I am posting this in case other people are writing non-PASCAL subroutines
called  from PASCAL  so you  don't waste  an hour  finding out  what your
problem was.
 
I don't know of any formal description of the machine format of VS PASCAL
booleans in  the manuals, other than  saying that they are  unaligned and
one  byte long.  Of course,  I  may just  have  missed it.  The code  the
compiler generates for IF instructions and the like checks for a value of
zero, anything else is TRUE. And of  course when you code IF NOT, it also
checks  for a  value of  zero and  just inverts  the branches.  It seemed
straightforward enough. But  it turns out that VS  PASCAL booleans really
have  three values:  FALSE  (zero), TRUE  (one), and  TRUE-NO-MATTER-WHAT
(anything else).  These booleans of the  third type work fine  as long as
you just  test them,  but once  you use  them in  an expression  which is
stored in a variable  or passed as a function argument, both  X and NOT X
turn out to be  true. This is because VS PASCAL does not  see fit to flip
the higher order bits, so the result  is nonzero no matter what. Your non
PASCAL subroutines had better return 1 and  not 255, or it will only work
most of the time.
 
  Eric