9 State, Change, and Equality

Although we have considered both the “stepped” nature of digital data and the steps taken by an executing process, we haven't yet looked very closely at how those ideas interact. We have already seen that a data item can change its value: the “scratch pad” or “tape” of the Turing machine has cells that can be read or written (chapter 3). As we start to consider multiple processes, it's useful to understand this changeability and its implications more carefully.