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082-Engineering-Mathematics-Anthony-Croft-Robert-Davison-Martin-Hargreaves-James-Flint-Edisi-5-2017

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190 Chapter 5 Discrete mathematics

HenceA·B+A·BsimplifiestoA.Notethatthisisthefirstminimizationlawgiven

inTable 5.9.

(b) A+A ·B=(A+A)·(A+B) by the distributive law

=1·(A+B)

by the complement law

=A +B using the identity law

(c) Note thatA +A·B·C can be written asA + (A·B)·C using the associative laws.

Then

A + (A·B)·C=(A+A·B)· (A +C) by the distributive law

=(A+B)· (A +C) usingpart(b)

=A +B ·C by the distributive law

(d) A·B·C+A·B·C+A·B·C+A·B ·C +A ·B·C

can be rearranged usingthe commutative lawtogive

A·B·C+A·B·C+A·B·C+A·B ·C +A ·B·C

This equals

A·B·(C+C)+A·B·(C+C) +A ·B·C by the distributive law

=A·B·1+A·B·1+A·B·C usingthe complement law

=A·B+A·B +A·B·C usingthe identitylaw

=A·(B+B) +A ·B·C usingthe distributive law

=A +A ·B·C usingthe complement and

identitylaws

Usingthe resultof part(c)this can be furthersimplified toA+B·C.

Engineeringapplication5.3

Designofabinaryfull-addercircuit

The binary adder circuit is a common type of digital logic circuit. For example, the

accumulator of a microprocessor is essentially a binary adder. The term full-adder

is used to describe a circuit which can add together two binary digits and also add

the carry-out digit from a previous stage. The outputs from the full-adder consist of

thesumvalueandthecarry-outvalue.Byconnectingtogetheraseriesoffull-adders

itispossibletoaddtogethertwobinarywords.(Abinarywordisagroupofbinary

digits, such as 0111 1010.) For example, adding together two 4-bit binary words

would requirefourfull-addercircuits.This isshown inFigure 5.16.

The inputs A0--A3 and B0--B3 hold the two binary words that are to be added.

TheoutputsS0--S3holdtheresultofcarryingouttheaddition.ThelinesC0--C3hold

the carry-out values from each of the stages. Sometimes there will be a carry-in to

stage 0 as a result of a previous calculation. Let us consider the design of stage 2 in

more detail. The design of the other stages will be identical. First of all a truth table

forthe circuitisderived. This isshown inTable 5.16.

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