Word (computer architecture)
A data word or just word processing is the basic data size on a computer. Synonymous with the terms binary word or English word are also used. The size of the data unit in bits is referred to as word length or word length, or as bus width.
Often, especially in the context of programming languages, the term word is also used in deviation. Platform independent - - fixed data unit called a fixed size as a word then.
The two times a word - in context - is called a double word (English double word, short DWord) or long word. For four times a word in English, there are also still the name quadruple word, short quadword or QWord. The unit of data with half the word length is accordingly called half-word.
Smallest addressable unit
Depending on your system may differ considerably, the word width, it always corresponds to a power of two and the multiples of a byte today. In addition to the maximum number that can be processed in a calculation step, the word length determines primarily the size of the maximum directly addressable memory. Therefore, a tendency to larger word lengths can be recognized.
The first main processors had only 4- bit data words (nibbles ). With 4 bits can represent 16 states, this ranges easily from the representation of the ten digits 0 through 9. Many digital clocks and simple pocket calculator today have 4-bit CPUs.
In the 1970s, the 8 -bit CPUs, which dominated the market for home users between 1970 and 1990 established. Due to the 8-bit one now could save 256 different characters in a data word. The computer used now alphanumeric inputs and outputs, which were much more readable and opened up new possibilities. The resulting 8- bit character sets such as EBCDIC, or a variety of 8 -bit extensions to the 7-bit ASCII codes have been preserved in part to the 32 -bit era, and plain text files are still often in a the 8- bit formats before.
Since then, the word width has been doubled several times within the x86 processor family. Thus, the doubling of the 8 -bit architecture initially led to the 16- bit architecture, which soon after, the 32- bit architecture, many still common PC processors such as the Pentium, Athlon, G4 and Core Duo processors followed. Currently, this architecture is in turn at the transition from 32 -bit to 64 -bit architecture, and so a series of newer PC processors already has a word length of 64 bits, such as the Athlon 64, G5, newer Pentium 4 - Core 2 Duo and various server processors (eg, Itanium, UltraSPARC, Power4, Opteron, Xeon newer ).
Different meanings
In Programming Languages for x86 systems is the size of a word, partly out of habit, but especially to obtain compatibility with previous processors, not grown with, but now referred to colloquially a bit string of 16 bits, so the status of the 8086 processor. For later x86 processors, the term double word / DWORD was introduced (also long word / long). In the change from 32 -bit architectures and operating systems to 64-bit, the importance of Long between Linux and Windows has separated: In the Windows world, the width of such a longword stayed with 4 bytes ( = 32 bits), has been in the Linux world, they to 8 bytes ( = 64 bits ) broadened.
In other computer architectures (eg PowerPC, Sparc ) is a word often meant a bit string of 32 bits ( the original word width of these architectures), which is why the term halfword for sequences of 16 bits is common there.
Examples
IA-32 processors, the 80x86 architecture
The term Word (or word) is also used in the Windows API for a 16- bit number.
PLC
During the programming of programmable logic controllers ( PLC), IEC 61131-3 standard defines the word sizes follows that:
If the letter U brought to an integer data types (eg UDINT ) so this means "unsigned " ( unsigned), otherwise the integer values are signed.