Reversible and Irreversible Logic Circuits

Logic circuit is said to be reversible if the “input of the circuit can be derived from its output”. For example, from a NOT gate output we can construct the input of the gate form its output. Unlikely gates such as AND, OR, X-OR, NAND and NOR are irreversible logic gates i.e., form its output it is impossible to construct the input. Such types of gates are called irreversible logic gates.

Reversible Computing

Reversible computing in a general sense means, computing using reversible operations that is, the operation that can be easily and exactly reversed or undo. In technical terms a reversible operation performs a bijective transformation of its local configuration space. By maintaining the reversibility of the circuit at low level (transistor level) , it avoids dissipating the energy that is associated with the bits of information that is being manipulated. This can help to reduce the overall energy dissipation of computations. There are two kind of reversibility is used for computing:- "physical reversibility and logical reversibility".

Physical reversibility:- A process is said to be physically reversible, if it results in no increase in physical entropy. These types of circuits are referred as charge recovery logic circuits or adiabatic circuits.

Logical reversibility:- A discrete, deterministic computational process is said to be logically reversible if the transition that maps old computational states to new one is a one-to-one function; i.e. the output logical state uniquely defines the input logic state of the computational operation.