Dos box opening

For all of my projects I have build as 64 bit executables I get a dos box opening when I launch the executables.

The corresponding 32 bit executables don't have this behaviour.

What might be the reason? Where should I search in my sources?

If it is a Windows application then you either need the directive WINAPP or the FTN95 command line option /windows or the SLINK64 option windows. It depends on the number of files and how you do your build.

Now it is working with the linker option windows. But I don't understand why this option is necessary now in 64bit, whilst it was working without such an option in 32bit.

I don't know exactly why without looking at the code but SLINK64 currently does not have all of the functionality of SLINK. At the moment, the 'windows' attribute is passed from FTN95 to SLINK64 only when using /LINK or /LGO.

On the other hand, SLINK is able to pick up this attribute from object code for the main program so it also works when SLINK is called explicitly by the user (or Plato etc.) rather than when it is spawned by FTN95.