Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!SUN.COM!wmb
From: wmb@SUN.COM (Mitch Bradley)
Newsgroups: comp.lang.forth
Subject: Re: Standard follies
Message-ID: <8907181620.AA07088@jade.berkeley.edu>
Date: 18 Jul 89 13:43:03 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Reply-To: Forth Interest Group International List <FIGI-L%SCFVM.bitnet@jade.berkeley.edu>
Organization: The Internet
Lines: 45

Many thanks to Rainer Woitok for reminding me that:

> on a 1's complement machine, negative zero is FFFF
> (16 bit  words assumed). 8000  is the smallest ("most  negative") number

Sorry for the error; what I said was correct for sign/magnitude representation
rather than one's complement.

In any case, the principle still holds; "0=" returns true for two distinct
bit patterns, one of which contains nonzero bits.  Worse yet, the standard
defines "true" (i.e. that number which is returned by comparison operators)
as "-1" (it used to be "1" in Forth 79) so that all the bits will be set
(which is not correct for one's complement, where "-1" is hex fffe).

The reason for having all the bits set was supposed to be to allow flags
to participate in AND and OR expressions with less chance for error.

For example, suppose I have the following code:

 ( value ) 40 and    num1 num2 <   and  if

This code would work in Forth-83 but not in Forth-79.  The intention is
obviously to test whether the 40 bit in value is set and num1 is less than
num2.  In Forth-79, < returns 1 , but AND'ing 40 with 1 gives 0.
In Forth-83, < returns -1 (= ffff on 2's complement machines), so you can
AND a flag with any non-false value and still get non-false.  This code
happens to work on one's complement machines too, but it wouldn't work
of you were masking with 1 instead of 40.  ( "1 -1" and  is 0 in
1's-complement)

The point is, if you want to write portable code, make sure that you
follow bit-masking operations with a test for nonzero, to turn the
bit mask into a real, honest-to-goodness, true flag.

But wait!
There is no operator to test for nonzero!  The closest is  0= 0= ,
and as we've seen, that doesn't work on one's-complement machines.

The bottom line here is that Forth has a problem.  Other languages
have bitten the bullet and have distinguished between boolean operators
and bitwise logical operators (C: && vs &   Fortran: .AND. vs AND() ).
Forth uses the same operator for different purposes, just because it
happen to work on most machines in most cases.

Mitch (give me precise operators!) Bradley