What to do if subtracting in binary overflows

Question

I am trying to do this equation -122 -(7) and in the result i'm getting = 01111111

I used 2's complement for -122 and got = 10000110. And I used 2's complement for 7 so I can make addition and got = 11111001. And when I am adding both I am getting = 01111111 with carry 1. I don't think this is correct answer for -129

I am not sure, but is that correct answer? Normally I would say its 127 — Max, Sep 14 '19 at 04:06

score 0 · Answer 1 · answered Sep 20 '19 at 19:48

Ignore the Overflow Bit

Two's complement with a bit-width of 8 has a range of [-128, 127].

This binary math shows what (-122) + (-7) is equal to.

  122 =>    01111010
  7   =>    00000111

  -122 =>   10000110
  -7   =>   11111001
+         +
------    ----------
  127     1 01111111

01111111 => 127

In this example, we see that there is an overflow bit designating -256. -256 is outside of the 8-bit range of [-128, 127]. Most implementations will drop this overflow bit, and so the result would be 127.

What to do if subtracting in binary overflows

1 Answers1

Ignore the Overflow Bit

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