fcbs are supported to some extend.
Dunno what your application exactly needs. You can't change a thing in the configfile to improve it.

No, I don't know what my application exactly needs cos' it wasn't written by me. All I know is that I have to run it with old Dos (with Win95) with config.sys like this :
device=c:\windows\himem.sys
device=c:\windows\emm386.exe noems
dos=high,umb
Lastdrive = Z
files=60
buffers=60
FCBS = 50
device=c:\windows\command\ANSI.SYS
device=C:\WINDOWS\COMMAND\display.sys con=(ega,,1)
Country=033,850,C:\WINDOWS\COMMAND\country.sys

I still don't understand why it bugs with dosbox.

you are using 0.63 i presume.
Well it's hard to say.
FCBS is indeed very old. (was the official way for accesing files in dos 1.1)
abandonded in dos 2.0 i think

That's it, FCBS was abandonned very early.
I actually use 0.63.
Do u think I could use an older version of DosBox ?

I've just seen the error message given with 0.63 :

Exit Error Dos Illegal : 0*33 Call 3

http://www.delorie.com/djgpp/doc/rbinter/id/38/27.html

seems to be a never really implemented call in dos itself

Weird application you got there 😀
If you can recompile dosbox try the following patch against 0.63
file dos.cpp

1--- dos.cpp     2004-11-16 15:24:52.000000000 +0100
2+++ dos.new.cpp 2004-12-01 09:50:35.000000000 +0100
3@@ -346,6 +346,10 @@
4                        case 0:reg_dl=dos.breakcheck;break;                     /* Get the breakcheck flag */
5                        case 1:dos.breakcheck=(reg_dl>0);break;         /* Set the breakcheck flag */
6                        case 2:{bool old=dos.breakcheck;dos.breakcheck=(reg_dl>0);reg_dl=old;}break;
7+                       case 3: /* Get cpsw Falltrough */
8+                       case 4: /* Set cpsw */
9+                               /* Ignore for now */
10+                               break;
11                        case 5:reg_dl=3;break;//TODO should be z                                                /* Always boot from c: :) */
12                        case 6:                                                                                 /* Get true version number */
13                                reg_bl=dos.version.major;
14

You could also try making a dummy config.sys file in the mounted hard disk drive which contains the above mentioned fcbs line. No hurt in trying, although that unsupported dos call is most suspicious... (i mean, it will probably still fail) 😳

Thanks a lot for your help. I'll try to make it run and tell u what happens.