Digital circuits implement core functionality of the architecture. They implement the Instruction Set Architecture (ISA) in hardware, and also implement storage and control functionality throughout the system. Designing digital circuits involves applying multiple levels of abstraction: circuits implementing complex functionality are built from smaller circuits that implement partial functionality, which are built from even simpler circuits, and so on down to the basic logic gate building blocks of all digital circuits. Figure 1 illustrates a circuit abstracted from its implementation. The circuit is represented as a black box labeled with its functionality or name and with only its input and output shown, hiding the details of its internal implementation.

There are three main categories of circuit building blocks: arithmetic/logic, control, and storage circuits. A processor integrated circuit, for example, contains all three types of subcircuits: its register set uses storage circuits; its core functionality for implementing arithmetic and logic functions uses arithmetic and logic circuits; and control circuits are used throughout the processor to drive the execution of instructions and to control loading and storing values in its registers.

In this section, we discuss these three types of circuit, showing how to design a basic circuit from logic gates, and then how to build larger circuits from basic circuits and logic gates.