- Some computers consider the C bit to be a borrow bit after a subtraction operation A — B. A borrow does not occur if A ^ B, but a bit must be borrowed from the next most significant position if A < B. The condition for a borrow is the complement of the carry obtained when the subtraction is done by taking the 2's complement of B. For this reason, a processor that considers the C bit to be a borrow after a subtraction will complement the C bit after adding the 2's complement of the subtrahend and denote this bit a borrow.