Magnetic tape
Magnetic tape is a thin plastic strip covered with a magnetic coating.
Data is stored as:
magnetised areas → represent 1
demagnetised areas → represent 0
Magnetic tape uses serial access.
Storage Devices and Media
Secondary storage is non-volatile storage used to keep data and programs permanently. It has a much larger capacity than RAM and ROM, although access is generally slower. This chapter covers magnetic, optical and solid-state storage.
Magnetic storageOptical storageSolid-state storageFuture of storage
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3.1 Magnetic media and magnetic storage devicesMagnetic tape, HDDs, serial/direct access, FAT and latency.3.2 Optical media and optical storage devicesCD, DVD and Blu-ray, including R, RW and ROM formats.3.3 Solid-state media and solid-state storage devicesSSDs, flash memory, pen drives and memory cards.3.4 The future of storage devicesDeclining use of optical media and increasing use of streaming, cloud and flash storage.
3.1 Magnetic media and magnetic storage devices
Magnetic storage uses magnetised areas to represent binary data.
The two main magnetic storage devices covered are magnetic tape and hard disk drive (HDD).
Serial access
With serial access, data has to be read in the same order in which it was stored. For example, to reach the fifth item on a tape, the system must first pass through items 1, 2, 3 and 4. This makes accessing individual pieces of data quite slow.
Uses of magnetic tape
backing up very large amounts of data
long-term archiving
batch-processing applications
Advantages
very large storage capacity
low cost per byte
useful for long-term storage
Disadvantages
very slow data access
updating data is inconvenient
magnetic fields can damage/corrupt the data
Data access time vs data transfer rate
Data access time = time needed to locate specific data.
Data transfer rate = speed at which data is transferred between the storage device and the computer.
A magnetic tape can therefore have a slow access time but a relatively fast transfer rate once the required data has been found.
Hard disk drives (HDD)
An HDD stores data magnetically on rotating disks called platters.
Important parts include:
platters
read/write heads
read/write arm
actuator
The platters rotate very quickly while the read/write heads move across their surfaces.
Data is organised into:
tracks – circular areas on the disk
sectors – smaller sections of each track
Unlike magnetic tape, an HDD uses direct access. This means the system can move directly to the required area without reading all previous data first.
Platter → Tracks → Sectors → Data
File Allocation Table (FAT)
The HDD maintains information showing where data is stored and which sectors are available. The book refers to this as a file allocation table (FAT). This helps the computer locate free areas when storing new data.
Latency
Latency is the time required for the required section of a rotating disk to move underneath the read/write head. This causes a small delay when accessing data from an HDD.
Fixed HDD
A fixed HDD is normally installed inside the computer.
Uses
storing the operating system
storing applications
storing files and working data
file servers
real-time and online systems
Advantages
large storage capacity
fast data access and transfer
Disadvantages
moving parts can fail
can be damaged by physical shock
noisier than solid-state drives
Portable HDD
A portable HDD is an external hard drive, usually connected using USB.
Uses
backing up files
transferring data between computers
Advantages
large capacity
fast access
portable
Disadvantages
can be damaged if dropped
slower transfer rate than many fixed HDDs
3.2 Optical media and optical storage devices
Optical storage uses laser light to read or write data.
The main types covered are CD, DVD and Blu-ray.
CDs and DVDs store data along a single spiral track running from the centre towards the outside of the disc.
Data is represented using pits and lands.
Centre → spiral track → pits and lands → edge
CD and DVD formats
R – Recordable
Can normally be written to once.
CD-R
DVD-R
RW – Rewritable
Can be written to many times.
CD-RW
DVD-RW
ROM – Read Only Memory
Can only be read. The user cannot normally overwrite the data.
CD-ROM
DVD-ROM
CD-R and DVD-R
Uses:
storing music or films
transferring files
storing data permanently
Advantage:
relatively inexpensive
relatively inexpensive
Disadvantage:
can only be recorded once
can only be recorded once
CD-RW and DVD-RW
Uses:
recording data that may need to be changed
backups
Advantage:
can be rewritten many times
can be rewritten many times
Disadvantage:
data can accidentally be overwritten
data can accidentally be overwritten
CD-ROM and DVD-ROM
These are read-only discs.
CD-ROM uses include:
music
software
computer games
reference material
DVD-ROM uses include:
films
larger computer programs and games
ROM discs are useful where data should not be changed or deleted.
Why does a DVD store more than a CD?
their pits are smaller
their tracks are closer together
they use a shorter-wavelength laser
DVDs can use dual-layer technology
Dual-layering means that two recording layers can be used within the disc.
Blu-ray discs
Blu-ray is another optical storage medium.
The main difference is that Blu-ray uses a blue laser, while CDs and DVDs use red lasers. The shorter wavelength of the blue laser allows smaller pits and closer tracks, meaning much more data can be stored.
