The following proggi does the job: #include <xmmintrin.h> #define _MM_DENORM_ZERO_ON 0x0040 main() { // enable flush to zero _mm_setcsr(_MM_FLUSH_ZERO_ON | _MM_MASK_UNDERFLOW | _mm_getcsr()); // enable denormals are zero _mm_setcsr(_MM_DENORM_ZERO_ON | _mm_getcsr()); } Note: you will need compiler option -mfpmath=sse and -march=pentium4 (works for both P4 and Athlon64) After reboot, both FZ and DAZ flags are reset to zero, so you should run this program in your /etc/profile. But as said earlier, works only for applications that use (are compiled to use) the SSE unit instead of the old style X86 FPU.