Word (computing)

In computing, word is a term for the natural unit of data used by a particular computer design. A word is simply a fixed sized group of bits that are handled together by the system. The number of bits in a word (the word size or word length) is an important characteristic of computer architecture.

The size of a word is reflected in many aspects of a computer's structure and operation; the majority of the registers in the computer are usually word sized and the amount of data transferred between the processing part computer and the memory system, in a single operation, is most often a word. The largest possible address size, used to designate a location in memory, is typically a hardware word (in other words, the full sized natural word of the processor, as opposed to any other definition used on the platform).

Modern computers usually have a word size of 16, 32 or 64 bits but many other sizes have been used, including 8, 9, 12, 18, 24, 36, 39, 40, 48 and 60 bits. The slab is an example of a system with an earlier word size. Several of the earliest computers used the decimal base rather than binary, typically having a word size of 10 or 12 decimal digits and some early computers had no fixed word length at all.

The size of a word is sometimes defined to be a particular value for compatibility with earlier computers. The most common microprocessors used in personal computers (for instance, the Intel Pentiums and AMD Athlons) are an example of this; their IA-32 architecture is an extension of the original Intel 8086 design which had a word size of 16 bits. The IA-32 processors still support 8086 (x86) programs, so the meaning of word in the IA-32 context was kept the same, and is still said to be 16 bits despite the fact that they at times (especially when the default operand size is 32 bits) operate largely like a machine with a 32 bit word size, similarly in the newer x86-64 architecture a word is still 16 bits, although 64-bit (quadruple word) operands may be more common.

Contents

Uses of words

Depending on how a computer is organized, units of the word size may be used for:

Word size choice

When a computer architecture is designed, the choice of a word size is of substantial importance. There are design considerations which encourage particular bit-group sizes for particular uses (e.g. for addresses), and these considerations point to different sizes for different uses. However, considerations of economy in design strongly push for one size, or a very few sizes related by multiples or fractions (submultiples) to a primary size. That preferred size becomes the word size of the architecture.

Character size is one of the influences on a choice of word size. Before the mid-1960s, characters were most often stored in six bits; this allowed no more than 64 characters, so alphabetics were limited to upper case. Since it is efficient in time and space to have the word size be a multiple of the character size, word sizes in this period were usually multiples of 6 bits (in binary machines). A common choice then was the 36-bit word, which is also a good size for the numeric properties of a floating point format.

After the introduction of the IBM System/360 design which used eight-bit characters and supported lower-case letters, the standard size of a character (or more accurately, a byte) became eight bits. Word sizes thereafter were naturally multiples of eight bits, with 16, 32, and 64 bits being commonly used.

Variable word architectures

Early machine designs included some that used what is often termed a variable word length. In this type of organization, a numeric operand had no fixed length but rather its end was detected when a character with a special marking was encountered. Such machines often used binary coded decimal for numbers. This class of machines included the IBM 702, IBM 705, IBM 7080, IBM 7010, UNIVAC 1050, IBM 1401, and IBM 1620.

Most of these machines work on one unit of memory at a time and since each instruction or datum is several units long, each instruction takes several cycles just to access memory. These machines are often quite slow because of this. For example, instruction fetches on an IBM 1620 Model I take 8 cycles just to read the 12 digits of the instruction (the Model II reduced this to 6 cycles, but reduced the fetch times to 4 cycles if both address fields were not needed by the instruction). Instruction execution took a completely variable number of cycles, depending on the size of the operands.

Word and byte addressing

The memory model of an architecture is strongly influenced by the word size. In particular, the resolution of a memory address, that is, the smallest unit that can be designated by an address, has often been chosen to be the word. In this approach, address values which differ by one designate adjacent memory words. This is natural in machines which deal almost always in word (or multiple-word) units, and has the advantage of allowing instructions to use minimally-sized fields to contain addresses, which can permit a smaller instruction size or a larger variety of instructions.

When byte processing is to be a significant part of the workload, it is usually more advantageous to use the byte, rather than the word, as the unit of address resolution. This allows an arbitrary character within a character string to be addressed straightforwardly. A word can still be addressed, but the address to be used requires a few more bits than the word-resolution alternative. The word size needs to be an integral multiple of the character size in this organization. This addressing approach was used in the IBM 360, and has been the most common approach in machines designed since then.

The power of 2

Different amounts of memory are used to store data values with different degrees of precision. The commonly used sizes are usually a power of 2 multiple of the unit of address resolution (byte or word). Converting the index of an item in an array into the address of the item then requires only a shift operation rather than a multiplication. In some cases this relationship can also avoid the use of division operations. As a result, most modern computer designs have word sizes (and other operand sizes) that are a power of 2 times the size of a byte.

