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From: seymour@milton.u.washington.edu (Richard Seymour)
Newsgroups: comp.lang.fortran
Subject: Re: How do compilers handle dimensions?
Message-ID: <5190@milton.u.washington.edu>
Date: 17 Jul 90 20:27:26 GMT
References: <90198.114258FC138001@ysub.ysu.edu>
Organization: University of Washington, Seattle
Lines: 41

In article <90198.114258FC138001@ysub.ysu.edu> FC138001@ysub.ysu.edu (Phil Munro) writes:
>  All of my Fortran has been with mainframes and for the past year or
>so I am using the VS2 compiler.  Probably my questions are simple, but
>perhaps they will be of interest to others also.
>
>1. The normal dimension byte size on the mainframe machine (which I
>presume is a 64-bit machine) is *4. 
   i think you're misusing the word "dimension" -- what you appear to
    be asking is the size of a data item (or variable).
   Four bytes (32 bits) is what many systems have settled on because:
     IBM did first (before 1967, at least)
     16-bit and 32-bit computers handle it easily.
     Many pre-1978 systems (Data General for one) did not support
       integers greater than 16-bits.
> Does this size, and the resulting
>size of numbers, have anything to do with the bit size of a machine?
    i think you mean "word size" instead of "bit size"
   and the answer to that is "yes".  CDC (for example) has some
   60-bit machines, and all of their items were 60 bits (or controllable
   subsets thereof) long.  Hence you could have BIG integers, and
   lots of REAL precision.
>For example, a PC would have 8, 16, or 32 for the bit size depending
>upon whether it is an XT-type, AT-type, or 386-type machine, as I
>understand it.  If I transport a program from a 64-bit mainframe, will
>the numerical precision for REAL numbers, and the maximum size for
>INTEGERS, be less?  Would it be necessary to change variables to
>DOUBLE PRECISION, or something like that, to get the same precision?
   i think you'll find that most PC (all types) compilers use REAL*4
   as their default.  Whether you gain or lose a bit of precision
   depends up[on how the manufacturer chops the REAL data type into
   mantissa and exponent regions.
>
>2. Are the dimension statements REAL*8, etc., part of Fortran 77, 90
>or whatever, or are they just a VS extension?
     they are an extension (the ANSI standard doesn't show "*8")
     but many compiler suppiers support the syntax (again, IBM did
    it first, and everyone needed to be sort of compatible)
>
>  Thanks for any discussion on these things.  --Phil
    good luck
    --dick