2-7 Sign Function

The sign, or signum, function is defined by

It may be calculated with four instructions on most machines [Hop]:

If you don't have shift right signed, then use the substitute noted at the end of Section 2-6, giving the following nicely symmetric formula (five instructions):

Comparison predicate instructions permit a three-instruction solution, with either

Equation 3

Finally, we note that the formula almost works; it fails only for x = -2³¹.