|        Some time ago, there has been a discussion about multitasking and
|solutions have been proposed with interrupts ...
|Does any of you has an idea of a multitask kernel (2 tasks should cover
|most applications) without usong interrupt facilities in order to be put
|on a PIC 12C509 for example ?

Here's a nice simple method, assuming one of the tasks will fit within
256 bytes of codespace, won't call subroutines more than 1-deep, and no
task switch will need to occur within a subroutine.

Define a register called "VirtPC".  When it starts up, your main task
should set it equal to the start address of your "alternate" task.  You
should also have a code address called "Springboard" at which is coded:

Springboard:
        movf    VirtPC,w
        movwf   PC

>From the main program, when you want to yield control to the alternate
task you just use the two instructions:

        call    Springboard
        movwf   VirtPC

When your alternate program wants to yield control, it should either do
a

        retlw   $+1

if its execution should resume at the next instruction the next time the
main program yields, or else

        retlw   label

if execution should resume at /label/.

Aside from the restrictions on call depth and the fact that W is trashed
on a context switch, this is probably just about the cheapest little multi-
tasker for ANY CPU platform (5 cycles to switch to the alternate task; 3
to switch back to the main line).  Cute, eh?