Size families

As computer designs have grown more complex, the central importance of a single word size to an architecture has decreased. Although more capable hardware can use a wider variety of sizes of data, market forces exert pressure to maintain backward compatibility while extending processor capability. As a result, what might have been the central word size in a fresh design has to coexist as an alternative size to the original word size in a backward compatible design. The original word size remains available in future designs, forming the basis of a size family.

In the mid-1970s, DEC designed the VAX to be a successor of the PDP-11. They used "word" for a 16-bit quantity while they used the term "longword" to refer to a 32-bit quantity. This is in contrast to earlier machines, where the natural unit of addressing memory would be called a word, while a quantity that is one half a word would be called, if anything, a halfword. As well, a VAX "quadword" is 64 bits.

Another example is the x86 family. The original 8086 architecture used a word size of 16 bits. The 80386 was based around units of 32 bits. If it were an unencumbered design, it would have had a 32-bit "word", but as an extension of the 8086, its "word" continued to be considered as 16 bits. The x86-64 architectural extensions bring the 64-bit size into a major position without dropping any of the 16- and 32-bit support.[1]

Thus one sees that today a computer architecture is based on a family of closely related sizes more than on a single omnipresent word size. The sizes are related by integral factors. Calling any one of them the architecture's word size may be somewhat arbitrary, and a word size may be designated for historical reasons.

Dword, qword, and oword

Some computer architectures define the term dword (double word) to be a unit of data that is twice the size of a word. The x86 platform originally had a word size of 16 bits (2 bytes) and that usage of the term is confusingly retained even though the actual processor word size is now 32 bits or even 64 bits. In that platform, a dword designates a 32-bit (4-byte) unit.

Similarly qword (quadruple word) is a unit of data that is four times the size of a word. On the x86 platform, this unit of data is 64 bits long because the definition of word on an x86 system is 16 bits, as discussed above.

Finally, Intel uses the term double quadruple word, or dqword, to denote 128 bits, found in the implementation of Streaming SIMD Extensions, whereas Microsoft Macro Assembler uses oword (octuple word) for the same data size.

Table of word sizes

key: b: bits, d: decimal digits, w: word size of architecture, n: variable size
Year Computer
Architecture
Word Size
w
Integer
Sizes
Floating Point
Sizes
Instruction
Sizes
Unit of Address
Resolution
Char
Size
1837 Babbage
Analytical engine
50 d w 5 different cards were used for different functions, exact size of cards not known w
1941 Zuse Z3 22 b w 8 b w
1942 ABC 50 b w
1944 Harvard Mark I 23 d w 24 b
1946
(1948)
{1953}
ENIAC
(w/Panel #16)
{w/Panel #26}
10 d w, 2w
(w)
{w}

