Complements the sign bit of ST(0). This operation changes a positive value into a negative value of equal magnitude or vice versa. The following table shows the results obtained when changing the sign of various classes of numbers.
SignBit(ST(0)) = ~SignBit(ST(0));
C1 Set to 0 if stack underflow occurred; otherwise, set to 0.
C0, C2, C3 Undefined.