Implement a 1-bit Full Adder (FA) from scratch (from truth-table and using basic gates), with input a, b, c_in, and output s, c_out. 

2. Implement a 4-bit Adder using 4 of the 1-bit FA subcircuit.

3. Implement a 4-bit Subtractor with the previous components

4. Two 7-seg displays showing the sign and value for any 4-bit signed number.

5. Putting the system together including function input f.