Well, interesting news: I have mounted the floppy image on dosbox on my desktop PC and it works! So the problem is with DosPad, most likely a cfg issue ? I paste my cfg file below.
In the meantime, do you know of any atari ST emulator that runs under dos (well, more precisely under dosbox) so that I try to run the ST version instead? (I need an ST emulator in DOS because I want to try to run it on the iPad).

By the way here's my iPad DOSPAD config:
[dosbox]
memsize=128
[sblaster]
sbtype=sb16
sbbase=220
irq=7
dma=1
hdma=1
sbmixer=false
oplmode=auto
oplemu=compat
oplrate=44100

[cpu]
# Try core=dynamic to improvement performance
# however, it doesn't always work.
# so we are using simple core by default
core=dynamic
cycles=10000
[speaker]
#pcspeaker=false

[render]
# we are using faster scale outside dosbox
scaler=none
frameskip=2

[joystick]
joysticktype=2axis

[gamepad.keybinding]
button0=CTRL,CTRL
button1=ALT,ALT
button2=SPC,SPACE
button3=ENTR,ENTER
button4=ESC,ESC
button5=F1,F1
D9008388
F24976b1
Ee053ce4
Var.stash.apps

Memsize... Put that back to 16

It worked!!!! Culprit wasn't the mem size but the core that needs to be simple not dynamic.
Now that leaves me only with 2 questions:
- when I select mouse in the game config, my computer mouse is not recognized, so the arm will still only move with arrows. How to change that? Do I need to load a specific mouse driver before launching the game?
-(slightly off topic) is anybody aware of an atari St emulator that can be run under dosbox?

Thanks to all who helped, you really made my day!!

You will need the latest ctmouse.exe from http://cutemouse.sourceforge.net/ to get mouse support when you're booting from a floppy. (Note that old versions of ctmouse do not support the PS/2 mouse that DOSBox emulates.)

As for emulators, there's really nothing particularly special about them – any Atari ST emulator you can find that has a DOS port will probably work. (I hear the game known as Sundog is magnificent, btw.)

Thanks for this I will try cutemouse
Sundog: Dos game? I will Check it out, never heard
Atari st emulator on dos: I looked for it and couldn't find one unfortunately, seems they are all win programs... The search goes on!

bsauvage wrote:

Thanks for this I will try cutemouse
Sundog: Dos game? I will Check it out, never heard
Atari st emulator on dos: I looked for it and couldn't find one unfortunately, seems they are all win programs... The search goes on!

Gemulator Classic 3.8

http://www.emulators.com/freefile/gem380.zip

bsauvage wrote:

Sundog: Dos game? I will Check it out, never heard

No, that's an ST exclusive. There aren't a lot of ST-exclusive games out there that people still talk about, so if you're going to go through the trouble of setting up an ST emulator, I thought I might suggest it.

Thanks for this. I will try!

Trying to make gemulator 3.8 but having problems. The readme file is vert short and refers to the 3.0 user manual that I can't find anywhere. Do you also have a link for the 3.0 version?