Should be ieee 754, 64 bits, ie. 8 bytes. I think maybe some of the crays
in the past had a slightly different meaning; these days practically
everything is intel ia32-or a derivative thereof-based, or itanium
(disappearing), or ibm power chips. You specify intel below I believe.
Intel does have an internal 80 bit format when you are in the registers; I
expect that compiler options determine when this is used, but it can cut
rounding error further. The thing is, once you store in memory or send dp
over the net, you are back to 8 bytes. I have not recently done a lot of
research on this, but that was the state of the world last time I looked...
It was also true for the various now-essentially-defunct risc chips like
MIPS and Alpha.
Regards - Bob Duke
----- Original Message -----
From: "sop dadoun" <dadoun.sop.gmail.com>
To: "AMBER Mailing List" <amber.ambermd.org>
Sent: Saturday, May 01, 2010 6:14 PM
Subject: Re: [AMBER] size of real
> Hi,
>
> Thank you for your answers.
> In fact, my question is about the width of "double precision" (64, 128 or
> more bits ?)
> Note : amber 8 , sander , ramd
> Station : Intel(R) Pentium(R) 4 CPU 3.20GHz
>
> Best regards,
> Sophie
>
>
> 2010/5/1 Robert Duke <rduke.email.unc.edu>
>
>> All of pmemd just uses "double precision" instead of "real". The use of
>> "double precision" is I think deprecated in the latest fortran standards,
>> but it is really clear what you are doing, and they may deprecate the
>> usage,
>> but it is unlikely to go away, given the wide use. The whole "kind"
>> system
>> focuses on the range of values you can represent, which certainly is
>> mathematically useful. But for a lot of computer-related issues, it is
>> actually (at least to me) also useful to know how much storage is
>> required.
>> The two are related, but not precisely - depends on the exact nature of
>> the
>> number representation, especially for floating point, and also I think
>> that
>> there is an "at least" sense to "kind", so the compiler maker can just
>> use
>> more storage than required to represent a specified range.
>> Regards - Bob Duke
>> ----- Original Message ----- From: "Jason Swails"
>> <jason.swails.gmail.com>
>> To: "AMBER Mailing List" <amber.ambermd.org>
>> Sent: Saturday, May 01, 2010 10:25 AM
>> Subject: Re: [AMBER] size of real
>>
>>
>>
>> Hello,
>>>
>>> On Sat, May 1, 2010 at 9:03 AM, sop dadoun <dadoun.sop.gmail.com> wrote:
>>>
>>> Dear amber users,
>>>>
>>>> I would like to know the size of real for a fortran program used in
>>>> amber
>>>> with Pentium 4 CPU.
>>>>
>>>>
>>> Most amber programs include the file $AMBERHOME/include/dprec.h or .fh,
>>> depending on what version of amber you're talking about. The precision
>>> of
>>> _REAL_ depends on the preprocessor flags that are given to (u)cpp.
>>> Typically, though, they're double unless you modify something.
>>>
>>> All the best,
>>> Jason
>>>
>>> Thanks you for rplying
>>>
>>>> Best regards,
>>>> Sophie
>>>> _______________________________________________
>>>> AMBER mailing list
>>>> AMBER.ambermd.org
>>>> http://lists.ambermd.org/mailman/listinfo/amber
>>>>
>>>>
>>>
>>>
>>> --
>>> Jason M. Swails
>>> Quantum Theory Project,
>>> University of Florida
>>> Ph.D. Graduate Student
>>> 352-392-4032
>>> _______________________________________________
>>> AMBER mailing list
>>> AMBER.ambermd.org
>>> http://lists.ambermd.org/mailman/listinfo/amber
>>>
>>>
>>>
>>
>> _______________________________________________
>> AMBER mailing list
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>> http://lists.ambermd.org/mailman/listinfo/amber
>>
> _______________________________________________
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>
>
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Received on Sat May 01 2010 - 16:00:03 PDT
