In computer hardware terminology, word size (word length) is the number of bits that a CPU can process at one time (the word). Processors with many different word sizes have existed, where the word size was selected as a trade off between precision and range of numeric results and the cost of the processing hardware. A 12-bit data word was used by some minicomputers such as the Digital Equipment Corporation PDP-8 and is still used on special-purpose microprocessors such as microcontrollers and digital signal processors. Some early mainframe systems used 35, 36, 40, 44 or 45-bit words, or words that contained between 6 and 16 decimal digits. Word lengths in powers of two (8, 16, 32, 64) have predominated for many years, roughly since the introducionn of the IBM System 360. See 8-bit, 16-bit, 32-bit, 64-bit, 128-bit .

A processor's word size for arithmetic is often equal to the width of its external data bus though sometimes the bus is made narrower than the CPU (often half as many bits) to economise on packaging and circuit board costs, space, and power consumption. For machines of the Von Neumann type, a word size equal or greater than the logical address size is convenient, though the physical address range of a processor may be greater or less than the word size for arithmetic.

A word n bits long can hold 2ⁿ distinct values. If these values represent integers, by far the most common ranges are −2ⁿ⁻¹ to 2ⁿ⁻¹−1 inclusive, or 0 to 2ⁿ−1 inclusive.

Reference

Anthony Ralston and Edwin D. Reilly, Encyclopedia of Computer Science Third Edition, Van Nostrand Reinhold, New York, 1993 ISBN 0442276795