The controller for physical memory supports two operations: read and write. In
the case of a read operation, the processor specifies an address; in the case of a write
operation, the processor specifies an address as well as data to be written. The crucial
idea is that the controller always accepts or delivers an entire word; physical memory
hardware does not provide a way to read or write less than a complete word (i.e., the
hardware does not allow the processor to access or alter part of a word).