Chapter 2 notes (ICT EDEXCEL)

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What is Data Storage?
When we talk about `storing' data, we mean putting the data in a known place. We can later come
back to that place and get our data back again.
`Writing' data or `saving' data are other ways of saying `storing' data.
`Reading' data, `retrieving' data or `opening' a file are ways of saying that we are getting our data
back from its storage location.
Next Up Backing Storage vs Main Memory
Backing Storage vs Main Memory
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Theory Notes
3. Storage Devices and Media
Backing Storage vs Main Memory
Main Memory
Main memory (sometimes known as internal memory or primary storage) is another name for RAM
(and ROM).
Main memory is usually used to store data temporarily. In the case of RAM, it is volatile (this means that
when power is switched off all of the data in the memory disappears).
Main memory is used to store data whilst it is being processed by the CPU. Data can be put into
memory, and read back from it, very quickly.
Memory is fast to access, but only holds data temporarily...

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Backing Storage
Backing storage (sometimes known as secondary storage) is the name for all other data storage
devices in a computer: hard-drive, etc.
Backing storage is usually non-volatile, so it is generally used to store data for a long time.
Backing storage devices are slower to access, but can hold data permanently...
Next Up Storage Media and Devices
Storage Media & Devices
The device that actually holds the data is known as the storage medium (`media' is the plural).…read more

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Accessing Stored Data
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Theory Notes
3. Storage Devices and Media
Accessing Stored Data
We refer to a collection of data stored in a computer system as a `file'. Files are often organised into
Whenever you click `Save' in an application, burn files to a CD-R, copy music onto your MP3 player, or
drag and drop a file onto memory stick, you are using storage devices - devices that can store and
retrieve data.…read more

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If you want to watch a show that you recorded earlier, you have to rewind / fast-forward through all
other shows until you find it.
The shows are only accessible in the same order that you recorded them. This type of one-by-one
storage and access is called serial access.
Systems that store things on tape (video, music, computer data, etc.…read more

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Some storage media can only store a very limited amount of data, whilst others can store vast
Data storage capacity is measured in bytes (B).…read more

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A million million bytes is called a terabyte (TB)
1,000,000,000,000B = 1TB
Even a very basic storage devices like a floppy disc can storage over a megabyte of data that's over 1
million letters or numbers!
And modern hard drives can store a terabyte of data or more that's more words than you could type
even if you started now, and typed until your old age!
Data Access Speeds
Some storage devices can access data very quickly, whilst others are extremely slow...…read more

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Access speeds are measured in bytes per second (Bps).
Slow devices have speeds measured in thousands of Bps (kBps).
E.g. a floppy disc can save/read data at a speed of 60kBps
Fast devices have speeds measured in millions of Bps (MBps).
E.g. a harddrive can save/read data at a speed of 300MBps (5000 times quicker than the
Magnetic Storage Devices / Media
Why Magnetic?
Magnetic storage media and devices store data in the form of tiny magnetised dots.…read more

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Data is stored inside a hard-drive on rotating metal or glass discs (called `platters').
Fixed Hard Drive
A hard-drive built into the case of a computer is known as `fixed'. Almost every computer has a fixed
Fixed hard-drives act as the main backing storage device for almost all computers since they provide
almost instant access to files (random access and high access speeds).…read more

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Many portable music players (such as the iPod classic) contain tiny hard-drives. These miniature devices
are just not much bigger than a stamp, but can still store over 100MB of data!
Magnetic Tape
Magnetic tape is a large capacity, serial access medium. Because it is a serial access medium, accessing
individual files on a tape is slow.
Tapes are used where large amounts of data need to be stored, but where quick access to individual
files is not required.…read more

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Removeable Media Magnetic Discs
Floppy Disc
A removable, portable, cheap, low-capacity (1.44MB) storage medium. Floppy discs are random
access devices used for transfer small amounts of data between computers, or to back-up small files,
etc. Access times are slow.
Almost every PC used to have a floppy disc drive. These are obsolete now, having been replaced by
higher capacity technology such as CD-ROMs, DVDs and USB memory sticks.…read more


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