MCQ-made_eassay

Question

How many full adder are needed to construct m-bit parallel adder ?

A. m/2

B. m

C. m-1

D. m+1

Comments

m-1 full adder and 1 half adder .
so m-1 will we more appropriate since you cannot do with less than m-1.

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Yeah, I assume the same...thanks .. :)

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And according to book ans is B

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its depent in which context they are asking. :)

Answer 1

If you have to construct m-bit parallel adder only with the Help of Full Adder => m Full Adder

If you have to construct m-bit parallel adder with the Help of Full Adder and half adder is also Allowed => (m-1)Full Adder + 1 Half Adder

So, This question is little bit ambiguous. They should mention terms like Only Full Adder or Use of Both Full and half Adder.

So, B and C Both Could be Answer to this Question.

But for a generalized case you can go with option B.

Comments

Nice... :)

Answer 2

You need m-1 full adders.

Except the first bit rest all m-1 bits require the full adder.

Let, A= a1 a2 a3 a4

       B= b1 b2 b3 b4

For 1st bit we need only input a1 and b1  no need of carry so we can satisy by using half adder which req 2 i/p.

2nd bit for a2 + b2 we need to add carry generated for 1st bit. So here we have 3 i/p, i.e a2+b2+c1

So we use FULL ADDER from b2 to b4 (3 full adder for 4 bit input)

So, overall we can manage addition with M-1 full adders..