Design a 4 bit combination circuit decrementor using 4 full addre circuit?

Question

Design a 4 bit combination circuit decremantor using 4 full adder circuit?

Answer 1

Use the regular 4 bit full added, but make one of the inputs 1111 = 2's compliment representation of -1. This will serve to decrement the other number by 1. Throw away the 5th bit, the carry bit.

Example

If 5 is entered:

0101
+ 1111
______

0100 = 4