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From: cunniff@hpfcso.HP.COM (Ross Cunniff)
Newsgroups: comp.lang.fortran
Subject: Re: What is the FORTRAN for ?
Message-ID: <9080007@hpfcso.HP.COM>
Date: 29 Aug 90 15:01:21 GMT
References: <1990Jul25.174153.16896@ecn.purdue.edu>
Organization: Hewlett-Packard, Fort Collins, CO, USA
Lines: 40

>> ... Beyond the requirement that LOGICAL variables take the
>>same amount of storage as INTEGER and REAL variables, the standard
>>places no restrictions on the internal representation and there is
>>no single "best" way to implement the type.  For example, one
>>implementation may focus on the least significant bit, using 0 for
>>.FALSE. and 1 for .TRUE.  Another may focus instead on the sign
>>bit, using +1 for .FALSE.  and -1 for .TRUE. ...
>
> The universal convention, so far as I know, is that .FALSE. is
> always represented by all bits zero, while *any* bit on must be
> recognized as .TRUE. and detected by the code generated by a
> conforming compiler.  Switching must always be done by moving a
> full word (or whatever) of zero bits into a variable to set it
> .FALSE., or a full word or whatever containing at least one one
> bit to set it .TRUE., never by clever bit flipping.

Well, no.  Take the following code fragment:

	logical l
	integer i
	equivalence (l,i)
	i = 2
	print *, l
	end

This results in the following output:

	T	(HP9000 Series 300 workstation)
	F	(HP3000 minicomputer)
	F	(DEC VAX)

Most UN*X compilers apply the same rule you quoted; however, there are
quite a few non-UN*X compilers out there.  Lest you think that, just
because this is not a standard conforming program, there is no problem,
consider the fact that most UN*X compilers have an option to treat
logicals as the VAX does...

				Ross Cunniff
				Hewlett-Packard Colorado Language Lab
				cunniff@hpfcla.hp.com