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5 Answers

Best answer
15 votes
15 votes

To add two decimal digits we need two 4 bit adders.

First one for adding the two BCD number and second one to make the result a valid BCD number.

Now for the first one we need a simple 4 bit adder which we can construct using 3 full adders and 1 half adder.

For the second one we need to observe how it is working,

Suppose the result obtained from 1st 4 bit adder be D3 D2 D1 D0 and also suppose this number is greater than 9. So we have to add 0110 to make it a proper BCD

Carry   C3 C2 C1 0  
Result from 1st 4 bit adder +   D3 D2 D1 D0
Adding 6 +   0 1 1 0
Final result           D0

Now D0 will always be added with 0.So we dont need any adder for this.

Since we dont need adder for LSB ( D0) , no carry will be generated by LSB. That means D1 will be simply added with 1 only and this can be done by half adder.

Also Dwill be added with C2 only. So here too we can use half adder.

To add D2 we will need full adder.

So total full adders = 3+1 =4  and Half adders = 1+2 = 3

So I think answer should be C)

3 votes
3 votes


The final carry generated by adding Ones digits will be made carry-in for addition of Tens digits, hence, the A0 + B0 is a full adder. So you require 5 Full adders and 2 Half adders. Option D is the right answer.

1 votes
1 votes
1 votes
1 votes

For Binary addition, 3 full adders and 1 half adder is required. And then BCD conversion needs 6 to be added, which in turn requires 2 more full adders and 1 half adder.

Thus, Final Answer is "D" - 5 Full adders and 2 Half adders

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