@corona -

Yes. I think I mentioned the least signficant 8 bits.

I might as well inject the whole sysexits thing here, too:

Man Page for sysexits (freebsd Section 3) - The UNIX and Linux Forums

The idea here is to use predefined exit values for various error conditions.
When you see a unix command return an error code like 64, chances are the exit codes conform to the sysexits model. sysexits is a model, or a suggestion. But it does allow the program to convey intelligence when an error occurs.

For example the diff command (by POSIX standards) has it's own little protocol. It does not use sysexits because in part the information ( or meaning ) associated with two return codes of the three are not errors.

Code:

return value    meaning
0                  differences found
1                  no differences found
2                  some other error occurred

This parallels in part alister's comment about the "range" of return values.