# How do adders work? (AQA)

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#1
I'm revising Logic Gates for computer science, currently on the adders part, but I cant work out how the addition works. I understand the half adder addition (0+0 = 0, 0+1 = 1, 1+1 = 1 carry 1), that all makes sense to me, but with full adders, looking at the truth table I don't understand it. What I mean is like 1 + 1 + 0 should, by half adder logic, equal 1 carry 1 (because + 0 doesn't add anything, right?) , but in reality it's 0 carry 1. very confused. if someone could explain how it works i'd be very thankful
Last edited by tShephard; 1 year ago
0
1 year ago
#2
(Original post by tShephard)
I'm revising Logic Gates for computer science, currently on the adders part, but I cant work out how the addition works. I understand the half adder addition (0+0 = 0, 0+1 = 1, 1+1 = 1 carry 1), that all makes sense to me, but with full adders, looking at the truth table I don't understand it. What I mean is like 1 + 1 + 0 should, by half adder logic, equal 1 carry 1 (because + 0 doesn't add anything, right?) , but in reality it's 0 carry 1. very confused. if someone could explain how it works i'd be very thankful
A full adder circuit basically works the same as a half adder.

So 1+1 in a half adder is the same as 1+1+0 in a full adder (=0 carry 1).

1+1 = 2
1+1+0 = 2

Both the operations have the same result and therefore the same output (0 carry 1)

The only real difference is that you can have the extra operation 1+1+1 = 1 carry 1.

I think you've looked at it wrong, you said that 1+1+0 should equal 1 by half adder logic, but by half adder logic 1+1+0 would still equal 0 carry 1 because 1+1+0 is the same as 1+1
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#3
(Original post by Hoc est Bellum)

I think you've looked at it wrong, you said that 1+1+0 should equal 1 by half adder logic, but by half adder logic 1+1+0 would still equal 0 carry 1 because 1+1+0 is the same as 1+1
I said that because the notes on mathsandphysicstutor.com show this

Is it just straight up wrong or is there something im missing?
0
1 year ago
#4
(Original post by tShephard)
I said that because the notes on mathsandphysicstutor.com show this

Is it just straight up wrong or is there something im missing?
Just think about it this way

If the answer is 0, it is 0 carry 0
If the answer is 1, it is 1 carry 0
If the answer is 2, it is 0 carry 1
If the answer is 3, it is 1 carry 1

This is the same for both half and full adders

For example
1+1+0 = 2 so it is 0 carry 1
1
#5
(Original post by Hoc est Bellum)
Just think about it this way

If the answer is 0, it is 0 carry 0
If the answer is 1, it is 1 carry 0
If the answer is 2, it is 0 carry 1
If the answer is 3, it is 1 carry 1

This is the same for both half and full adders

For example
1+1+0 = 2 so it is 0 carry 1
thanks man
0
1 year ago
#6
(Original post by tShephard)
I'm revising Logic Gates for computer science, currently on the adders part, but I cant work out how the addition works. I understand the half adder addition (0+0 = 0, 0+1 = 1, 1+1 = 1 carry 1),
Check your understanding: 1+1 = 0 carry 1.
1
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