Subtracting a large unsigned binary number from a smaller one

Question

I\'m taking a computer organization and assembly language course. The written part of our lab this week has a question on it that has me stumped. The question reads...

Answer 1

You do it the same way irrespective of which number is bigger and which is smaller.

 bb b      bb   <- borrows
 0101 0111 1101 (1405)
-1110 1011 0110 (3766)
 --------------
 0110 1100 0111 (1735?)

Now, if you want a proper difference, you need to take into account the overflow since the above result doesn't include the sign bit:

 b bb b      bb   <- borrows
 0 0101 0111 1101 (1405)
-0 1110 1011 0110 (3766)
 ----------------
 1 0110 1100 0111 (-2361 signed 2's complement)

Really, the CPU doesn't care what gets subtracted from what. It uses the same algorithm for integer addition/subtraction, moreover, this algorithm is the same for signed and unsigned integers. You only have to correctly interpret the result and the carry and overflow flags. That's all.