1.3 Storage
Secondary storage
Non-volatile, long-term storage.
There are many types of secondary storage that are different and suit different scenarios. E.g. a USB stick is relatively small in capacity, but fast and portable, making it easy for transfer between different devices.
Not all computers need secondary storage i.e. embedded systems like digital watches.
Types of storage
Optical
Solid state
Magnetic
Involves using magnetic fields with magnetised and demagnetised sections to represent bits of data (magnetised = binary 1 and demagnetised = binary 0). In a hard disk drive, a read-write head on the disk will interpret the data by noting which sections are magnetised or demagnetised.
They are fairly cheap, high in capacity and durable. However, they can be easily damaged through being dropped and are affected by magnetic interference.
Uses a laser to scan the surface of a disc divided into tracks with lands and pits. When the beam is pointed, lands (binary 1) reflect the light back and pits (binary 0) scatter the beam.
Optical media comes in different forms.
R media - blank data which can be written to the device with a laser. Can only be written once, but read many times.
ROM media - pre-written data i.e. music, films etc.
RW media - same as R media but can be written to more than once.
Non-volatile RAM. Have fast access times. They have no moving parts, so are durable and portable.
Quite expensive so tend to be rather small in capacity than other types of storage.
Require little power.
Choosing secondary storage
Here are some examples of different types of storage mediums and how they compare to one another.
CD-ROM/R - optical, capacity of 640MB, very slow access, very portable, not very durable, reliable if looked after, normally used for backups and distribution of media e.g. music.
Hard disk - magnetic, capacity between 500GB and 12TB, slow access speeds, internal disks are not portable but external ones are, quite durable but open to damage, normally used for long-term storage.
USB - flash, capacity between 2GB and 2TB, fast access, very portable, robust, resilient and reliable, used for copying data and transferring it from computer to computer.