logic circuits.. help..

[potonski]

Design a combinational circuit that will add or subtract 2 digit BCD numbers and produces a sum or difference number also in BCD. The circuit must accept “1” for addition and “0” for subtraction (A control switch must be employed to switch from addition to subtraction and vice versa.) and display the operands as well as the result using a 7 segment display. For a negative, LED may be used as an indicator.

[admin]

What you need is BCD adder. Here is a couple of links that google will give you:
http://en.wikipedia.org/wiki/Binary-coded_decimal
http://electronicsinourhands.blogspot.c ... adder.html
http://tams-www.informatik.uni-hamburg. ... adder.html
http://www.ijetch.org/papers/229-JT336.pdf

Good luck