6.2 Physical Realisations of Classical Bits and Logic Gates
We have so far defined bits and classical logic gates from a theoretical computer science point of view. We now provide an overview of the most efficient hardware techniques used to effectively implement such operations.
6.2.1 Implementation of the NAND gate
The NAND gate (together with the fan-out operator) is a universal gate in classical digital computing. Therefore, it should be sufficient to find a practical physical implementation of the NAND Boolean function in order to build a universal computer. Figure 6.7 displays several possible realisations of the NAND gate using different technologies, from electrical switches to semiconductors.
Relay Logic: Switches are interpreted as bits with 0 = open and 1 = closed. When switches A and B are both closed, an electromagnet opens switch C. If either or both of switches A and B are open, the circuit is broken and an electromagnet...
