs during run time.

Common RAM packages http//en.wikipedia.org/wiki/R
andom_access_memory
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Devices For Storing Data
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Organisation of Main Memory

• Physical each bit is stored by a small
  electrical circuit.
• Logical a list of cells or words addressed from
  0 to 2n-1, 8ltnlt30.
• All cells contain the same number of bytes, e.g.
  1 byte.

34 35 36 37 38
39 40 41 42 43 44
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File Storage in Main Memory


File1 File2 ?
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Definition of Random Access

• The cells in the memory can be accessed
  individually.
• Read times and write times do not vary from cell
  to cell.
• Example main memory RAM.
• The following are not random access M. Disk, CD,
  M. Tape

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Main Memory

• Mostly DRAM (dynamic RAM) power is needed to
  maintain the memory.
• Small non-volatile part for booting (start up).
• The CPU reads from and writes to the main memory.
• Fast read and write (nano seconds)

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Volatile and Non-Volatile Memory

• Volatile memory contents lost if the power is
  switched off, e.g. main memory.
• Non-volatile memory contents retained if the
  power is switched off, e.g. M. Disk.

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Representing Information as Bit Patterns

• Integers twos complement notation.
• fractionsfloating point notation.
• Text

• ASCII (1 byte/symbol)
• Unicode (2 bytes/symbol)
• ISO (4 bytes/symbol).

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Images

• Grey level image grid of pixels each with a 1
  byte grey level.
• Colour image each pixel has 3 bytes of colour
  values, R/G/B.
• A colour image of size 1024x1024 contains 3.1
  MB.

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Example of a Digitised Image
Detail
Original image
http//www.scantips.com/basics1b.html