Because when you need to do subtraction beyond the word length of the processor, such as 8086 with long shaping (32 bits, 4 bytes) subtraction calculation, you can only calculate the subtraction of high-order words and low-order words respectively, and the low-order words can be directly subtracted, but if the subtraction is not enough, there will be a borrow (CF= 1), which should be borrowed from the high-order words, that is to say, the high-order words borrowed 650.