3.3 Data storage

Primary Memory

The CPU directly accesses primary storage. Some examples include RAM, ROM, Cache Memory

Secondary Memory

The CPU does not directly access secondary storage. Examples: HDD, SSD, DVD, Bluray disk. The secondary memories are all non-volatile or persistent. These are necessary for more permanent storage of data

Hard Disk Drive (HDD)

• Have platters whose surfaces are covered with a magnetisable material.
• Platters are mounted on a central spindle and rotated at high speed
• The surface of platters is divided into concentric tracks & sectors, where data is encoded as magnetic patterns
• Each surface is accessed by read/write heads
• Reads/Writes data using electromagnets
• When writing, current variation in the head causes magnetic field variation on the disk
• When reading, magnetic field variation from the disk produces current variation in the read head
• Very slow data access compared to RAM

Solid State Drive (SSD)

• Most use NAND/NOR-based flash memory
• Uses EEPROM technology
• Non Volatile Secondary Storage
• Has no moving parts
• Consist of a grid of columns & rows that has 2 transistors at each intersection
• Stores data by flashing it onto the chips
• Data stored by controlling the flow of electrons through transistors
• The electric current reaches the control gate and flows through to the floating gate to be stored
• When data is stored the transistor is converted from 1 to 0
• Two transistors:
  • Floating Gate: stores electrons, and the presence or absence of charge (electrons) represents either 1 or 0
  • Control Gate: controls charge (electrons) flow for read/write
• Benefits:
  • More reliable
  • Lighter
  • Low power consumption
  • Run much cooler than HDDs
  • Very thin
  • Data access is faster than HDD
  • Quiter
• Drawback:
  • Limited longevity
  • Expensive
  • Storage lower than HDD

Optical disk Reader/Writer

• The disk surface has a reflective metal layer and is spun
• The tracking mechanism moves the laser assembly
• The lens focuses laser onto the disk
• A laser beam shone onto a disk to read/write
• Tracks have sequences of pits and land states on the metallic layer
• When reading, the reflected light from the different states on the track is encoded as bit patterns
• When writing, the laser changes surface to pits and land along the track, corresponding to 1s or 0s

CD/DVD Disks

• Laser (red) light is used to read and write data on the surface of the disk
• A thin layer of metal alloy is used to store data.
• Both systems use a single spiral track that runs from the center of the disk to the edge
• DVD uses Dual-Layering, which increases the storage capacity (two individual recording layers)

Blu-ray Disks

• Uses a blue/violet laser to carry out read-and-write operations
• The wavelength of laser light is less than CD and DVD
• Automatically come with secure encryption (prevent piracy and copyright infringement)
• Used as backup systems
• Faster transfer rate than CD/DVD
• Has single layer of polycarbonate
• Larger Storage capacity compared to CD/DVD

USB (Universal Serial Bus) Flash Memories/ SD (Secure Digital) Card

• Uses NAND or NOR technology
• Transistor are used as control gates and floating gates
• Very small, lightweight, and suitable for transferring files
• Small back-up devices for photo, music
• Solid state, so needs to be treated with care

Cloud Storage:

• A collection of servers storing data in a remote location

Virtual Memory

• Memory that appears to exist as main storage although most of it is supported by data held in secondary storage. It is a paging mechanism that allows a program to use more memory addresses than are available in main memory
• Process split into pages, memory split into frames of same size
• Not all pages of the program need to be initially loaded.
• Pages swapped in/out of memory as required
• No need for all pages to be in memory
• CPU address space is thus larger than physical space
  • Addresses resolved by the memory management unit
• Benefits
  • Not all of the program has to be in memory at once
  • Large programs can be run with or without large physical memory
• Process
  • Load all pages on the disk initially
  • One/more loaded into memory when process ‘ready’
  • Pages replaced from disk when needed
  • This can be done with a FIFO queue or usage-statistics-based algorithm
• Disadvantage
  • Expensive
  • Disk thrashing: Perpetual loading/unloading of pages due to a page from disk immediately requiring the page it replaced.