Because limit gives the address of the last addressable
byte in the segment, the limit argument to SEG16 and SEG
needs to have 1 subtracted from it.

Cannot use "g" as an arg constraint because
some memory references aren't actually valid
and gcc is a bit too smart in parsing (%0) and
a bit too dumb in knowing which memory refs
are not valid.  Using "r" seems to constrain gcc
to use a register as in (%eax) and not substitute
a memory reference like (-36(%ebp)).

No one really understands these things.