Talk:Adder (electronics)

Carry-Skip and Carry-Select

We should probably also link to Carry-skip and Carry-select adders too, since they lie between ripple adders and full lookahead adders.Symmetry 14:47, 18 January 2007 (UTC)[reply]

carry-lookahead vs carry-propagate

two questions: what is the difference between a carry-lookahead and a carry-propagate, which I have also heard of? Are they the same thing? Also, is the carrry select adder the same as the carry save adder? these can be confusing because they share the same acronym.

jUst wondering. Thanks for good work.

A carry-save adder is actually an architecture for adding more than 2 binary numbers together, like in a multiplier. I'm not certain, but I think a carry-propagate adder might be as well.

Ckape 03:03, 3 Jan 2005 (UTC)

GG and PG

What's the GG and PG leaving the carry lookahead at the end of the carry lookahead block? Why do you not just have a single carry out from the lookahead block? Cheers.

suggest changes

can i suggest some changes to the third paragraph:

The other type of single bit adder is the full adder which is like a half adder, but takes an additional input carry C_i. A full adder can be constructed from two half adders by connecting A and B (i.e. the inputs of the full adder) to the input of one half adder, connecting the sum from that to an input of the second adder, connecting C_i to the other input and taking the carry output of the full adder to be the or of the carry outputs of the two half adders. Equivalently, S could be made the three-bit xor of A, B, and C_i and C_o could be made the three-bit majority function of A, B, and C_i. The output of the full adder is the two-bit arithmetic sum of three one-bit numbers.

i haven't changed the file myself as this is not really my area of expertise, but i think my changes make clearer the distinction between the full adder and the two half adders. otherwise there is some ambiguity. User:halio 13th dec 2005

i just looked further down the page and realised that the points i found confusing are better expressed by the diagrams than they are in the intro. perhaps this page needs a slight re-edit to create a more succinct intro, and to better integrate the more detailed explaination with the diagrams. User:halio 13th dec 2005

gah! why, on the image of the ripple carry adder, is the first line to the first 'B' crooked!?

Brent & Kung adder

I came to this page on a redirection from "Brent-Kung adder", but I see no reference to B&K. First of all, since the B&K cells are a way to implement a carry lookahead, the redirection should go there. Second, if it's not going to be discussed in the CLA adder page, then what's the point in a redirection?

BTW, a carry propagate adder is an adder where the carry is immediately added, as opposite to the carry save adder where two words (carry and sum) are kept separate till the end. All of the adders here described (ripple carry or lookahead) are carry propagate adders. And the carry select adder is a carry-propagate adder.

Full adder

full adder has three inputs inp1,inp2 and carry.

The one u have stated is a half adder .

In my Logic Design book, it says a full adder has two inputs - not three like this article says. Is this article wrong on that? Fresheneesz 07:28, 13 March 2006 (UTC)[reply]

Full adder picture OK?

Is the picture for the full adder really ok? For C_out it implements C_out = (A and B) or (C_in and (A xor B)), which is not equivalent to the stated expression C_out = (A and B) or (C_in and (A or B)). The latter puts a 1 for C_in in the last row of the truth table, whereas the former puts a 0 there.

Note also that the note under the truth table does not concern this problem; A and B can both be 1 simultaneously.

—Preceding unsigned comment added by 129.16.80.10 (talk) 10:40, 18 January 2008 (UTC)[reply]

Yes, it's OK, but I think the expression for C_in should be changed to C_out = (A and B) or (C_in and (A or B)) = (A and B) or (C_in and (A xor B)) since the last part of this is what's really implemented.