Secondary Storage (Cambridge (CIE) O Level Computer Science): Revision Note

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:

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?

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

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

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