Secondary Storage

• Secondary storage is not directly accessed by the CPU and is needed for permanent storage of data.
• Secondary storage is non-volatile, meaning it will keep data even if there is no power.
• Secondary storage devices may be internal or external to the computer.
• Storage types:

  • Primary storage.
  • Secondary storage.
  • Offline secondary storage.
• Primary storage:

  • RAM and ROM.
• Secondary storage:

  • Hard Disk Drive (HDD).
  • Solid State Drive (SSD).
• Offline secondary storage:

  • Compact Disc (CD), Digital Versatile Disk (DVD) or BluRay.
  • Flash memory, SD cards.
  • Removable HDD or SSD.
  • Magnetic tape.
• Storage methods:

  • Magnetic.
  • Optical.
  • Solid State.
• Magnetic storage: mechanical parts move over the disk's surface to read and write data magnetically, or a drive head reads a magnetic tape.
• Optical storage: lasers read and write data using light.
• Solid state storage: data is recorded onto solid memory chips without any moving parts.
• Features of magnetic disks:

  • Tracks: concentric circles.
  • Sectors: divisions of each track.
  • Disk heads mounted on mechanical arms to read and write data.
• A disk with a solid platter is a 'hard' disk. Soft plastic disks are known as 'floppy' disks.
• Features of hard disks:

  • Magnetic platter. Contains data, a hard disk may contain more than one.
  • Drive spindle. Rotates.
  • Drive read/ write head. Reads data on the drive using electromagnets.
  • Actuator. Moves the read/ write arm.
• Hard disks are magnetic storage.
• Hard disks have a very large storage capacity. They are very cheap compared to solid state drives.
• Portable hard disks can be connected to a computer via a USB port. They are used for backing up or transporting data.
• Magnetic storage: advantages, disadvantages, uses and capacity.

  A) Cheap

  B) Large storage capacities

  C) Relatively fast write speeds

  D) Lots of mechanical parts

  E) Lack of durability

  F) Not portable

  G) Personal computers

  H) Storage of large quantities of data

  I) 500 GB - 12TB or greater

  
• Features of optical storage:

  • Data is stored as pits and lands burnt or pressed into a spiral track circulating outwards from the centre.
  • A laser beam passes over the pits and lands. The level of reflection is measured.
  • From this signal, 0s and 1s can be derived.
• Optical storage: advantages, disadvantages, uses and capacity.

  A) Cheap

  B) Very easily portable

  C) Takes up little space physically

  D) Less storage capacity compared to other types

  E) Easily damaged/ scratched

  F) Requires a CD reader

  G) Slow write speeds

  H) Songs, videos and other multi-media storage

  I) Backup and archiving of data

  J) CD-ROM: up to 720 MB

  K) DVD: up to 8.4 GB (dual layered disk)

  L) Blu-Ray: up to 50 GB (dual layered disk)

  
• A CD has bigger pits and lands than Blu-Ray as red light has a larger wavelength.
• Blu-Ray pits can store more data than CDs because of its smaller pits and lands. This is because violet has a shorter wavelength than red.
• Features of Solid State Drives (SSD):

  • Non-volatile flash memory to store information.
  • Very fast read/ write speeds as it doesn't need to wait for a disk to spin to the correct location and an arm to move.
  • No mechanical or moving parts meaning these disks are very durable.
• Solid State Drive storage: advantages, disadvantages, uses and capacity.

  A) Highly durable

  B) No moving parts

  C) Very fast read/ write speeds

  D) No noisy fan or drive arm

  E) Faster start up times

  F) More expensive than magnetic hard disks

  G) Similar storage capacity to magnetic hard disks

  H) Higher end computers

  I) Laptops

  J) Smartphones and tablets

  K) 100 GB - 16 TB

  
• Flash memory is low cost, portable, has no moving parts, and is durable.
• Flash memory are ideal for a range of offline devices:

  • Cameras.
  • Mobile phones.
  • USB memory sticks.
• How flash memory stores data:

  • A floating gate may store or not store electrons.
  • If there are no electrons in the floating gate, the current will flow, representing a 1.
  • If there are electrons in the floating gate, the current doesn't flow, representing a 0.
• The isolation layers are semiconductors known as transitors.
• In order to write to the flash memory cell, the electrons need to pass through the isolation layer. The electrons only move through this layer when writing.
• The control gate is used to move electrons into or out of the floating gate:

  • A negative voltage forces electrons out of the floating gate.
  • A positive voltage forces electrons into the floating gate.
• How flash memory stores data, diagram:

  A) 1

  B) 0

  C) Control Gate

  D) Isolation Layer

  E) Isolation Layer

  F) Control Gate

  G) Isolation Layer

  H) Isolation Layer

  I) Electrons

  
• Two types of solid-state flash memory:

  • NOR.
  • NAND.
• NOR flash memory is used for programs:

  • Reading from NOR flash is random access like RAM.
  • It is used to store code such as the BIOS and possibly an operating system.
  • Executed directly by the CPU.
• NAND memory is used for data storage:

  • NAND memory works like other secondary storage.
  • Data is first copied to RAM before it is accessed by the CPU.
• A cache is a high-speed data storage layer which stores a subset of data, typically transient in nature, so that future requests for that data are served up faster than is possible by accessing the data's primary storage location.
• Secondary storage refers to storage devices that provide long-term, non-volatile storage for data that is not immediately needed by the computer's CPU.
• Offline storage refers to storage devices or media that are not continuously connected to the computer system and require manual intervention to access data.