--the mem_string
LIBRARY ieee;
  USE ieee.std_logic_1164.ALL;
  USE ieee.std_logic_arith.all;
  --unsigned.ALL;
  USE work.pkg_types.ALL;
  USE work.pkg_components.ALL;

ENTITY mem_string IS
PORT ( clk        :   IN  bit1 ;
       reset      :   IN  bit1 ;
       sel_read   :   IN  bit1 ;
       s_sel      :   IN  bit1 ;
       burst      :   IN  bit2_r ;
       s_req      :   OUT  bit1 ;
       s_ack      :   IN  bit1 ;
       s_valid    :   IN  bit1 ;
       data_in    :   IN  bit8 ;
       s_ad       :   IN  bit5 ;
       s          :   out  bit8 ;
       s_done     :   out  bit1 );
END mem_string;

ARCHITECTURE behavior OF mem_string IS
SIGNAL  a1,addr       :  bit5_r;
SIGNAL  sel_write     :  bit1;
BEGIN
 PROCESS
  --地址初始化
  PROCEDURE init (burst : IN  bit2_r;SIGNAL a1 : OUT bit5_r) IS
  BEGIN 
     addr <= burst & "000";
     a1   <= burst & "000";
  END init;
  --寫RAM，即更新相應(yīng)地址信號(hào)
  PROCEDURE write_ram ( sel_write : IN bit1; data : IN bit8) IS
  BEGIN
     a1   <= addr;
  END write_ram;

  BEGIN
   --初始化信號(hào)值
   s_req <= '0';
   s_done <= '1';
   addr <= "00000";
   sel_write <= '0';

   --直到mem_string為工作狀態(tài)且被選中，當(dāng)時(shí)鐘上升沿到來時(shí)，
   WAIT  UNTIL (s_sel = '1' AND rising_edge(clk) AND reset='1');
   
   --找到相應(yīng)burst的開始位置，并且置mem_string為寫入狀態(tài)
   init(burst,addr);
   s_done <= '0';
   s_req <= '1';
   
   WAIT UNTIL ((s_ack='1' AND rising_edge(CLK)) OR reset/='1');
   --得到應(yīng)答信號(hào)s_ack后，撤消s_req信號(hào)
   s_req <= '0';
   
   write_loop  : LOOP
       --等到addr有效且地址信號(hào)不為邊界
       --因?yàn)橐粋€(gè)burst設(shè)定為8個(gè)字符
       IF s_valid/='1' AND addr/=7 AND addr/=15 AND addr<23 THEN
              WAIT UNTIL ((s_valid='1' AND rising_edge(clk)) OR reset/='1');
              --復(fù)位則退出
              IF reset/='1' THEN 
                   EXIT write_loop;
              END IF;
       END IF;
       --寫入8個(gè)字符，一個(gè)字符為8位
       addr <= addr +1;
       write_ram(sel_write,data_in);
       --寫入完成后，寫信號(hào)置位防止寫入不正確的數(shù)據(jù)
       sel_write <= '1';
       --當(dāng)?shù)刂愤_(dá)到邊界或mem_string復(fù)位時(shí)退出
       IF addr=7 OR addr=15 OR addr>=23 OR reset/='1' THEN
              WAIT UNTIL rising_edge(clk);
              EXIT write_loop;
       END IF;

       WAIT UNTIL rising_edge(clk);
       sel_write <='0';
   END LOOP write_loop;
  END PROCESS;
 
  string_mem :mem_24x8
  PORT MAP (s,data_in,s_ad,a1,sel_read,sel_write,clk);

END behavior;