Hardware Devices: Secondary storage devices

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UNIT 2 - ⇑ Hardware Devices ⇑

← Output devices Secondary storage devices

This section will be looking at the various forms of secondary storage devices (media). For each device you should be familiar with the following details:

  • Seek time - The average time taken from requesting data to starting to read the requested data
  • Capacity - The amount of data it is possible to store on a medium
  • Write type - Whether it is read only, write only, or readable and writable
  • Cost - How much it costs per megabyte
  • Access type - Whether it uses Random Access or Serial Access
sequential access requires all memory locations before the one sought to be read, before reaching it. Random access memory allows you to jump directly to the memory location you are seeking

Pay special attention to devices with an orange background, you need to be able to describe exactly how they work!

Magnetic media[edit]

Magnetic media stores data by assigning a magnetic charge to metal. This metal is then processed by a read head, which converts the charges into ones and zeros. Historically, magnetic media has been very popular for storing programs, data, and making backups. It looks set to continue in this role for some time. However, solid state technology is starting to be used more and more, storing programs and data on new devices such as mobile phones and cameras.

Solid-state memory
Device Size

Hard Disk

Up to 4 Terabytes

Magnetic Tape

Up to 2 Terabytes

Hard disk[edit]

Video of exposed hard disk drive (HDD)

Hard disks are usually found inside computers to store programs and data. They are increasingly cheap and more and more companies are using them to back things up. Hard disks can vary in physical size with some disks getting as small as your thumb. The capacity of a commercial disk is currently up to about 4 terabytes allowing users to read and write to them. They are constructed from several key components:

  • Platter - Metallic disks where One or both sides of the platter are magnetized, allowing data to be stored. The platter spins thousands of times a second around the spindle. There may be several platters, with data stored across them
  • Head - The head reads magnetic data from the platter. For a drive with several platters there may two heads per platter allowing data to be read from top and bottom of each
  • Actuator Arm - used to move the read heads in and out of the disk, so that data can be read and written to particular locations and you can access data in a Random fashion, you don't need to read your way through the entire disk to fetch a particular bit of information, you can jump right there. Seek time is very low.
  • Power connector - provides electricity to spin the platters, move the read head and run the electronics
  • IDE connector - allows for data transfer from and to the platters
  • Jumper block - used to get the disk working in specific ways such as RAID
Hard drive-en.svg

For the exam you must be able to explain how a hard disk works:

  1. The platters spin around the spindle
  2. data is requested to be read from a particular area of a platter
  3. the actuator arm moves the read head to that track
  4. Once the data sector that is required has spun around and under the read head, data is read
  5. Read data is sent from the IDE connector to main memory
Description of a hard disk platter

Writing data is very similar:

  1. The platters spin around the spindle
  2. data is sent to the hard disk using the IDE connector
  3. the actuator arm moves the write head to the track that will be written to
  4. Once the data sector that is required has spun around and under the write head, data is written to the platter

Pros

plus pointFast seek times
plus pointRandom access
plus pointHigh capacities possible
plus pointLow cost per megabyte


Cons

minus point Very susceptible to damage from physical shocks


Magnetic Tape drive[edit]

DDS tape drive. Above, from left right: DDS-4 tape (20 GB), 112m Data8 tape (2.5 GB), QIC DC-6250 tape (250 MB), and a 3.5" floppy disk (1.44 MB)

Increasingly obsolete, the tape has been a medium to deliver software and back up data since the early days of computing. Nowadays they are used mostly for corporate backing up and archiving of data. Tapes are sequential data stores, meaning that if you had information stored at the end of the tape you would have to wind your way through the entirety of the tape before you could read it. There is no random access like with a hard disk! Tapes can be several terabytes in size and reading and writing can be very fast as long as you read or write continuous sections of the tape at once.

