Thanks to evryone who answered my question. I learnt a lot.
First, I didn't know that I can download the IDE without having a board. I'll try it as soon as possible. I'll also play with wokwi.com.
This first app will drive 4 points and 2 relays. Its main purpose is learning how it works. I'll look to the link about railroad mainly to see 'how far' can we go.
Now, there is a big difference between a subroutine call and a software interrupt : the adress of the routine to be called by the interrupt is stored in RAM (basically, the adress of the routine to call by interrup 80 is in the word 80 of the memory). It means that you can modify the routine to call without having to compile again the app; You can also 'insert' a routine if this routine stores the adress of the interrupt in a memory word, then stores its own adress in the interrupt word and call the previous routine instead of the 'return'. I hope you understand !
By the way, is there any french people among you ?
Best regards
Jean