Secondary Storage (Cambridge (CIE) IGCSE Computer Science)
Revision Note
Written by: James Woodhouse
Reviewed by: Lucy Kirkham
Secondary Storage
Storage devices are non-volatile secondary storage, that retain digital data within a computer system
They provide a means of storing, accessing, and retrieving data, which can include software applications, documents, images, videos, and more
There are 3 types of storage:
Magnetic
Solid-state (flash memory)
Optical
Computer systems need both primary and secondary storage to operate
Both types of storage play a crucial role in the operation of a computer system
A quick comparison of primary and secondary storage shows:
Primary | Secondary |
---|---|
Volatile (with the exception of ROM) | Non-volatile |
Small capacity | Large capacity |
Why do you need secondary storage?
A computer needs secondary storage for long term storage of programs and data that are currently not in use
Secondary storage is needed as ROM is read only and RAM is volatile
Secondary storage holds the programs and data whilst the computer is turned off (non-volatile)
Performance of secondary storage is slower than primary storage but capacity is much higher which makes it perfect for backup & archive of data files
What are the characteristics of secondary storage?
Capacity - What is the maximum amount of data that can be stored?
Speed - How fast can data be read from and written to? (R/W)
Cost - How much does it cost?
Portability - How easy is it to move around? What is the physical size? Weight?
Durability - How robust is the storage?
Reliability - What is the probability that the device will consistently perform its function?
Worked Example
A games console has secondary storage.
State, using an example, why the games console needs secondary storage [2]
How to answer this question
They keyword is 'why', why does a games console need secondary storage? give an example, do not just describe what secondary storage is!
Answer
To store data once the power is turned off / permanently // for non-volatile storage
Examples could include:
Storing games
Save games
User data/profiles
Downloaded content/applications/updates
Magnetic Storage
What is magnetic storage?
Magnetic storage is a type of non-volatile media that uses magnets (polarity) to store binary 0s and 1s
A magnetic hard disk is made up of several metal discs coated with a magnetic material
These are called platters
Iron particles on each platter are magnetised to represent a 0 or 1
Each platter is divided by concentric circles creating several tracks and wedge shaped sectors
Where they intersect is a track sector
The hard drive spins the metal disk(s) at a high speed (typically around 5400-7200 RPM) using a motor
A read/write arm, controlled by an actuator, moves the head over the surface of the disc to the location of the data
The data is read/written using electromagnets
Advantages | Disadvantages |
---|---|
Capacity - High storage | Durability - Moving parts can get damaged if dropped |
Cost - Low per gigabyte | Portability - Heavy & bulky making them less convenient for transport |
Speed - Moderate read/write access | Reliability - Prone to mechanical failure |
| Noise - Loud (spinning disks) |
Solid-state (Flash Memory)
What is solid state (flash memory)?
Solid-state (flash memory) storage is a type of non-volatile media that uses electronic circuits to store binary 0s and 1s
Examples of solid state storage devices include:
Solid state drives (SSD) - replacing magnetic hard drives as a computers primary secondary storage device as capacity increases and cost decreases
USB flash memory - most common form of portable storage device
It uses NAND and NOR gates in electrical circuits to persistently control the flow of electrons
Solid-state (flash memory) is memory made up of tiny cells that can contain one bit of data (1 or 0)
Each cell contains a transistor that acts as a switch that can be turned on or off
The transistor contains two main parts:
Control gate - top layer of transistor, connects to circuit and controls if current can flow through the transistor
Floating gate - can hold a charge (like a tiny rechargeable battery) and is sandwiched between two layers of insulating material (Oxide)
To store data a charge is placed on the floating gate
A high voltage is applied to the control gate, which allows electrons to be pushed through the oxide layer and onto the floating gate
To remove data, a high voltage charge is applied in the opposite direction, pulling the electrons off the floating gate
Advantages | Disadvantages |
---|---|
Capacity - Medium/high storage | Cost - Very high per gigabyte |
Speed - Very fast read/write access | Reliability - Limited read/write cycles |
Durability - No moving parts |
|
Portability - Small and no moving parts |
|
Noise - Silent |
|
Optical Storage
What is optical storage?
Optical storage is a type of non-volatile media that uses lasers to burn the surface of a disk, creating pits and lands suitable for storing binary 0s and 1s
Examples of optical storage include:
Blu-rays have the largest capacity
CDs have the lowest capacity
CD-R are read-only (you cannot save data on to them)
CD-RW can be written to and read from
DVD-RW can be written to and read from
All optical devices work by shining a laser at the disk and processing the reflection
An arm moves the laser across the surface of the disk
In CD-Rs a laser burns the data, permanently on to the disk, by creating pits and lands
The laser is also used to read the data from the pits and lands
When the laser light hits the point where the pit changes into a land or vice versa the light scatters and is not reflected back as well.
This is captured by a sensor and can be interpreted as a change in the binary value
Advantages | Disadvantages |
---|---|
Cost - Very low per gigabyte | Capacity - Very low |
Durability - No moving parts | Speed - Very slow read/write access |
Portability - Small and no moving parts | Reliability - Prone to scratches |
Noise - Silent |
|
Worked Example
Zarmeen has purchased a new tablet computer. The tablet has an internal secondary storage device.
The storage device is a solid state device.
Give three benefits of the tablet having a solid state device instead of a magnetic device [3]
Give two drawbacks of the tablet having a solid state device instead of a magnetic device [2]
Answers
Benefits
Faster access / read/write speeds
Smaller in physical size
More durable // no moving parts
Drawbacks
More expensive (per gigabyte)
Smaller capacity (usually)
Limited number of read/write cycles
Guidance
Portable is not enough on it's own
No moving parts is not enough on it's own
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