The control unit fetches instructions from memory and controls the processing unit to carry out the instruction. The PC contains the address of the next instruction to execute in the program. In the Von Neumann Model, the program is stored in memory and therefore the PC contains a memory address. More accurately, the PC points to the next instruction to be executed and does not count programs. In the diagram below, the PC is shown as a finger because it points to the next instruction.
All useful computation requires some form of output to be generated.
Most programs also require input.