组装-标志标志和奇偶校验标志

Question

I didn't understand when the sign flag is set, and when the parity.

As I know, the sign flag indicates the sign of the result of an operation, 0 for positive numbers and 1 for negative numbers.

So why in the next code:

mov al, -5 
sub al, 124

The SF is zero? The result is negative number.

About the PF, Why in a and b, the PF is set?

a) sub al, al.

b) mov al, 127
   add al, 129

Answer 1

The code

mov al, -5 
sub al, 124

calculates 256-5-124 . It's 127 and it's positive number

Both sub al, al and 127+129 evaluates to 0, it has 0 bits set, and 0 is even value, so PF is set.

Answer 2

您可以用8位表示的最大数字是2 ^ 8-1，即255，因此，因为127 + 129 = 256，al将保持0，并标记：CF PF AF ZF IF将被设置，PF被如上所述设置-因为只有且仅当设置的位数为偶数时，才设置PF。