128 bit addition using 4 bit adder

In one of the interviews that I attended, I was asked the following question:

Design a circuit that we can use to add two 128-bit numbers, you have only one 4-bit adder and no other adder IC. You are not allowed to use any microprocessor/microcontroller.

I gave an answer that we can do it by adding 4 bits at a time, storing it and going to the next 4 bits, and they asked how exactly will you go to the next 4 bits?

Can someone help me in finding a solution to this? (During the interview drawing the block diagram was enough, a detailed description of the block was not needed)

• The 74181 (4-bit ALU slice) is what I used to develop an 8-bit ALU. It's pretty simple to extend from 4-bit to 8-bit. What mental difficulties did you encounter? I'm curious and perhaps a little confused about what difficulties you feel you face.
– jonk
Jun 1 at 5:08
• I am sorry, I have made a small change in the question. I was asked to use only one 4 bit adder IC, I was confused how can I add two such large numbers using only one 4 bit adder. Jun 1 at 5:15
• It's an interview question which tells about how you approach problems. How to actually solve the problem depends how the bits are available to you to receive. Serially? Or parallel? How many bits at a time? Which is not specified. So there is no answer how to actually do it, there are details missing. Jun 1 at 5:16
• @NarsimhaKethiri Have a look at the RISC-V SERV. It is a bit-serial ALU. You can use the concept, adapting it for a 4-bit slice (instead of the 1-bit slice that the SERV uses.) This is not rocket-science.
– jonk
Jun 1 at 5:24
• They said you couldn't use other adders but other logic is fine. So chuck it in a couple of (shift) registers and do the job 4 bit at a time Jun 1 at 5:51