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Student notes 2.5
Secondary storage
A secondary storage device allows data to be stored when it is not actively used by the
processor.
The data will have already been input and so it will already be in binary form and only
readable by the computer.
Secondary storage is non-volatile so that the data can be used later, even after the
computer is turned off and on again. This also allows the users to use storage devices to
transfer data from one computer to another.
All desktop computers and most other computer devices need some secondary storage to
store the following items:
• The operating system.
• Any programs and applications that the user installs onto the computer.
• Any files and documents containing the user’s data.
These cannot be stored in the primary memory. If they were stored in the ROM, they will
not be alterable which means the user will not be able to install any programs or modify
any files. If they were stored in the RAM, they will be lost every time the computer is
switched off.
We saw earlier that when the data is being used, it has to be in the primary memory
because only the primary memory can be accessed by the processor. When the data in
secondary storage needs to be used, it needs to first be transfered from the secondary
storage to the primary memory. This is called ‘loading’ and it is the reason why we say
that we are loading a program or loading a file. If you are playing a game and you have to
wait for a level to load, you are waiting for the data about that level to be transfered from
secondary storage to primary memory, so that it can be played.
The opposite of ‘loading’ is ‘saving’. Saving means to copy the data from primary memory
to the secondary storage where it is ‘safe’ because it is non-volatile and will still be there,
even if the computer is turned off.
Internal and removable storage
Internal storage devices are those that are built into the computer. They are always used
with just one computer and so can be used to store programs and data that are needed on
that computer.
Removable storage refers to storage that can be taken away from the computer and
possibly used on another computer. It is also useful for backing up. Sometimes, with
removable storage, the whole device is not removed from the computer. Only the medium
on which the data is stored (e.g. a CD) is removed, but the device that reads this medium,
the CD drive, stays in the computer.
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Some people confuse removable storage with an input device. You can use a CD to input
data like a music file into your computer. This does not mean that a CD drive is an input
device because the data on the CD was already in binary format. It had been input using
microphones when the music was recorded and then stored on the CD to be played later.
Types of storage devices
Most storage devices used on modern computers can be categorised under three
categories:
• Magnetic devices, which use magnetised surfaces to store the data.
• Optical devices, which use patterns of reflection of a laser on a surface to store the data.
• Solid-state or flash devices, which use electronic components in a circuit to store the data.
We will look at these types of devices in turn.
Magnetic devices
The most common use of a magnetic storage device is a hard-disk drive (HDD). These
consist of one or more metal discs spinning very fast under a moving electromagnetic head.
The head can magnetise or demagnetise any part of the disc. It can also check whether
part of the disc has been magnitised or not. Binary data is written onto the magnetic disc
by magnetising or demagnetising given parts of the disc’s surface to represent a 1 or a 0,
respectively. These parts of the disc are then read by the head when the data needs to be
retrieved.
Hard disk storage capacity is usually very large: hundreds, if not thousands, of gigabytes.
Hard disks are often used as the main internal storage in the computer to store the
operating system. They are also used as removable storage (when they are then called
external hard drives). Their large size means that they are very useful for backing up large
amounts of data, or for transfering large files (such as video files) from one computer to
another.
Other magnetic devices are magnetic tape and floppy disks, but these are rarely used
nowadays.
Optical devices
There are several different types of optical device, but they all work in a similar way. The
data is stored on a medium that is usually an approximately 12 cm diameter reflective disc.
Binary data is stored by altering parts of the disc so that they do not reflect a laser beam.
Parts of the disc that have not been altered still reflect a beam. The two states, reflective
and non-reflective, are used to represent the two binary states, 0 and 1.
The different types of optical storage vary by their storage capacity and how they are used.
CDs usually store about 800 MB of data. DVDs store up to 9 GB of data and Blu-ray® discs
(BD) can store 25 GB or more.
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Each of these type of disc can either be read-only, recordable or rewritable. A read-only CD
is also called a CD-ROM and is used, for example, when storing software to be purchased
by the public. A recordable CD allows the user to store data onto the disc once. This data
cannot be altered again, but it can be read many times. A rewritable CD allows the user to
store and delete data on the disc as many times as required. The same principles apply for
recordable and rewritable DVDs and Blu-ray® discs.
Optical storage is always removable storage (although the device which reads the discs
is usually built into the computer) because the data itself can easily be moved from one
computer to another.
Solid-state/flash devices
In general, solid-state devices refer to all devices that use flash memory (i.e. erasable electronic
circuits of transistor switches) to store the data, because they all work in a similar way.
The term ‘solid state’’ is however most often used to refer to the internal ‘solid-state drive’
(or SSD). Removable storage that uses the same solid-state principles is usually called
‘flash storage’ and include devices such as USB memory sticks and memory cards in digital
cameras.
They are called ‘solid state’ because they have no moving parts, unlike discs which need to
spin. This is very useful in devices that need to be carried around (e.g. for use with portable
computers and MP3 players). Moving parts can cause the data to be incorrectly written or
read because the read/write head could skip and jump around the disc.
Solid-state technology is rapidly evolving, so the maximum storage capacity of different
types of solid-state or flash drives is always being increased.
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