### 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 = -2^{31}.