I expected that the result of bintoc(0,1) would be chr(0) - in fact it is chr(128), or 80 hex. Further tests showed that all bintoc() results have 128 added to the most significant byte.
Ex:
bintoc(255) results in chr(128)+chr(255) (hex 80 FF)
bintoc(256) results in chr(129)+chr(0) (hex 81 00)

Why is this?

TIA