Changing floating point value in exe

Question

I have a Windows program (exe) that was written in Delphi (Delphi 7 I think) several years ago and that program is still used occasionally. It contains a Single type variable that is multiplied by 0.9 at some point in the code.

I would like to change 0.9 to 0.8 right in the exe file using a hex editor and I need help to find the relevant part of the code to change.

The variable is declared as

private  myValue: Single;

and when a button is clicked a component's edit value gets set to Round(myValue * 0.9)

procedure MyForm.buttonClick(Sender: TObject);
begin
  if button.Down then
    myEditComponent.EditValue := Round(myValue * 0.9);
end;

The value 0.9 is only used once in the whole code so I thought it should be easy to find it. I learnt that the hex representation of 0.9 as a DWORD should be 0x3f666666 but I could not find that value in the exe.

So it's either that I am wrong and 0.9 is represented with a different hex string or the compiler formulated this calculation in a different way (like myValue * 9 / 10 or some other way) or ..

Before you suggest that I recompile the project: I have the source code but it is a large project with several dependencies. The code relies on multiple component pack libraries and dozens of third party components. It could take a couple of days just to get and install an old version of Delhi and register all the components in order to recompile the code. Not something I would like to do for such a small change of an infrequently used program.

If I had Delphi installed I would just write these few lines of code and disassemble / debug it in order to see the assembly code. Having the assembly I could figure out what needs to be changed in the exe. I hope someone could point me in the right direction so I do not have to install an old version of Delphi.

Answer 1

Floating point literal constants are stored as 80-bit extended in the 32 bit versions of Delphi.

A value 0.9 is stored as 66 66 66 66 66 66 66 E6 FE 3F

For 0.8 replace it with CD CC CC CC CC CC CC CC FE 3F