Buses are typically designed to transfer fixed-sized chunks of bytes known aswords. The number of bytes in a word (the word size) is a fundamental system parameter that varies across systems. Most machines today have word sizes of either 4 bytes (32 bits) or 8 bytes (64 bits).
《Computer system: a programmer‘s perspective》
原文：https://www.cnblogs.com/lightmonster/p/11000697.html