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**n-bit comparator using 1-bit comparator**

Our focus is on making n-bit circuit using 1-bit building block Comparing two n-bit numbers for their equality A ----> An-1 An-2 …………A0 B ----> Bn-1 Bn-2………….B0 First, we shall compare the two numbers for equality Start with MSB and compare one bit at a time If the bit of one number is greater than the corresponding bit of the other number, then the number is greater. This becomes the solution Else, i.e if the two bits are equal, then compare the next bit. Repeat the last two steps

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**Basic building block for n-bit comparator**

1-bit comparator forms the building block for comparing two n-bit numbers. E is the cascading signal Ei+1 is the Cascading input, Ei is the cascading output Ei+1 = 1 implies that the two numbers are not equal so far Ei+1 = 0 implies that the two numbers are equal so far If Ei+1 = 1, then Ei = 1 Else, Ei = Ei+1 + (Ai XOR Bi ) Ai Bi Ei+1 Ei

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**Using 1-bit building blocks to make n-bit circuit**

Using the building block, we can build an n-bit comparator circuit What is the value of input En ? The final result is, if E0 = 0, then A = B and if E0 = 1, then A != B An-2 Bn-2 En En-1 E1 E0 An-1 Bn-1 A0 B0

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**1-bit complex comparator**

Next, we compare the two numbers for A > B, A = B or A < B The basic building block for the purpose is as shown The primary inputs are Ai and Bi Cascading inputs are (A>B)in, (A=B)in and (A<B)in, and Cascading outputs are (A>B)out, (A=B)out and (A<B)out The equations can be directly written as (A>B)out = (A>B)in + (A=B)in . (Ai.Bi’) (A<B)out = (A<B)in + (A=B)in . (Ai’.Bi ) (A=B)out = (A=B)in . (Ai Bi + Ai’.Bi’) Ai Bi (A>B)in (A=B)in (A<B)in (A>B)out (A=B)out (A<B)out

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**1-bit simplified complex comparator**

The 1-bit complex comparator block is modified to have fewer cascading signals It has two primary inputs, two cascading inputs, and two cascading outputs The equations for the outputs are (A>B)out = (A>B)in + (A<B)in’ . ( Ai Bi’ ) (A<B)out = (A<B)in + (A>B)in’ . ( Ai’ Bi ) If both (A>B)out and (A<B)out are 0 then the two bits are equal Ai Bi (A>B)in (A<B)in (A>B)out (A<B)out

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**1-bit maximizer building block**

The 1-bit maximizer block is similar to the simplified complex comparator It has two primary inputs, one primary output, two cascading inputs, and two cascading outputs The primary output M is 1 if depending on which input is greater The equations for the outputs are (A>B)out = (A>B)in + (A<B)in’ . ( Ai Bi’ ) (A<B)out = (A<B)in + (A>B)in’ . ( Ai’ Bi ) M = (A>B)in . A + (A<B)in . B + (A>B)in’ . (A<B)in’ . (Ai + Bi) Ai Bi (A>B)in (A<B)in (A>B)out (A<B)out M

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Designing Combinational Logic Circuits

Designing Combinational Logic Circuits

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