🚀 go-pugleaf

Your program would work fine in a system with no other interrupts, however
that is not the case on the ST.  There are a number of interrupts that occur
on a regular basis (vertical retrace, keyboard etc.)  It is quite likely that
your timer will go off while one of these routines is being serviced.  It is
also quite likely that one of these routines is using D7.

What you need to do is establish a different mechanism for communicating
between the interrupt routine and the main program.  The easiest method is 
to declare some space in the interrupt procedure, preceeding it with a label,
and use PC relative addressing to access it.  If you save A4 in the space
then you can directly access C global variables from your interrupt procedure.

The following listing includes these modifications to your original program.
========================================================
#include <stdio.h>
#include <osbind.h>

extern int my_timer_code ();
long count;

asm {
    my_timer_code:
        move.l  A4, -(A7)   
        move.l  save_a4(PC), A4     /* Restore A4 */
        addq.l  #1, count(A4)       /* Just count interrupts */
        move.b  #0xDF,0xFFFA0F      /* Clear ISRA Timer A bit */
        move.l  (A7)+, A4
        rte

    save_a4:
        dc.l    0
    }


main()
{
    register int i;
    int (*my_interrupt_handler) ();
 
    asm {   /* Save A4 for use in interrupt routine */
        lea     save_a4(PC), A0
        move.l  A4, (A0)
    }
    my_interrupt_handler = my_timer_code; /* point to my interrupt */

    Xbtimer (0,1,100,my_interrupt_handler);  /* Enable timer A, prescale 4
                                                for 100 counts (100us ?) */ 

    for (i = 0; i < 30000; i++)
        ;

    Xbtimer (0,0,0,0L);
    
    printf ("Timer timed out %ld times.\n", count); 
}