N-bit Vor- Rückwärtszähler in VHDL notieren?

Google: vhdl n bit counter down up

Xilinx ISE Software -> Edit -> Language Templates -> VHDL -> Synthesis 
Constructs -> Coding Examples -> Counters -> Binary -> Up/Down Counters

process (<clock>)
begin
   if <clock>='1' and <clock>'event then
      if <count_direction>='1' then
         <count> <= <count> + 1;
      else
         <count> <= <count> - 1;
      end if;
   end if;
end process;

Ich würde dir auch das "xst.pdf" Dokument von Xilinx empfehlen.