How can I set and restore FPU CTRL registers?

Question

I can reset FPU's CTRL registers with this:

http://support.microsoft.com/kb/326219

But how can I save current registers, and restore them later?

It's from .net code..

What I'm doing, is from Delphi calling an .net dll as an COM module. Checking the CTRL registers in delphi yield one value, checking with controlfp in the .net code gives another value. What I need, is in essential is to do this: _controlfp(_CW_DEFAULT, 0xfffff); So my floatingpoint calculations in the .net code does not crash, but I want to restore the CTRL registers when returning.

Maybe I don't?, maybe Delphi is resetting them when needed? I blogged about this problem here: http://blog.neslekkim.net/2008/10/fpu-issues-when-interoping-delphi-and.html

Answer 1

Same function you use to change them: _controlfp(). If you pass in a mask of 0, the current value won't be altered, but it will be returned - save it, and use a second call to _controlfp() to restore it later.

Answer 2

uses
   SysUtils;

var
   SavedCW: Word;

begin
   SavedCW := Get8087CW;

   try
         Set8087CW($027f);

         // Call .NET code here

      finally
         Set8087CW(SavedCW);
      end;
end;