Pros

plus pointFast
plus pointHigh capacity
plus pointCheap per megabyte


Cons

minus point Serial read and write capabilities


Optical media[edit]

Optical media works by creating a disc with a pitted metallic surface. There are several different types of disk out there ranging from 650 MB to 128 GB, with the pits and lands getting closer together for higher volume disks. The principle behind how each of them works is the same.

pitted surface visible on the surface of a CD. Massively zoomed in!
Optical media
Device Type Size Image
  • Read Only
  • Write once then Read only
  • re-Writable
650 - 900 MB CD logo.png
  • DVD-ROM
  • DVD-R
  • DVD-RW
  • DVD-RAM
  • Read Only
  • Write once then Read only
  • re-Writable
  • re-Writable
4.7 - 9.4 GB DVD logo.svg
  • Blu-ray (BD) disc
  • HD DVD (obsolete)

Re-Writable and Read Only versions available. Uses a blue laser, that is able to recognise smaller pits and lands, which allows for the pits and lands to be more closely packed, and so store more data

25 - 128 GB Blu-ray 200GB.png

CD-ROM[edit]

close up of the surface of a CD being read

A CD-ROM is a metal disc embedded into a plastic protective housing. Each disc has to be 'mastered'; this is the process of creating the CD and placing the data on it. CDs are WORM (Write Once, Read Many) media; this refers to the fact that once they have been mastered, there is no way to change the data on them.

Reading from a CD-ROM

  1. A single track runs in a spiral pattern from the centre of the disc to the outside, this track is made of pits and lands to represent the ones and zeroes of binary data
  2. A low-powered laser is shone on the metallic surface and the reflection is captured in a photodiode sensor, the lands reflect differently to the pits, meaning it can tell the difference between a 1 and a 0
  3. The disc spins and the laser follows the track
  4. The binary data (the 1s and 0s) are put together and the CD-ROM has been read

Pros

plus pointCheap
plus pointData cannot be written over by the consumer

Cons

minus point Slow seek time
minus point Data degrades with time, discs from 20 years ago might not work!
minus point Can only be written to with a very high powered laser, which is not usually available in home computers
minus point Data cannot be written over


CD-R[edit]

The CD-R is made of a reflective metal disk with a layer of (usually green, opaque) dye on top.

Writing to a CD-R

  1. A single track runs in a spiral pattern from the centre of the disc to the outside.
  2. A high-powered laser is shone onto the CD-R, changing the transparency (permanently) of the dye above. The transparent and opaque parts represent binary 1s and 0s
  3. The disc spins and the laser follows the track, putting the binary data onto the CD-R in a spiral track
  4. The data has been written

Reading from a CD-R

  1. A single track runs in a spiral pattern from the centre of the disc to the outside, this track is made of pits and lands to represent the ones and zeroes of binary data
  2. A low-powered laser is shone on the surface and the reflection is captured in a photodiode sensor. The opaque dye will reflect differently to the transparent dye (which would just reflect the metal underneath it), meaning it can tell the difference between a 1 and 0
  3. The disc spins and the laser follows the track
  4. The binary data (the 1s and 0s) are put together and the CD-R has been read

Pros

plus pointCheap
plus pointCan be written to using a conventional home computer

Cons

minus point Slow seek time
minus point Data degrades with time, discs from 20 years ago might not work!
minus point Data cannot be written over


CD-RW[edit]

The CD-RW is made of a reflective metal disk with a layer of a special ('phase change') metal on top.

Writing to a CD-RW

  1. A single track runs in a spiral pattern from the centre of the disc to the outside.
  2. A high-powered laser is shone onto the CD-ROM. Depending on whether this is very high powered or heats at a slightly lower temperature, the top layer of metal cools differently. These will result in different amounts of reflectivity, which represent the 1s and 0s.
  3. The disc spins and the laser follows the track, putting the binary data onto the CD in a spiral track
  4. The data has been written

Reading from a CD-RW

  1. A single track runs in a spiral pattern from the centre of the disc to the outside, this track is made of pits and lands to represent the ones and zeroes of binary data
  2. A low-powered laser is shone on the surface and the reflection is captured in a photodiode sensor. The different ways the metal has cooled reflect different amounts, meaning it can tell the difference between a 1 and 0
  3. The disc spins and the laser follows the track
  4. The binary data (the 1s and 0s) are put together and the CD-ROM has been read

