Input/Output assignments for external memory

Obviously, the data pins must have inout type and your design has to perform some multiplexing as in the example below (used for a static RAM):

IF Wave_RD = '1' THEN
  RAM_nWEi <= '1';
  RAM_Data <= "ZZZZZZZZZZZZZZZZ";
  RAM_Adr <= Wave_Adr;
  RAM_nOE <= '0';		
  RAM_nCE <= '0';
  RAM_nLB <= '0';
  RAM_nUB <= '0';
ELSIF Osc_WR = '1' THEN
  RAM_nCE <= '0';
  RAM_nOE <= '1';		
  RAM_nWEi <= '0';
  RAM_nLB <= '0';
  RAM_nUB <= '0';
  RAM_Adr <= Osc_Adr;
  RAM_Data <= Osc_Data;
ELSE 
  RAM_nCE <= '1';
  RAM_nOE <= '1';		
  RAM_nWEi <= '1';
  RAM_Data <= "ZZZZZZZZZZZZZZZZ";
END IF;