Approximate storage capacities given in the book
| Medium | Capacity |
|---|---|
| Single-layer DVD | 4.7 GB |
| Single-layer Blu-ray | 27 GB |
| Dual-layer Blu-ray | 50 GB |
Uses of Blu-ray
storing high-definition films
games consoles
computer backups/data storage
video recording
Advantages
very large optical storage capacity
faster transfer and access than other optical media
built-in encryption helps reduce piracy
Disadvantages
relatively expensive
encryption can sometimes cause compatibility problems
CD vs DVD vs Blu-ray
| Feature | CD | DVD | Blu-ray |
|---|---|---|---|
| Laser | Red | Red | Blue |
| Relative capacity | Lowest | Higher | Highest |
| Common use | Music/software | Films/software | HD films/large files |
| Layers | Usually single | Can be dual-layer | Can be single/dual-layer |
For exams, remember: CD < DVD < Blu-ray in storage capacity.
3.3 Solid-state media and solid-state storage devices
Solid-state storage:
has no moving parts
does not use magnetism
stores data electronically
is non-volatile
generally gives very fast data access
Data is stored as binary values inside large numbers of tiny electronic components called transistors.
Floating gate and control gate
Solid-state memory uses floating gate and control gate transistors.
a charged floating gate can represent one binary value
an uncharged floating gate represents the other
the charge remains even when power is removed
This is why flash memory is non-volatile.
Students do not need to overcomplicate this: remember that solid-state storage keeps data by controlling electrical charge rather than magnetising a disk.
Solid-state drives (SSD)
An SSD performs a similar role to an HDD and can store:
operating system
software
documents
photographs
other files
SSDs have become common in laptops, smartphones and tablets because they have no moving mechanical parts.
Advantages of SSD over HDD
much faster access
more reliable because there are no moving parts
lighter and thinner
lower power consumption
produces less heat
Disadvantage:
Solid-state memory has a limited number of write operations over its lifetime, although improvements are continually being made.
Solid-state memory has a limited number of write operations over its lifetime, although improvements are continually being made.
HDD vs SSD
| HDD | SSD |
|---|---|
| Uses magnetic storage | Uses solid-state/flash memory |
| Has moving parts | No moving parts |
| Slower access | Faster access |
| Heavier | Lighter |
| More power consumption | Lower power consumption |
| Can be noisy | Silent |
This is an important exam comparison; the chapter's exam questions specifically ask why SSDs are replacing HDDs.
Pen drives
A pen drive, also called a memory stick, is a small portable solid-state device that connects to a computer using a USB port.
Uses
transferring files between computers
backing up files
storing photos or music
sometimes used as a security device/dongle
Advantages
small and portable
robust because there are no moving parts
not affected by magnetic fields
easy to use
Disadvantages
very easy to lose
incorrect removal during a read/write operation can corrupt the data
Pen drive vs flash drive: A flash drive is any storage device using flash/solid-state technology. A pen drive is a particular type of flash drive with a USB connector. Therefore, all pen drives are flash drives, but not all flash drives are pen drives.
Memory cards
Memory cards also use solid-state technology.
Examples covered in the book include:
SD card
XD card
CFast card
Uses
digital cameras
smartphones
music players
portable devices
Advantages
very small and portable
durable because there are no moving parts
capable of storing large amounts of data
Disadvantages
easier to lose because they are very small
lower capacity than many hard drives
not every computer has a built-in memory-card reader
3.4 The future of storage devices
The book explains that CDs and DVDs are gradually becoming less important.
Many modern computers no longer include an internal CD/DVD drive and instead rely more heavily on:
USB/flash storage
HDDs and SSDs
cloud storage
internet streaming
Music
Rather than buying or using CDs, users increasingly:
store music on computers or smartphones
use portable devices
store music in the cloud
stream music over the internet
Movies and television
DVD and Blu-ray use has also declined as streaming has become more common.
Smart televisions can connect to the internet and stream films and programmes directly without needing a DVD or Blu-ray player.
Important Chapter 3 comparisons
Serial access vs direct access
| Serial access | Direct access |
|---|---|
| Data is read in sequence | Data can be located directly |
| Used by magnetic tape | Used by HDD and optical media |
| Slow to locate individual records | Much faster for locating data |
| Good for backups/batch processing | Good for everyday computer use |
Magnetic vs optical vs solid-state
| Magnetic | Optical | Solid-state |
|---|---|---|
| Uses magnetism | Uses laser light | Uses electronic memory |
| HDD, magnetic tape | CD, DVD, Blu-ray | SSD, pen drive, memory card |
| Often large capacity | Useful for distributing/storing media | Very fast |
| HDD has moving parts | Disc and drive have moving parts | No moving parts |
Chapter 3 revision checklist
Students should be able to:
explain the difference between storage media and a storage device
identify magnetic, optical and solid-state storage
explain serial access and direct access
describe common uses of magnetic tape
explain the basic structure of an HDD
define latency
distinguish between CD-R, CD-RW and CD-ROM
explain why DVD stores more data than CD
explain why Blu-ray stores more data than DVD
compare CD, DVD and Blu-ray
compare HDD and SSD
explain why SSDs are becoming more common
explain what pen drives and memory cards are
understand why CD/DVD use is declining
explain the increasing use of streaming, flash storage and cloud storage