I understood the 1 bit full subtractor, but i don't know how to complete the truth table for 2 bit full subtractor. The difference is on 2 bits: D0, D1 and there is also a Bout? What's the logic in completing this table? Thank you, guys!

 

So if you are subtracting two bits from two bits, and have a possible borrow from the next-least significant pair of bits, what will be the result and will you need to borrow from the next-most significant pair of bits?

Work a toy problem by hand and see how you do it manually. Consider the following six-bit problem:

X = (10)(01)(10)
Y = (01)(10)(11)

Do the math and focus on what happens with that middle pair of bits.

 

Ok, something like this? Is right?

 

Yes -- aside from the fact that the answer is wrong. Your answer is larger than X, the value you started with!

But we can still work with this. Look at the inner pair of bits. What there a borrow-in? What did you do with it? Was there a borrow-out? How did you determine it?

 

If nothing else, you say that you understand a 1-bit full subtractor. Well, isn't a 2-bit subtractor just two 1-bit subtractors hooked up appropriately?