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 machine. The number of bits in a word
(the word size or word length) is an important characteristic of a
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. The typical numeric value manipulated by the
computer is probably word sized. The amount of data transferred between the
processing part of the computer and the memory system is most often a word. An
address used to designate a location in memory often fits in a word.
Modern computers usually have a word size of 16, 32, or 64
bits. Many other sizes have been used in the past, including 8, 9, 12, 18, 24,
36, 39, 40, 48, and 60 bits; the slab is an example of an early word size. Some
of the earliest computers were decimal 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.
Sometimes the size of a word is 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-bit) 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.
Uses of words
Depending on how a computer is organized, units of the word size may be used
for:
Integer numbers Holders for integer numerical
values may be available in one or in several different sizes, but one of the
sizes available will almost always be the word. The other sizes, if any, are
likely to be multiples or fractions of the word size. The smaller sizes are
normally used only for efficient use of memory; when loaded into the
processor, their values usually go into a larger, word-sized holder.
Floating point numbers Holders for floating
point numerical values are typically either a word or a multiple of a word.
Addresses Holders for memory addresses must be
of a size capable of expressing the needed range of values, but not be
excessively large. Often the size used is that of the word, but it can also
be a multiple or fraction of the word size.
Registers Processor registers are designed with
a size appropriate for the type of data they hold, e.g. integers, floating
point numbers, or addresses. Many computer architectures use "general
purpose" registers that can hold any of several types of data; those
registers are sized to allow the largest of any of those types, and
typically that size is the word size of the architecture.
Memory-processor transfer When the processor
reads from the memory subsystem into a register, or writes a register's
value to memory, the amount of data transferred is often a word. In simple
memory subsystems, the word is transferred over the memory
data bus, which
typically has a width of a word or half word. In memory subsystems that use
caches, the word-sized transfer is the one between the processor and the
first level of cache; at lower levels of the memory hierarchy larger
transfers (which are a multiple of the word size) are normally used.
Unit of address resolution In a given
architecture, successive address values designate successive units of
memory; this unit is the unit of address resolution. In most computers, the
unit is either a character (e.g. a byte) or a word. (A few computers have
used bit resolution.) If the unit is a word, then a larger amount of memory
can be accessed using an address of a given size. On the other hand, if the
unit is a byte, then individual characters can be addressed (i.e. selected
during the memory operation).
Instructions Machine instructions are normally
fractions or multiples of the architecture's word size. This is a natural
choice since instructions and data usually share the same memory subsystem.
In Harvard architectures the word sizes of instructions and data need not be
related.
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.
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.
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.
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.
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 AMD64 architectural extensions bring the 64-bit size into a major position
without dropping any of the 16- and 32-bit support.
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.
In computer science, a dword (double word) is a unit
of data that is twice the size of a word. On the x86 platforms, which have a
word size of 16 bits, a dword unit of data is 32 bits (4 bytes) long.
A qword (or quadword, or quadruple word) is a unit of
data that is four times the size of a word. On the common x86 platforms, this
unit of data is 64 bits because the size of a word on an x86 system is defined
to be 16 bits (whether the particular machine works primarily with 16, 32, or 64
bit items).
Finally, Intel uses the term double quadruple word, or
DQWord, to denote a 128-bit datum, found in the implementation of Streaming SIMD
Extensions and its ancestors. Microsoft Macro Assembler uses oword (octuple
word) for the same data size.