-- Behavioural model of a 16-word, 8-bit Random Access Memory
-- download from: www.fpga.com.cn & www.pld.com.cn

LIBRARY ieee;
USE ieee.std_logic_1164.ALL;

ENTITY ram16x8 IS
        PORT(address : IN STD_LOGIC_VECTOR(3 DOWNTO 0);
             csbar, oebar, webar : IN STD_LOGIC;
             data : INOUT STD_LOGIC_VECTOR(7 DOWNTO 0));
END ram16x8;

ARCHITECTURE version1 OF ram16x8 IS
        
BEGIN

PROCESS(address, csbar, oebar, webar, data)

        TYPE ram_array IS ARRAY (0 TO 15) OF BIT_VECTOR(7 DOWNTO 0);
        VARIABLE index : INTEGER := 0;
        VARIABLE ram_store : ram_array;
BEGIN

      IF csbar = '0' THEN

        --calculate address as an integer
                index := 0;
                FOR i IN address'RANGE LOOP
                        IF address(i) = '1' THEN
                        index := index + 2**i;
                        END IF;
                END LOOP;

         IF rising_edge(webar) THEN
            --write to ram on rising edge of write pulse
            ram_store(index) := To_bitvector(data);
         ELSIF oebar = '0' THEN
            data <= To_StdlogicVector(ram_store(index));
         ELSE
            data <= "ZZZZZZZZ";
         END IF;            
      ELSE
         data <= "ZZZZZZZZ";
      END IF;

   END PROCESS;
                
END version1;