6.2 Storage Devices

Nov 10, 2019

Overview of this chapter

  • Difference between primary, secondary, and off-line storage
  • Primary storage
    • RAM / Dynamic RAM / Static RAM
    • ROM / EEPROM
  • Secondary storage
    • HDD
    • SSD
  • Off-line storage
    • CD / VCD / DVD
      • DVD-RAM
      • Blue-ray
    • Hard disk
    • Flash memory

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Primary, secondary, and off-line storage

PURPOSE:

  • Primary: faster, stores the data currently in use (software data, file handles, etc.)
  • Secondary & Off-line: relatively slower but cheap, requiring less power, stores the data not currently in use (unopened photos, software executable files, etc.)

PORTABILITY:

  • Primary & Secondary: built into the computer, cannot be easily removed
  • Off-line: plugged into or connected to the computer, can be easily removed
primary storagesecondary storageoffline storage
SpeedFast, direct access from CPUSlow, no CPU direct access
VolatilityVolatile (RAM) / Involatile (ROM)Involatile
ExamplesRAM, ROMSSD, HDDCD (DVD, blue-ray), USB disk
UseData currently used by computer, such asData not currently in use, such as 
1. Software running1. Files and softwares not opened
2. Operating System
3. Boot-up information (ROM)

Primary storage

RAM (RANDOM ACCESS MEMORY)

  • holds data and instructions
  • computer systems can temporarily store data to Secondary Memory to avoid running out of built-in RAM

Dynamic RAM

  • main memory in memory unit
  • more dependent on refresh (every 15 microsecond)
  • less expensive
  • higher storage capacity

Structure

  • capacitor — this holds the bits of information
  • transistor — this allows control circuit to read the capacitor or change its value

Static RAM

  • memory cache in the memory unit
  • no need to refresh and able to be read multiple times, hence less power consumptive
  • faster IO speed

ROM (READ ONLY MEMORY)

  • holds fundamental instructions (BIOS)
  • determines fatal properties: MAC, frequency, etc.
  • current ones are in fact able to be rewritten but such things do not constantly occur (EEPROM)

Secondary

HARD DISK DRIVES (HDD)

big servers storages

  • contain a number of magnetic surfaced disks that spins 7000 times per second
  • data is read and written by a number of read-write heads that is able to move from the edge of the disks to the center back and forth 50 times per second
  • slow access speed (significant latency)

SOLID-STATE DRIVES (SSD)

personal computer built-in storages

  • Advantage: Without moving parts, hence
    • physically smaller and lighter
    • more reliable
    • releases less heat
    • consumes less power
    • requires less booting time
    • operates faster than HDD
  • Disadvantage: With special electronic structure, hence
    • in long-term, less durable than HDD
    • more expensive than HDD

classification

  • NAND-based — Flash Drive (cheap, half-duplex, operates based on sections)
  • NOR-based — EEPROM (faster, full-duplex, operates accurately to bytes)

Off-line

must be ‘ejected’ before been physically removed from the computer system, or data inside can be corrupted

OPTICAL STORAGE DEVICES

  • CD (Compact Disk)
  • VCD (Video Compact Disk)
  • DVD (Digital Video Disk)
  • DVD-RAM (Digital Video Disk Random Access Memory)
  • Blue-ray Disk

Function

  • a thin layer of metal alloy or light-sensitive organic dye to store the data
  • single, spiral track which runs from the center of the disk to the edge
  • ‘pits’ and ‘bumps’ on the spiral track
  • read or write using a red laser

Special cases

  • DVD
    • dual layers of data, hence larger storage capacity, but suffer from birefringence
  • DVD-RAM
    • concentric tracks
    • allow simultaneous read and write
    • high number of total read and write times the durability
  • Blue-ray Disk
    • a blue laser has a shorter wavelength than a red laser, hence
      • able to store more data
      • able to hold a security encryption system
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Comparison

Optical storage devices will be replaced soon by other storage methods such as flash drives.

HARD DISK

  • can be understood as removable HDD

MEMORY STICKS / FLASH MEMORIES

  • can be understood as removable SSD
  • SSD structure (NAND gates)
  • portable — small and light
  • used as:
    • small backup memory
    • dongle (software key)

Raymond Wu

Founder and current leader of SCIE Programmers

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