地址译码器.vhd

含有各类寄存器

-- Address decoder 
-- asbar must be '0' to enable any output
-- csbar(0) : X"00000" to X"01FFF"
-- csbar(1) : X"40000" to X"43FFF"
-- csbar(2) : X"08000" to X"0AFFF"
-- csbar(3) : X"E0000" to X"E01FF"

library ieee;
use ieee.std_logic_1164.all;
entity addrdec is
      port(
      asbar : in std_logic; 
      address : in std_logic_vector(19 downto 0);
      csbar : out std_logic_vector(3 downto 0)
      );
end entity addrdec;


architecture v1 of addrdec is
begin

      csbar(0) <= '0' when 
              ((asbar  = '0') and 
              ((address >=  X"00000") and (address <=  X"01FFF"))) 
              else '1';
                                                              
      csbar(1) <= '0' when 
              ((asbar  = '0') and 
              ((address >=  X"40000") and (address <=  X"43FFF"))) 
              else '1';
                                                              
      csbar(2) <= '0' when 
              ((asbar  = '0') and 
              ((address >=  X"08000") and (address <=  X"0AFFF"))) 
              else '1';
                                                              
      csbar(3) <= '0' when 
              ((asbar  = '0') and 
              ((address >=  X"E0000") and (address <=  X"E01FF"))) 
              else '1';               
                                                                                                                                                                              
end architecture v1;