Bits and logic gates

A classical computer codes information as sequences of bits, each bit having one of two values – 0 or 1. Of course, if you looked inside the circuitry, you wouldn’t see things shaped like ovals for 0s and vertical lines for 1s. Instead, you might observe electrical voltages. Any voltage below 0.8 volts might stand for a 0 bit, and any voltage over 2.0 volts might stand for 1 bit. A voltage between 0.8 and 2.0 would be treated as an error condition.

As bits flow from one part of the computer to another, they pass through the circuitry’s gates. A gate is a piece of hardware (often made of transistors) that operates on bits. Each kind of gate is named after the operation that it performs.

Let’s see a few examples:

• A NOT gate performs a NOT operation. The gate outputs 1 if and only if the gate’s input is not 1:

Figure 1.1 – Behavior of the NOT gate

• An AND gate performs the...