NOR gates are one of the two basic logic gates (the other being NAND gates) from which any other logic gates can be built. Due to this property, NOR and NAND gates are sometimes called "universal gates". However, modern integrated circuits are not constructed exclusively from a single type of gate. Instead, EDA tools are used to convert the description of a logical circuit to a netlist of complex gates (standard cells) or transistors (full custom approach).
A NOT gate is made by joining the inputs of a NOR gate. Since a NOR gate is equivalent to an OR gate followed by a NOT gate, joining the inputs of a NOR gate leaves only the NOT part.
If the truth table for a NOR gate is examined or by applying De Morgan's Laws, it can be seen that if any of the inputs are 0, then the output will be 1. However to be an AND gate, only when both inputs are 1, the output is 1. Therefore, if the inputs are inverted, both high inputs will trigger a high output.
An OR gate is simple a NOR gate with an inverted output (a NOT gate). The NOT gate is simply a NOR gate with its inputs tied together. Therefore a cascade of two NOR gates, the second one with its inputs connected together will result in an OR gate.
An XOR gate is constructed using five NOR gates. Three of the gates are connected in an AND topology. A logical expression for this arrangement is ((A AND B) NOR (A NOR B)).
