Switching from a 53-bit to 64-bit FPU in Fortran

Question

Hi,

I am porting Fortran code from Fortran PowerStation(version 4.0) to the Fortran 11(2003) compiler. The old compiler (PowerStation) has 53-bit precision. When porting to the new compiler, I am not achieving proper or exact values for my real/float variables. I hope the new compiler is 64-bit precision. So I think I need to change the FPU (floating point unit) from 53-bit to 64-bit precision. Is this correct? If so, how do I go about changing 53-bit to 64-bit precision using the properties of the new compiler? If not, what should I be doing?

Thanks in advance.

Answer 1

Hi

You hope the new compiler is 64-bit precision ? I sort of expect that you read the manual and figure that out yourself but if you can't do that, tell us which compiler you are using and someone might help.

How different are the results of the old code and the code compiled with the new compiler ? Of course the results won't be exactly the same if the precision has changed -- how could they be unless you take special steps to ensure sameness.;

Regards

Mark

Answer 2

The portable way to request floating point precision in Fortran 90/95/2003 is with the selected_real_kind intrinsic function. For example, *integer, parameter :: DoubleReal_K = selected_real_kind (14)* will define a integer "DoubleReal_K" that specifies a floating point variable with at least 14 decimal digits: *real (DoubleReal_K) :: MyFloat*. Requesting 14 decimal digits will typically produce a double-precision float with 53 bits -- but the only guarantee is 14 decimal digits. If you need more precision, use a larger value than 14 to specify a longer type -- 17 decimal digits might get extended precision (64 bits), or it might get quadrupole precision, or nothing, depending on the compiler. If the compiler has a larger type available, it will provide it... Otherwise, get a better compiler. Why are you using such an old and unsupported compiler? Also, don't expect exact results from floating point calculations -- it is normal for changes to cause small changes in the results.