Secondary Storage AMS I2'1'7 Fall 2005

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Secondary Storage AMS I-2.1.7 Fall 2005

• Greg Phillips
• greg.phillips_at_rmc.ca
• Royal Military College of Canada
• Electrical and Computer Engineering

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Hardware module

• topics
• fundamentals of digital systems
• computer system organization and CPUs
• gate logic (lab)
• parallel and serial interconnects
• core memory
• secondary storage
• ROM and flash memory
• disk drives
• cd roms
• display and input devices

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Memory Hierarchy
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Read Only Memory (ROM)

• like RAM, is random access
• five major types
• ROM (read only memory)
• custom fabricated for each application grid of
  diodes
• PROM (programmable ROM)
• grid of diodes with fuses, initially all zeros,
  set to one by blowing fuses
• EPROM (erasable PROM)
• like PROM but erasable by exposure to UV light
  (in essence, UV light restores fuses uses metal
  oxide chemistry)
• EEPROM (electrically EPROM)
• like PROM but electrically erasable (while
  installed)
• can erase/rewrite one byte at a time
• flash memory
• a type of EEPROM that allows blocks of memory
  (typically 512 bytes) to be erased and rewritten
  in a single operation

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Magnetic disks

• Disks are organized by cylinders, tracks, and
  sectors

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Magnetic disks

• Each disk is divided into a number of annular
  regions called tracks
• Each track is divided into a number of sectors
• Up to several hundred sectors per track on a hard
  disk!

Sectors
Track
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Magnetic Disk Review

• All of the tracks for a given arm position form a
  cylinder.
• So, the number of cylinders is equal to the
  number of tracks per platter side
• A disk location is specified by (cylinder, head,
  sector) but sometimes misleadingly as (cylinder,
  track, sector)

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Magnetic Disks

• On older disks, the number of sectors per track
  was a constant for all cylinders
• This wastes potential storage space on the outer
  cylinders of the disk
• Modern disks decrease the number of sectors per
  track towards the center of the disk. Some in
  stepwise fashion, others at a linear rate
• Often, the modern controllers attached to the
  drives present a virtual geometry to the
  operating system, but have a much more
  complicated physical arrangement...

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Magnetic Disks

• Virtual Organization Physical Geometry

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Disk Formatting

• After manufacturing, a hard disk platter is
  simply a metal or alloy disk covered in a thin
  magnetizable metal oxide
• to ensure reliability, the manufacturer performs
  a low-level format consisting of
• Preamble Bit pattern that allows the hardware
  to recognize the start of a sector. Also
  contains cylinder and sector number
• Data Actual data in the sector. Typically 512
  bytes
• ECC Error Correction Code can identify and
  recover from read errors. 16 bytes is not an
  unusual size

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Disk Formatting

• Usually, some amount of cylinder skew is applied
  during the format
• Without cylinder skew, the head will miss sector
  zero of the next track due to the time it takes
  to properly position the head (seek time)
• Cylinder skew is an offset where each track is
  some number of sectors offset from the previous
• This allows multiple tracks to be read
  sequentially without having to wait for sector 0
  of the next track to pass under the head
• This will only work in one direction of cylinder
  travel

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Disk Formatting

• Illustration of cylinder skew

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Disk Buffers

• Disks need buffers for two main reasons
• Buffer data that is arriving faster than it may
  be transferred to the operating system
• Read ahead for data that has not yet been
  requested, but may soon be (data following the
  last request)
• Disks with small buffers may need to interleave
  their sectors on the disk

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Disk Buffers/Interleaving
Single Interleaving
Double Interleaving

• Interleaving allows for the disk to spin past the
  head while data is transferred from the
  controller to memory
• Buffer space to hold a track or more would
  eliminate this requirement

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CD-ROMs

• Physical makeup
• Data stored in a spiral from the inner ring to
  the outer ring
• Surface is composed of Pits (depressions) and
  Lands (unburned areas). When the laser reflects
  off a pit it returns ½ a wavelength out of phase
  with the light reflecting off the surrounding
  surface, resulting in a weaker return
• A pit/land transition represents a 1 and its
  absence represents a 0

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CD-ROMs

• CD-ROM issues
• The spiral makes 22,188 revolutions around the
  disk (600 per mm)
• When rotated at a constant angular velocity, more
  data comes off the outer tracks than the inner
  tracks
• Since tracks are not delineated by cylinders,
  seeking to a particular track is much more
  complicated than on a magnetic disk

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RAM disk cache

• most operating systems reserve a portion of RAM
  for use as a disk cache
• stores copies of recently accessed files or parts
  of files in RAM
• typically works for hard disks and removable
  media
• easily observable impact on system performance
• experiment
• time how long it takes to open a large file
  (e.g., 300 kB PPT) from a floppy by
  double-clicking in Explorer
• close PowerPoint, and repeat

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On-disk sector cache

• modern hard disks have integrated drive
  electronics (IDE)
• typically includes 512kB to 2MB of RAM, used as a
  read-ahead cache
• when a read request comes in, the accessed sector
  is cached
• subsequent sectors cached as well
• this has essentially no cost, since the drive
  heads need only stay over the drive surface for
  one revolution (approx. 8 ms at 7200 rpm) to read
  all sectors in a given track
• typically takes heads several milliseconds to
  move between tracks anyway

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Next class

• Devices for
• Human Computer
• Interaction