HARDWARE. Storage devices.
STORAGE DEVICES
MEMORY
The smallest unit of memory is 1 bit. A bit can only be 0 or 1. Most modern computers
organise data into bytes consisting of 8 bits.
Numbers
1 Byte:
Place value
128
64
32
16
8
4
2
1
Bit value
0
1
0
0
0
0
0
1
Each place value in a byte is worth twice as much as the place to its right. In the byte
above, we have a 1 in the 64 place and a 1 in the 1 place. That means that the number
stored is 64 + 1 = 65. This number can also be used as a code for different things
Characters
The ASCII code (American Standard Code for Information Interchange) uses a number
to represent each character or letter. For example, 65 represents the capital letter 'A'.
Another system, called Unicode, allows the storage of characters from all languages.
Pictures
Bytes can be used to store a set of dots of different colours (pixels). These pixels can be
mapped to a screen display or a printer. This is called a bit-mapped image and it can
take over a million bytes to store an image.
Music
Sounds can be sampled for pitch, loudness and other qualities. The samples can be
stored as numbers. The more numbers that are collected, the better the quality of the
sound. Digital sound can be played back on a digital device, e.g. an MP3 player.
Instructions
The instructions in a computer program are also stored as bytes. AII stored data is in the
same form. The computer knows what to do with the data according to the program
instructions and where the data is stored.
1 byte is enough data to code for 1 character. Modern computers store vast amounts of
data, so we use other units to measure groups of bytes.
Unit
Kilobyte (KB)
Megabyte (MB)
Gigabyte (GB)
Terabyte (TB)
Meaning
210 bytes = 1024 bytes
220 bytes = 1 048576 bytes
230 bytes = 1073 741824 bytes
240 bytes = 1 099511 627776 bytes
PRIMARY STORAGE
There are two types of primary storage:
• RAM
• ROM
RAM
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HARDWARE. Storage devices.
RAM (Random Access Memory) stores the data and instructions that are currently being
used. Data must be in RAM before it can be processed. RAM loses its contents when the
power is switched off.
Cache Memory
Cache memory is a small store of fast memory that copies data from the most frequently
accessed parts of main memory. The processor checks the cache first to see if the data
or instruction it needs next is there. If it is, it saves the time it would take to access the
slower main RAM.
ROM
The data in ROM (Read-Only Memory) can be accessed by the processor, but it can't
put new data into it.
ROM keeps its data when the computer is switched off. It stores the basic instructions to
allow the computer to boot up, i.e. load the operating system software. ROM is used to
store programs in embedded systems such as mobile phones, washing machines and
MP3 players.
Flash memory is a type of ROM that can be erased and reprogrammed thousands of
times. It’s used in secondary storage. Embedded systems increasingly use flash memory
because it can be updated.
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