I'd like to mix C and assembler language. Why? Bit manipulation is easier with assembler and I already have some code in assembler. Inline assembler seems quite clumsy to me, I prefer assembler libraries. Some explanation is in thread As I understood, including assembler library should be implemented in 0018, but that is not mentioned in 0018 version news.

So, are S. files supported by default in current version 0018 ? If not, are there some good instructions how to do it ?

Have you tried dropping a dot-S file into a Sketch directory?

Yes, I have assembler file “on.s” in sketch folder (set up in arduino file-preferences) and subfolder “libraries”.
I also put file “on.s” in folder arduino\0018\libraries.
Also, assembler file is in sketch folder (CTRL+K or meni sketch-show sketch folder). In this sketch folder I have folder “libraries” with assembler file in it.

if I put in first line of the sketch:
#include <on.s>

I get an error :16 no such file or directory

I'm afraid that's all I have for suggestions. Hopefully, someone else can offer some ideas.

It looks like the arduino IDE wants assembler files to have an UPPPERCASE ".S" suffix, but you said you named your file "on.s"... Try "on.S" instead...

(I haven't tried this; I just browsed the Arduino source code a bit. (app/src/processing/app/debug/ (sigh.))

If you are not afraid of assembler, then maybe you are advanced enough that you might want to consider to dump this IDE anyway. I just stick to the libraries and never use the IDE anymore. Got me rid of all these strange problems immediately. Of course you may have constraints that makes this step impossible for you, but this would be my suggestions.

Cheers, Udo