Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Valve RF amplifier wikipedia , lookup
Analog-to-digital converter wikipedia , lookup
Schmitt trigger wikipedia , lookup
Operational amplifier wikipedia , lookup
Crossbar switch wikipedia , lookup
Power MOSFET wikipedia , lookup
Opto-isolator wikipedia , lookup
Flip-flop (electronics) wikipedia , lookup
Rectiverter wikipedia , lookup
Reversible Logic Models: Billiard Ball and Optical Lecture 4 Marek Perkowski Reversible and Quantum Logic Fundamentals from the Logic Synthesis Point of View Review: Atom-scale computation: • What will be the difficulties when we will try to build classical computers (Turing machines) on the atomic scale? • One of the toughest problems to scale down computers is the dissipated heat that is difficult to remove. • Physical limitations placed on computation by heat dissipation were studied for many years (Landauer, 1961). Computing at the atomic scale: a survey made by Keyes in 1988 R. W. Keyes, IBM J. Res. Develop. 32, 24 (1988). • Plot showing the number of dopant impurities involved in logic with bipolar transistors with year. – (Copyright 1988 by International Business Machines Corporation) Information loss = energy loss • The loss of information is associated with laws of physics requiring that one bit of information lost dissipates k T ln 2 of energy, – where k is Boltzmann’ constant – and T is the temperature of the system. • Interest in reversible computation arises from the desire to reduce heat dissipation, thereby allowing: – higher densities – speed R. Landauer, “Fundamental Physical Limitations of the Computational Process”, Ann. N.Y. Acad.Sci, 426, 162(1985). When will IT happen? Logarithmic scale Power for switching one bit Related to information loss k T ln 2 Assuming Moore Law works 2001 2020 2010 In our lifetime Information is Physical • Charles Bennett, 1973: There are no unavoidable energy consumption requirements per step in a computer. • Power dissipation of reversible circuit, under ideal physical circumstances, is zero. • Tomasso Toffoli, 1980: There exists a reversible gate which could play a role of a universal gate for reversible circuits. A B C Reversible and universal A B C AB Reversible computation: • Landauer/Bennett: almost all operations required in computation could be performed in a reversible manner, thus dissipating no heat! • The first condition for any deterministic device to be reversible is that its input and output be uniquely retrievable from each other. – This is called logical reversibility. • The second condition: a device can actually run backwards - then it is called physically reversible – and the second law of thermodynamics guarantees that it dissipates no heat. Billiard Ball Model Reversible logic Reversible are circuits (gates) that have oneto-one mapping between vectors of inputs and outputs; thus the vector of input states can be always reconstructed from the vector of output states. INPUTS OUTPUTS 000 000 001 001 36 010 010 42 011 011 100 100 101 101 110 110 111 111 2 4 53 65 (2,4)(365) Reversible logic Reversible are circuits (gates) that have the same number of inputs and outputs and have one-toone mapping between vectors of inputs and outputs; thus the vector of input states can be always reconstructed from the vector of output states. INPUTS OUTPUTS 000 000 001 001 2 4 36 010 not allowed 010 Feedback 011 011 42 100 100 Fan-out not allowed 101 101 65 110 110 111 111 53 (2,4)(365) Reversible logic constraints Feedback not allowed in combinational part In some papers allowed under certain conditions Fan-out not allowed In some papers allowed in a limited way in a “near reversible” circuit Observations • Every reversible gate can be described by a permutation. • Every reversible circuit can be described by a permutation. • Synthesis of a reversible circuit can be considered as decomposition of a permutation to a sequence of elementary permutations. • Group theory has been successfully used for designing cascades of reversible gates. • To understand reversible logic, it is useful to have intuitive feeling of various models of its realization. Our examples will illustrate reversible gates, conservative gates and synthesis principles Conservative Reversible Gates Definitions • A gate with k inputs and k outputs is called a k*k gate. • A conservative circuit preserves the number of logic values in all combinations. • In balanced binary logic the circuit has half of minterms with value 1. 