Pros

plus pointCheap
plus pointCan be written to using a conventional home computer

Cons

minus point Slow seek time
minus point Data degrades with time, discs from 20 years ago might not work!
minus point Data can be changed after writing
minus point Not all CD players (mostly older ones) can read CD-RWs, as opposed to CD-ROMs and CD-Rs


Solid-state memory[edit]

Solid-state memory
Device Description

USB flash drive

Up to 256 GB

Memory card

Up to 256 GB

USB(memory stick) Flash Drive[edit]

Usbkey internals.jpg
Internals of a typical USB flash drive

1 USB Standard-A plug
2 USB mass storage controller device
3 Test points
4 Flash memory chip
5 Crystal oscillator
6 LED
7 Write-protect switch (Optional)
8 Space for second flash memory chip

USB Flash drives are solid state, that means that there are no moving parts. This is very useful for seek times as we don't have to wait for mechanical movement, meaning seek time is very low and it allows for fast Random Access Memory. Flash drives can be set to read only mode, but they will always allow for reading and writing. The size of flash drives is not as great as a Hard Disk and they are generally much more expensive per megabyte

  1. put drive into USB socket
  2. USB driver loads, providing the computer with code on how to read and write from the USB
  3. The USB is read, giving information on the file and folder structure (File Allocation Table) to the Computer
  4. [Reading] The user chooses to open a file, the Computer sends the address wanted to the USB port
  5. [Reading] The USB returns the data at the location requested
  6. [Writing] The computer sends data to the USB port where it is place into empty space on the drive
  7. [Writing] The computer then requests a new version of the file and folder structure

Pros

plus pointVery fast seek times
plus pointVery portable


Cons

minus point Limited capacity
minus point expensive per MB when compared to Hard Disks


Memory cards[edit]

Work in much the same way as a Flash drive and can often be converted into Flash Drives. They have different connectors and are generally smaller than USB Flash drives allowing for them to be used in cameras, mobile phones and game consoles.

comparison of different memory cards
Exercise: Secondary Storage

Choose the correct secondary storage device for each of the following scenarios:

A company needs to backup some HD movie files for 4 years.

Answer :

  • DVD-ROM
  • Blu-Ray
  • Tape
  • Hard Disk

NOTE: not CD-ROM, it is too small and USB might be too expensive an option

You need to store files for a school project taking up about 70 MB that you can then transport to and from school

Answer :

  • USB
  • CD-RW

NOTE: nothing that is ROM!

Why do some people reckon that Blu-Ray will be the last optical disk format?

Answer :

As broadband gets faster and people store more and more of their data online, everything will be transmitted over the internet and there will be no need to have large physical storage at home

Describe how a Hard Disk writes a file

Answer :

  1. The platters spin around the spindle
  2. data is sent to the hard disk using the IDE connector
  3. the actuator arm moves the write head to the track that will be written to
  4. Once the data sector that is required has spun around and under the write head, data is written to the platter

Describe how a computer reads from a usb

Answer :

  1. put drive into USB socket
  2. USB driver loads, providing the computer with code on how to read and write from the USB
  3. The USB is read, giving information on the file and folder structure (File Allocation Table) to the Computer
  4. The user chooses to open a file, the Computer sends the address wanted to the USB port
  5. The USB returns the data at the location requested

What optical disk format would you recommend for the following:

  • Making long term backups of some photos

Answer :

  • CD-R
  • DVD-R

NOTE: there is no need for RW or RAM here

  • Distributing a software product that includes 20 minutes of HD video footage

Answer :

  • DVD-ROM
  • Blu-Ray

NOTE: CD-ROM would probably be too small for this

  • Backing up school work at the end of each week to take home

Answer :

  • USB Flash Drive
  • DVD-RW / RAM
  • CD-RW

NOTE: Rs wouldn't be suitable as you would have to keep throwing them away!