I need your assistance. I am taking a Modern Digital Design course, and I am stuck on our lab for this week. We are to construct a Four bit 2s complement adder circuit, using logic gates. The circuit must be connected through a 4-bit adder, then connected through a BCD - 7 segment converter and displayed on a 7 segment display. If the sum is negative the overflow will feed through another 4-bit adder, through another BCD - 7 segment converter and the symbol (0 for positive, 1 for negative) will display on a second 7 segment display. However I ran into a few issues. We are to use Multi-sim, and Multi-sim does not have a 4 bit adder available on the student provided version, so i had to cascade four 1 bit adders. Im assuming since A0 A1 A2 A3, B0, B1, B2, B3 will be connected directly to the first 4 bit adder, i will need to build another adder/subtractor for the second 4 bit adder. Therefor I am unsure of a couple things.

1. Connecting AB inputs of the second 4 bit adder, since I would need them to provide a sum for the Inputs of the BCD - 7 segment converter.

2. I am also not getting any reading on the first seven segment display when pressing switches. The circuit i am using is straight from our textbook (Digital Electronics Principals and Applications Roger Tokhiem).

I am using CMOS XOR gates, with a XOR gate as the over flow going into the second cascaded 4 bit full adder. any ideas or advice would be appreciated. I have no idea where i went wrong. I uploaded a screenshot, and tried to upload the actual file however the actual file is an unallowed extension.