0 1 1 0 1 variable 1 1 0 1 0 xor3 Shannon Conservative circuit = the same number of ones in inputs and outputs Davio majority xor2 Examples of balanced functions = half of Kmap are ones Billiard Ball Model DEFLECTION DELAY SHIFT • This is described in E. Fredkin and T. Toffoli, “Conservative Logic”, Int. J.Theor. Phys. 21,219 (1982). Billiard Ball Model (BBM) A Input output A B 1234 0 0 1 1 0 1 0 1 0 0 0 1 0 1 0 0 0 0 1 0 A and B B and NOT A 0 0 0 1 A and NOT B B A and B This is called Interaction Gate This illustrates principle of conservation (of the number of balls, or energy) in conservative logic. Interaction gate Input A B output A Z1= A and B z1 z2 z3 z4 Z2 = B and NOT A 0 0 1 0 1 0 0 0 0 0 0 1 0 0 0 0 1 0 1 1 1 0 0 1 Z3 = A and NOT B B Z4 = A and B A Z1= A and B Z2 = B and NOT A B Z3 = A and NOT B Z4 = A and B Inverse Interaction gate input z1 z2 z3 z4 output A Z1= A and B A B Z2 = B and NOT A 0 0 0 0 0 1 0 0 0 0 1 0 0 0 1 0 1 0 1 0 0 1 1 1 Other input combinations not allowed Z3 = A and NOT B B Z4 = A and B A B Only disjoint inputs are OR-ed z1 z3 z2 z4 Designing with this types of gates is difficult Billiard Ball Model (BBM) switch A Input A B 0 0 1 1 0 1 0 1 output z1 z2 z3 0 0 1 0 0 0 0 1 0 0 1 1 1 2 B 3 Z1 = NOT A * B Z2 = A * B Z3 = A Switch Gate A 1 2 Input A B 0 0 1 1 0 1 0 1 output z1 z2 z3 0 0 1 0 0 0 0 1 0 0 1 1 B 3 Z1 = NOT A * B Z2 = A * B A B Z3 = A Inverse Switch Gate A z1 z2 input z1 0 1 0 0 z2 output z3 B z3 A B 0 0 0 0 0 0 0 1 0 1 1 0 1 1 1 1 A B Z3 Z1 Z2 Inverter and Copier Gates from Switch Gate Z1 = NOT A * 1 garbage A Inverter realized with two garbages garbage 1 garbage Input constants Copier realized with one garbage V2 = B * 1 B 1 V3 = B Garbage outputs shown in green Feynman Gate P + • When A = 0 then Q = B, when A = 1 then Q = B. • Every linear reversible function can be built by composing only 2*2 Feynman gates and inverters • With B=0 Feynman gate is used as a fan-out gate. (Copying gate) A Q A B A A A + A + 0 A 1 Feynman Gate from Switch Gates Z1 = NOT A * B Fan-out > 1 B Z2 = A * B A Z3 = A AB B V1 = NOT B * A V2 = B * A B A V3 = B Garbage outputs Z2 and V2 shown in green Fredkin Gate – Fredkin Gate is a fundamental concept in reversible and quantum computing. – Every Boolean function can be build from 3 * 3 Fredkin gates: P = A, Q = if A then C else B, R = if A then B else C. Q Q R R P P 0 0 1 0 1 1 A B A B C A circuit from two multiplexers C A B C C AC’+B BC’+AC Its schemata This is a reversible gate, one of many Notation for Fredkin Gates Yet Another Useful Notation for Fredkin Gate Fredkin Gate C P C Q Inverse Fredkin Gate C’P+CQ C C’P+CQ C P CP+C’Q CP+C’Q Q In this gate the input signals P and Q are routed to the same or exchanged output ports depending on the value of control signal C Fredkin gate is conservative and it is its own inverse Operation of the Fredkin gate C A B A 0 B C AC’+BC BC’+AC A AB A B 1 0 0 A B A B A A+B A 0 1 1 A B 1 B A A A A’ A 4-input Fredkin gate X A B C 1 A B C X 0 A B C AX’+CX BX’+AX CX’+BX 1 C A B A B 0 1 0 A B C A A+B AB A’ Fredkin Gate from Switch Gates CQ CP+CQ Q CQ CP+ CQ CP P C CP C Another Illustration of Conservative Property of a Circuit Operation of a circuit from Switch Gates One red on inputs and outputs Conservative property CQ CP+CQ Q=0 CQ CP P=1 C=1 CP+ CQ CP Red signals are value 1 C Two red on inputs and outputs Minimal Full Adder using Interaction Gates AB+AXC +BXC 0 Carry AB A AB A’B (A+B)C B AB’ B AB AB A’B C AB’ AB Garbage signals shown in green 3 garbage bits Sum Reversible logic: Garbage • A k*k circuit without constants on inputs which includes only reversible gates realizes on all outputs only balanced functions. • Therefore, k1 * k1 circuit can realize nonbalanced functions only with garbage outputs. Switch Gate Switch Gate CP Inverse Switch Gate CP P C C P C C’P C’P C In this gate the input signal P is routed to one of two output ports depending on the value of control signal C Minimal Full Adder Using Switch Gates AB A carry B B A’B sum C Garbage signals shown in green 7 garbage bits Minimal Full Adder Using Fredkin Gates A B carry C 1 0 sum In this gate the input signals P and Q are routed to the same or exchanged output ports depending on the value of control signal C 3 garbage bits Fredkin Gate from Interaction Gates PQ C C PQ P PQ Q PQ C CP+ CQ CP+ CQ Swap gate from three Feynman Gates 0 0 1 1 0 1 1 1 1 0 1 1 1 0 0 1 1 1 1 1 1 1 0 0 Thus every non-planar function can be converted to planar function Fredkin Gate from Interaction Gates PQ C C PQ P Q PQ C CP+ CQ CP+ CQ To verify conservative property signal ON is shown in red Fredkin Gate from Interaction Gates C P Q C CP+ CQ CP+ CQ To verify conservative property signal ON is shown in red Fredkin Gate from Interaction Gates C P Q C CP+ CQ CP+ CQ To verify conservative property signal ON is shown in red Concluding on the Billiard Ball Model • The Interaction and Switch Gates are reversible and conservative, but have various numbers of inputs and outputs • Their inverse gates required to be given only some input combinations • Logic Synthesis methods can be developed on the level of k*k gates such as Fredkin and Toffoli (quantum) • Logic synthesis methods can be developed on level of simpler gates such as Interaction gate and Switch gate that have direct counterparts in physical processes. Concluding on the Billiard Ball Model • INVERTER, FREDKIN and FEYNMAN gates can be created from Billiard Ball Model. • There is a close link of Billiard Ball Model and quantum gates and other physical models on micro level • Many ways to realize universal (for instance – optical) gates, completely or partially reversible but conservative