Quick Sign Magnitude Negation question.

Question

Ok ive learned negation in Sign magnitude but I hit a bump in the road with one problem that seems to make the basic rules not work. So if someone would be so kind as to tell me how you subtract this sign magnitude problem out I would be very thankful.
01100100 (Which is 100) minus
00111100 (Which is 60) Seems very simple and I know all you have to do is
subtract but how do you subtract from 0? Where the 0's are above the 1's. In basic arithmetic that would be negative 1 but not in Sign magnitude. Anyone see what im saying?

Answer 1

Another way is called 2s complement addition. If you want to subtract 60 from 100 then you can add minus 60 to 100. In 2s complement the high order bit is the sign bit. To represent minus 60 in 2s complement binary notation you first write +60 in binary, then form the 1s complement (by inverting all the bits), and then add 1.
So,
0 0 1 1 1 1 0 0 = 60
1 1 0 0 0 0 1 1 = 1s complement. (-128 sign bit, + 64 + 2 + 1) = -61
1 + 1 to form the 2s complement
1 1 0 0 0 1 0 0 = 2s complement (-128 + 64 + 4) = -60
So, add minus 60 to 100, which is the same as 100 - 60.
0 1 1 0 0 1 0 0 =100
1 1 0 0 0 1 0 0 = -60
0 0 1 0 1 0 0 0 = 40
Notice that the carry from the left most column is ignored, because we are using 8 bit binary 2s complement addition. Also the sign bit in the answer is cleared, so the answer is positive.