A computer word is a group of bits or bytes that may be manipulated and stored as a unit. It is a critical concept, because the internal circuitry of virtually every computer system is designed around a certain word size.
The Apple Macintosh IIX and IBM PS/2 Model 80 computers, for example, use the Motorola 68030 and Intel 80386 chips, respectively. Both the 68030 and 80386 chips have a 32-bit-word internal architecture, which means that data are transferred within each CPU chip itself in 32-bit chunks.
Both chips also have a 32-bit word I/O bus, meaning that there is a 32-bitwide data path from each CPU to external devices.
Besides speed, there are many other important reasons to consider word size when buying a computer. For one thing, the longer the word size, the greater the RAM capacity. A 32-bit machine theoretically can address about 4 gigabytes of memory, although most operating systems designed around them use at most several megabytes.
A 16-bit machine, in contrast, often is practically limited to several hundred thousand addresses. Longer word sizes also generally make for greater precision. A big number that occupies one word in a large computer may occupy two words in a smaller machine, resulting in some loss in accuracy, not to mention reduced speed.
On very large, scientific computers, where speed and accuracy are extremely important, word sizes as great as 60 or more bits are not unusual.
The greater the size of the word the machine manipulates, the greater the number of bits available to represent machine language instructions. For example, most 32-bit machines are designed such that the first 8 bits are reserved for the instruction type.
This permits a total of 28, or 256, different instructions. Sixteen-bit machines, on the other hand, have a smaller number of bits available for this purpose. Typically, these computers reserve 6 bits for instructions, which permits only 26, or 64, different instructions. Thus, the bigger the word size, the larger the set of possible instructions available to the computer.
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