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Various units of measurement exist to describe the capacity of storage devices. The smallest unit of storage is the bit. A computer can only store data as binary numbers (0s and 1s). A bit of storage can hold either a 0 or a 1. Devices such as computers which can only use binary information are described as digital devices.
Eight bits are grouped together to form a byte. Using 8 bits it is possible to represent any character on the keyboard with a byte. The characters are represented in 0s and 1s using a code called the ASCII code. Here are some example codes :
| Character | ASCII Code
| A
| 01000001
| B
| 01000010
| |
To store the sentence "Hello World" in a computer would require 11 bytes (one for each character), which is 11*8 = 88 bits.
Describing the capacity of a storage device in bytes would be silly. A typical home microcomputer can store 536870912 bytes in its immediate access store and 128849018880 bytes in its main backing store (probably a hard disk). Therefore there are a number of other related units that are used :
| Unit | Equivalent To
| 1 Bit | A single storage location which can hold either a 0 or a 1. | 1 Byte | 8 Bits | 1 Kilobyte (Kb) | 1024 Bytes | 1 Megabyte (Mb) | 1024 Kilobytes | 1 Gigabyte (Gb) | 1024 Megabytes | |
Typical Storage Capacity of a Microcomputer
A typical new home microcomputer has 512Mb of immediate access store. This is equivalent to :
The same computer is likely to have three backing storage devices, a floppy disk drive (capacity 1.44Mb), a CD/DVD drive (capacity 650Mb-18Gb) and a hard disk drive (capacity 120Gb+).
GCSE ICT Companion 04 - (C) P Meakin 2004