help! why?

my codes:

unsigned int i=0;
void Timer(void) interrupt 1
    {
    i++;
    }

void Main (void)
    {
    while (1) printf ("i=%d\n", i);

    }

I had thought that I will get below on serial window

"
i=0;
i=1;
i=2;
i=3;
...
"


but I got

"
i=0
i=-9472
i=-28416
i=29184
i=10240
i=-8960
i=-27904
i=29696
i=10752
i=-8192
i=-27392
i=30464
i=11264
i=-7680
......
"


these values are disorderly and unsystematic, why? You had better try it on your own keilc.