Secondary Storage (CIE IGCSE Computer Science)

• Secondary storage is not directly accessed by the CPU
• It is used for permanent storage of data
• Secondary storage is non-volatile. This means data is stored even when power is lost to the device
• Secondary storage is needed as primary storage (such as RAM ) is volatile and will not retain data when the computer is turned off
• Unlike ROM, secondary storage can be read from and written to. ROM is read only
• Common examples of secondary storage include hard disk drives (HDDs), solid-state drives (SSDs), external hard drives, USB flash drives, CDs, DVDs, and Blu-ray discs. These can be used to store data to transfer it to another computer
• Data access speeds for secondary storage devices are typically slower than primary storage (such as RAM)
• Secondary storage devices often have large capacity (can store large amounts of data)
• Software such as the operating system is saved permanently on secondary storage. The instructions and data that makes up the software then moves to RAM when it is in use
• Secondary storage devices can be internal (inside the computer) and external (outside the computer)

Diagram showing different internal and external secondary storage devices

Comparison of primary and secondary storage

Primary storage	Secondary storage
Volatile / temporary	Non-volatile /permanent
RAM and cache can be read from and written to ROM is read-only	Secondary storage can be read from and written to
Can be directly accessed by the CPU	Cannot be directly accessed by the CPU
Smaller capacity	Larger capacity
Data access speeds faster than secondary storage	Data access speeds slower than primary storage
Stores data and instructions	Stores data and instructions

• There are three types of secondary storage - magnetic, solid state and optical

Magnetic

• Magnetic hard disk drives have been the main type of internal secondary storage in personal computers for many years however solid state drives are increasingly popular due to their durability, low power consumption and high read/write speeds
• 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

A diagram showing the structure of a magnetic hard disk

• Each platter is divided by concentric circles creating several tracks and wedge shaped sectors. Where they intersect is a track sector

A diagram showing how each platter is divided by tracks and sectors

• 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

Solid State

• Solid State secondary storage is made of transistors that are arranged in a grid layout
• It uses NAND and NOR gates in electrical circuits to persistently control the flow of electrons
• Current flowing is 1, no current is 0 -  which is how electrical flow can represent binary values
• A NAND gate will take in two inputs and produce an output (1/electrical current) as long as both inputs are not 1

The truth table truth table for a NAND gate

InputA	InputB	Output
0	0	1
0	1	1
1	0	1
1	1	0

• A NOR gate will produce an output (1) only when both inputs are 0

The truth table for a NOR gate

InputA	InputB	Output
0	0	1
0	1	0
1	0	0
1	1	0

Optical

• Optical devices include CDs, DVDs and Blu-rays
  • 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

A diagram showing how data is burnt on to optical discs to create lands and pits which can then be used 

• In CD-RW the chemical composition of the disk is changed to represent 0s and 1s. This change can be overwritten