Question

Computer Architecture:
How many gate delays would be needed to propagate the carry bit all the way to the 41th bit, in a 41-bit ripple carry adder? ____

Answer 1

In case of ripple carry adder, processing of every bit takes place sequentially from right to left. So, the second bit is processed only after the first bit is processed and the third bit processed only after the second bit is processed and so on.

Also, in ripple carry adder, each bit is processed in a full adder. A full adder circuit needs 2 gate delays.

So, the carry bit reaches the second bit after 2 gate delays.

And similarly, the carry bit reaches the third bit after 4 gate delays.

In general, the carry bit reaches the n^th bit after 2*(n-1) gate delays.

So, the carry bit reaches the 41^th bit after 2*(41-1) = 2*40 = 80 gate delays.