If you wish to construct a low pass filter or high pass filter based upon a comb structure - this is very easy. Use RAM as fifo, make sure you physically clock zeros through ram at reset. Then simply subtract last FIFO sample from first FIFO sample. This will generate a simple comb filter which is good for decimation and interpolation. Rotate taps (can yu guess simple way how) degrees to make highpass. One can use two or more banks to make more sophisticated bpf or notch structures. Good way to go high speed decimation. Equivalent to simple CIC structure (with number of taps in comb equivalent to decimation rate in single stage CIC), but without the issues of infinite gain at DC or Nyquist.