(2d, 4d, 6d, 8d)
{2d, 4d, 6d, 8d}


{w}
1951 UNIVAC I 12 d w ½w w 1 d
1952 IAS machine 40 b w ½w w 5 b
1952 Fast Universal Digital Computer M-2 34 b w? w 34 b = 4 b opcode plus 3x 10b address 10 b
1952 IBM 701 36 b ½w, w ½w ½w, w 6 b
1952 UNIVAC 60 n d 1d, ... 10d 2d, 3d
1953 IBM 702 n d 0d, ... 511d 5d d 1 d
1953 UNIVAC 120 n d 1d, ... 10d 2d, 3d
1954
(1955)
IBM 650
(w/IBM 653)
10 d w
(w)
w w 2 d
1954 IBM 704 36 b w w w w 6 b
1954 IBM 705 n d 0d, ... 255d 5d d 1 d
1954 IBM NORC 16 d w w, 2w w w
1956 IBM 305 n d 1d, ... 100d 10d d 1 d
1957 Autonetics Recomp I 40 b w, 79 b, 8d, 15d ½w ½w, w 5 b
1958 UNIVAC II 12 d w ½w w 1 d
1958 SAGE 32 b ½w w w 6 b
1958 Autonetics Recomp II 40 b w, 79 b, 8d, 15d 2w ½w ½w, w 5 b
1959 IBM 1401 n d 1d, ... d, 2d, 4d, 5d, 7d, 8d d 1 d
1959
(TBD)
IBM 1620 n d 2d, ...
(4d, ... 102d)
12d d 2 d
1960 LARC 12 d w, 2w w, 2w w w 2 d
1960 CDC 1604 48 b w w ½w w 6 b
1960 IBM 1410 n d 1d, ... d, 2d, 6d, 7d, 11d, 12d d 1 d
1960 IBM 7070 10 d w w w w, d 2 d
1960 PDP-1 18 b w w w 6 b
1961 IBM 7030
(Stretch)
64 b 1b, ... 64b,
1d, ... 16d
w ½w, w b, ½w, w 1 b, ... 8 b
1961 IBM 7080 n d 0d, ... 255d 5d d 1 d
1962 UNIVAC III 25 b, 6 d w, 2w, 3w, 4w w w 6 b
1962 Autonetics D-17B
Minuteman I Guidance Computer
27 b 11 b, 24 b 24 b w
1962 UNIVAC 1107 36 b 1/6w, ⅓w, ½w, w w w w 6 b
1962 IBM 7010 n d 1d, ... d, 2d, 6d, 7d, 11d, 12d d 1 d
1962 IBM 7094 36 b w w, 2w w w 6 b
1963 Gemini Guidance Computer 39 b 26 b 13 b 13 b, 26 b
1963
(1966)
Apollo Guidance Computer 15 b w w, 2w w
1963 Saturn Launch Vehicle Digital Computer 26 b w 13 b w
1964 CDC 6600 60 b w w ¼w, ½w w 6 b
1964 Autonetics D-37C
Minuteman II Guidance Computer
27 b 11 b, 24 b 24 b w 4 b, 5 b
1965 IBM 360 32 b ½w, w,
1d, ... 16d
w, 2w ½w, w, 1½w 8 b 8 b
1965 UNIVAC 1108 36 b 1/6w, ¼w, ⅓w, ½w, w, 2w w, 2w w w 6 b, 9 b
1965 PDP-8 12 b w w w 8 b
1970 PDP-11 16 b w 2w, 4w w, 2w, 3w 8 b 8 b
1971 Intel 4004 4 b w, d 2w, 4w w
1972 Intel 8008 8 b w, 2d w, 2w, 3w w 8 b
1972 Calcomp 900 9 b w w, 2w w 8 b
1974 Intel 8080 8 b w, 2w, 2d w, 2w, 3w w 8 b
1975 ILLIAC IV 64 b w w, ½w w w
1975 Motorola 6800 8 b w, 2d w, 2w, 3w w 8 b
1975 MOS Tech. 6501
MOS Tech. 6502
8 b w, 2d w, 2w, 3w w 8 b
1976 Cray-1 64 b 24 b, w w ¼w, ½w w 8 b
1976 Zilog Z80 8 b w, 2w, 2d w, 2w, 3w, 4w, 5w w 8 b
1978
(1980)
Intel 8086
(w/Intel 8087)
16 b ½w, w, 2d
(w, 2w, 4w)

(2w, 4w, 5w, 17d)
½w, w, ... 7w 8 b 8 b
1978 VAX-11/780 32 b ¼w, ½w, w, 1d, ... 31d, 1b, ... 32b w, 2w ¼w, ... 14¼w 8 b 8 b
1979 Motorola 68000 32 b ¼w, ½w, w, 2d ½w, w, ... 7½w 8 b 8 b
1982
(1983)
Motorola 68020
(w/Motorola 68881)
32 b ¼w, ½w, w, 2d
(w, 2w, 2½w)
½w, w, ... 7½w 8 b 8 b
1985 ARM1 32 b w w 8 b 8 b
1985 MIPS 32 b ¼w, ½w, w w, 2w w 8 b 8 b
1989 Intel 80486 16 b ½w, w, 2d
w, 2w, 4w
2w, 4w, 5w, 17d ½w, w, ... 7w 8 b 8 b
1989 Motorola 68040 32 b ¼w, ½w, w, 2d w, 2w, 2½w ½w, w, ... 7½w 8 b 8 b
1991 Alpha 64 b ¼w, ½w, w w, 2w ½w 8 b 8 b
1991 Cray C90 64 b 32 b, w w ¼w, ½w, 48b w 8 b
1991 PowerPC 32-64 b ¼w, ½w, w w, 2w w 8 b 8 b
2000 IA-64 64 b 8 b, ¼w, ½w, w ½w, w 41 b 8 b 8 b
2002 XScale 32 b w w, 2w ½w, w 8 b 8 b
key: b: bits, d: decimal digits, w: word size of architecture, n: variable size

[2][3]

See also

References

  1. Fog, Agner (2010-02-16). "Calling conventions for different C++ compilers and operating systems: Chapter 3, Data Representation". http://www.agner.org/optimize/calling_conventions.pdf. Retrieved 2010-08-30. 
  2. Gerrit A. Blaauw & Frederick P. Brooks (1997). Computer Architecture: Concepts and Evolution. Addison-Wesley. ISBN 0-201-10557-8. 
  3. Anthony Ralston & Edwin D. Reilly (1993). Encyclopedia of Computer Science Third Edition. Van Nostrand Reinhold. ISBN 0-442-27679-6.