The programming languages we use must mirror the types of operations that a computer can perform. So let’s begin our discussion by repeating the definition of a computer: A computer is a programmable electronic device that can store, retrieve, and process data.

The operational words here are programmable, store, retrieve, and process. In a previous chapter we pointed out the importance of the realization that data and instructions to manipulate the data are logically the same and could be stored in the same place. That is what the word programmable means in this context. The instructions that manipulate data are stored within the machine along with the data. To change what the computer does to the data, we change the instructions.

Store, retrieve, and process are actions that the computer can perform on data. That is, the instructions that the control unit executes can store data into the memory of the machine, retrieve data from the memory of the machine, and process the data in some way in the arithmetic/logic unit. The word process is very general. At the lowest level, processing involves performing arithmetic and logical operations on data values.

Furthermore, the data has to get into the computer memory initially, and we need a way for the human user to see whatever results we want to present. So, there are also instructions that specify the interaction between an input device and the CPU and between the CPU and an output device.