The store of the central processor can contain up to 256,000 words,and it is necessary to select a certain number of these for use in a particular calculation.It is vitally important ,therefore that their exact positions are known otherwise the calculation could not take place.The store is in fact divided up into compartments or locations.Each locations holds a word and its position is identified by a serial number known as the address.
Computer words are of two types
1.INSTRUCTION WORDS and
2.DATA WORDS
The instruction words which tell the computer what to do and data words which represent the number the computer has to use in its calculations .An instruction word has itself got two parts ,the first part is the operation code or the "opcode",which describes in number form the operation to be performed .The second part contains one or more addresses of data words with which the computer is to carry out arithmetic operations.The number of addresses required for a particular calculation may vary between one and three but is more usually one or two.The tables opposite show how information may be contained in instruction words for the different systems
LET US SEE THE FORMAT OF INSTRUCTION
OPCODE ADDRESS ONE ADDRESS TWO ADDRESS THREE
EXAMPLE THREE ADDRESS SYSTEM
OPCODE OPERATION TO BE EXECUTED BY THE COMPUTER
ADDRESS 1 ADDRESS OF FIRST PIECE OF DATA
ADDRESS 2 ADDRESS OF SECOND PIECE OF DATA
ADDRESS 3 ADDRESS OF THIRD PIECE OF DATA
LET IS KNOW ABOUT EACH IN BRIEF
WORDS
"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 may in actuality (and especially when the default operand size is 32-bit) operate more 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.
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 one or 1 cycle 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
Data values may occupy differing sizes of memory, because, for instance, some numbers need to be capable of having greater precision than others. The commonly used sizes are usually chosen to be a power of 2 multiple of the unit of address resolution (byte or word). This is convenient because converting the index of an item in an array into the address of the item then requires only a shift operation (which is just a conductor routing in hardware) 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.
Dword and Qword
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 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.
Table of word sizes
| 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 | 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 | w | ½w, w, 2w | w | w | 8 b |
| 1991 | PowerPC | 32 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 | |||||||
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