address off the stack, so put one there for it.
(Bug was hidden by bad segment limits.)

byte in the segment, the limit argument to SEG16 and SEG
needs to have 1 subtracted from it.

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.