rdusedw not count in my dcfifo

Your FIFO is 256 deep. The used signal is 8 bits wide, so when the FIFO is full the used amount is 256 (1,0000,0000 in binary). Since you only have 8 bits for the used signal the 100000000 turns into 00000000 due to truncation. To get the "true" used amount you should concatenate the full and used signals together. i.e. something like this: assign true_used = {fifo_full, fifo_used}; // fifo_used width + 1