negative number of cycle CPU DSP C6678

hello,

I work with the C6678 DSP using CCS v5.3.0 tool and I want to calculate the number of cycle of function. I normally proceeds as follows:

unsigned long long beg, end, Ncycles;                                            // declaration

_CSL_tscEnable ();                                                                             // Enable the counter

beg _CSL_tscRead = ();                                                                     // Read the first counter value
pdtMat ((float *) A, (float *) B (float *) C, N, K, M);                           // function
_CSL_tscRead end = ();                                                                     // Read the last counter

Ncycles = end-beg;                                                                              // Subtract

The problem now is that if the size of the function arguments is large, the variable "" Ncycles "" takes negative values !!

I would like an explanation or solution for this problem.

Thanks,

Zakaria.