Summary
Summary Storage Devices
This document contains information on the content for the storage devices section in Unit 1 of the OCR specification for the A-Level Computer Science
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Storage devicesThere are two types of storage:
1. Primary – the primary storage is the RAM, ROM, Cache and the Registers of the computer, it
stores currently in use programs or data. When the power is turned off however this is cleared
as it is volatile.
2. Secondary – this are of storage is not in direct access with the CPU, so access times are much
slower, but the advantage is that it retains its data when the power is off (non-volatile)
Why is it needed:
Secondary storage is a means for the computer to permanently store the data that has been processed.
Primary storage is volatile so when the power is off it is deleted which is not ideal. Primary storage is
used as it has direct access to the CPU, so access times are much quicker. Before the primary storage is
cleared, the secondary storage is written too to save the wanted data.
Hard Disk:
- A hard disk uses a rigid rotating platter coated in a magnetic material. This material has ferrous
particles on it which are polarised to either face north or south (two states, 1 or 0). This platter is
divided into concentric tracks each which are subdivided into sectors. A metal arm with a
read/write head passes over the platter as it rotates (10,000 RPM) and data is read or written as
it passes over the track.
- Hard disk storage comes in cheaper capacity
- But the arm can be damaged from shock, so it is not robust
Optical disk:
- An optical disc works by shining a high-powered laser to ‘burn’ pits into the disc to make the
area less reflective. Then a laser of low power is shone over the disc to see the variation in
reflection. This variation of reflection is what is stored as a 1 or 0
- There are 3 types:
1. CD-ROM – READ ONLY – pits are pressed during manufacture; reflective and non-reflective areas
are 1 or 0
- CD-ROM stores about 700mb of data but Blu-Ray can store up to 50GB. This is done as a smaller
wavelength of laser which makes smaller pits and lands in the same area, so more can be stored
- Used for music, software distribution
2. CD-R – RECORDABLE – there is a reflective layer coated in transparent dye that becomes less
reflective when a spot laser ‘burns’ it
Used for copying games, video or audio files
3. CD-RW – REWRITABLE – use a laser and a magnet to heat a spot on a disk and use a magnet to
set it to a state before it cools
- Used for transferring data between computers
- Optical storage is very cheap and lightweight
- However, it is easily damaged by scratches and shocks
Solid-State disk:
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