Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wasatch!cs.utexas.edu!uunet!tektronix!tekcae!kurtk
From: kurtk@tekcae.CAX.TEK.COM (Kurt Krueger)
Newsgroups: comp.lang.fortran
Subject: Re: CHARACTER EQUIVALENCE (Was Re: fortran to C converter)
Message-ID: <2693@tekcae.CAX.TEK.COM>
Date: 24 Apr 89 16:03:54 GMT
References: <67044@pyramid.pyramid.com> <1390@hudson.acc.virginia.edu> <21338@prls.UUCP>
Reply-To: kurtk@tekcae.CAX.TEK.COM (Kurt Krueger)
Distribution: usa
Organization: Tektronix, Inc., Beaverton,  OR.
Lines: 22

There is a compiler that enforces separate common blocks for character and other
data.  A Fortran powerhouse - CDC.

There are some real sticky issues about word alignment.  Some machines HAVE to
word align (the older CDC's for examaple).  Other machines MAY or MAY NOT, but
some of these machines have boundary requirements that differ for different
data types.  Both Gould and IBM (at least the 370 style machines) have byte
addressibility but generate hardware faults if you try to fetch a full word
that is not on a full word boundary.  They may allow you to mix characters
and full word data in the same common block, but proper alignment is usually
up to you.  Any goof and the code crashes.  With all the possible archetectures,
it is virtually impossible to write code with mixed character/full word data
that won't cause grief on somebody's machine.

VAXen can handle non aligned data, but only because of a special feature of
the processor.  Memory fetches are always aligned, but the processor can fetch
twice and align it in a register.  It works, but it slows things down.

I know it's a pain to have two COMMON blocks where one should do.  An easy way
around this is to use whatever INCLUDE facility your system has.  That way the
code only has to mess with one INCLUDE, but both common blocks are defined in
the included file.