[转]How to set the control word of FPU in delphi

      On systems based on the IA-32 architecture, the FPU control word includes bits that control the FPU's precision, rounding mode, and whether exceptions generate signals if they occur, if you want to know more, please refer Floating-point Control Word Overview.

     In delphi, it provide us two function named “Set8087CW” and “Get8087CW” to set and read the control word of FPU. Function Set8087CW sets both the control word in the floating-point unit and the variable System.NoErrMsg declared in the system unit. The floating-point unit control word controls the precision of floating-point calculations, the rounding mode, and whether certain floating-point operations trigger exceptions. See the Intel processor documentation for details.

This routine allows you to have direct access to the CW. Be aware that using this routine to change the value of the 8087CW changes the behavior of the program's FP calculations. To avoid this, reset it.

It is  recommend that you disable all floating-point exceptions when using OpenGL to render 3D graphics. To do this, call Set8087CW(0x133f) in your main form's OnCreate event before calling any OpenGL functions.

Following is the example:

var
  Form1: TForm1;
  Saved8087CW: Word;
 
implementation
 
{$R *.dfm}
 
procedure TForm1.Button1Click(Sender: TObject);
begin
  Edit3.Text := FloatToStr(StrToFloat(Edit1.Text) / StrToFloat(Edit2.Text));
end;
 
procedure TForm1.RadioGroup1Click(Sender: TObject);
begin
  if RadioGroup1.Items[RadioGroup1.ItemIndex] = 'FPU Exceptions' then
    System.Set8087CW(Saved8087CW);
  if RadioGroup1.Items[RadioGroup1.ItemIndex] = 'No FPU Exceptions' then
    System.Set8087CW($133f); { Disable all fpu exceptions. }
end;
 
procedure TForm1.FormCreate(Sender: TObject);
begin
  RadioGroup1.Items.Add('No FPU Exceptions');
  RadioGroup1.Items.Add('FPU Exceptions');
  RadioGroup1.ItemIndex := 2;
  Saved8087CW := Default8087CW;  // Save this because Set8087CW changes it.
end;
 
procedure TForm1.FormDestroy(Sender: TObject);
begin
  System.Set8087CW(Saved8087CW); // Default value (with exceptions) is $1372.
end;

 

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