Книга: Introduction to Microprocessors and Microcontrollers

Bits, bytes and other things

All the information entering or leaving a microprocessor is in the form of a binary signal, a voltage switching between the two bit levels 0 and 1. Bits are passed through the microprocessor at very high speed and in large numbers and we find it easier to group them together.

Nibble

A group of four bits handled as a single lump. It is half a byte.

Byte

A byte is simply a collection of 8 bits. Whether they are ones or zeros or what their purpose is does not matter.

Word

A number of bits can be collected together to form a ‘word’. Unlike a byte, a word does not have a fixed number of bits in it. The length of the word or the number of bits in the word depends on the microprocessor being used.

If the microprocessor accepts binary data in groups of 32 at a time then the word in this context would include 32 bits. If a different microprocessor used data in smaller handfuls, say 16 at a time, then the word would have a value of 16 bits. The word is unusual in this context in as much as its size or length will vary according to the situations in which it is discussed. The most likely values are 8, 16, 32 and 64 bits but no value is excluded.

Long word

In some microprocessors where a word is taken to mean say 16 bits, a long word would mean a group of twice the normal length, in this case 32 bits.

Kilobyte (Kb or KB or kbyte)

A kilobyte is 1024 or 210 bytes. In normal use, kilo means 1000 so a kilovolt or kV is exactly 1000 volts. In the binary system, the nearest column value to 1000 is 1024 since 2⁹=512 and 2¹⁰=1024. The difference between 1000 and 1024 is fairly slight when we have only 1 or 2 Kb and the difference is easily ignored. However, as the numbers increase, so does the difference. The actual number of bytes in 42 Kb is actually 43 008 bytes (42?1024). The move in the computing world to use an upper case K to mean 1024 rather than k for meaning 1000 is trying to address this problem.

Unfortunately, even the upper or lower case b is not standardized so tread warily and look for clues to discover which value is being used. If in doubt use 1024 if it is to do with microprocessors or computers. Bits often help to confuse the situation even further. 1000 bits is a kilobit or kb. Sometimes 1024 bits is a Kb. One way to solve the bit/byte problem is to use kbit (or Kbit) and kbyte (or Kbyte).

Megabyte (MB or Mb)

This is a kilokilobyte or 1024?1024 bytes. Numerically this is 2²⁰ or 1 048 576 bytes. Be careful not to confuse this with mega as in megavolts (MV) which is exactly one million (10⁶).

Gigabyte (Gb)

This is 1024 megabytes which is 2³⁰ or 1 073 741 824 bytes. In general engineering, giga means one thousand million (10⁹).

Terabyte (TB or Tb)

Terabyte is a megamegabyte or 2⁴⁰ or 1 099 511 600 000 bytes (Tera = 10¹²).

Petabyte (PB or Pb)

This is a thousand (or 1024) times larger than the Terabyte so it is 10¹⁵ in round numbers or 2⁴⁰ which is pretty big. If you are really interested, you can multiply it out yourself by multiplying the TB figure by 1024.

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