1-22Step 4—Remember to discard any carry beyond the size of the original number. Our originalproblem had four digits, so we discard the carry that expanded the difference to five digits.This carry we disregard is significant to the computer. It indicates that the difference ispositive. Because we have a carry, we can read the difference directly without any furthercomputations. Let's check our answer:If we do not have a carry, it indicates the difference is a negative number. In that case, the differencemust be R's complemented to produce the correct answer.Let's look at an example that will explain this for you.Subtract 9_{10} from 5_{10} (1001_{2} from 0101_{2}):Step 1—Leave the minuend alone:0101_{2} remains 0101_{2}Step 2—R's complement the subtrahend:0111_{2} R's complement of subtrahendStep 3—Add the R's complement found in step 2 to the minuend of the original problem:Step 4—We do not have a carry; and this tells us, and any computer, that our difference (answer) isnegative. With no carry, we must R's complement the difference in step 3. We will thenhave arrived at the answer (difference) to our original problem. Let's do this R'scomplement step and then check our answer:0100_{2} R's complement of difference in step 3Remember, we had no carry in step 3. That showed us our answer was going to be negative. Makesure you indicate the difference is negative. Let's check the answer to our problem:

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