when multiplying a binary number by (-1), in other word changing the sign the technique is NOT the number then ADD 1 to it. Why is ADD 1 needed to complete the process?

That''s due to the way that signed numbers are stored. You probably know that unsigned chars are like this:

128 | 64 | 32 | 16 | 8 | 4 | 2 | 1

Signed chars are stored like this:

-128 | 64 | 32 | 16 | 8 | 4 | 2 | 1


Now, if I have 35, (32 + 2 + 1), if I NOT it I get (-128 + 64 + 16 + 8 + 4), but that''s only -36. To get -35, I have to add 1. So that''s why you need to add 1. The process is called the "two''s complement".

But wasn''t there an opcode to do that automatically?