Programming :: ANSI C Code Seg-faults When Unsigned Int Variable Is Used?

I'm writing a tridiagonal solver for sparse linear systems and I coded this decreasing loop:

PHP Code:

xx[NN - 1] = gamma[NN - 1]*BB[NN - 1];
ii = NN - 2;
/* Compute the x values: */
while (ii >= 0) {
xx[ii] = gamma[ii]*(BB[ii] + xx[ii + 1]);
ii = ii - 1;
} 

now... if ii is declared as an unsigned integer, the code gives me a segmentation fault, but if I declare it as an int, it doesn't. Using DDD I noticed that, as an unsigned int variable, when I finally decrease it, it underflows and blows up to a huge value, ergo giving me an unaccesible direction for xx in the following iteration! I taugh me not to use unsigned int variables for dereasing loops! XD