RickySu.Work

今天看ac97控制器的hdl，发现用natural类型的subtype在一个逻辑相量中寻3bit出来作地址译码，语法诡异，编了一个简单程序，验证果然如此，每见那本书上是这么用的。

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library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;


entity endian_2 is

end endian_2;

architecture Behavioral of endian_2 is

constant C_OPB_AWIDTH : integer := 32;
signal temp1 : std_logic_vector(31 downto 0) := x"9999_9999"; --1001,1001,...
signal temp2  : std_logic_vector(0 to 31) := x"5555_5555";  --...,0101,0101
signal result1 : std_logic_vector(2 downto 0);
signal result2 : std_logic_vector(2 downto 0);

subtype  ADDR_CHK1 is natural range C_OPB_AWIDTH-3 downto C_OPB_AWIDTH-5;  --29,28,27 bit
subtype  ADDR_CHK2 is natural range C_OPB_AWIDTH-5 to C_OPB_AWIDTH-3;  --27,28,29
bit

begin

  result1 <= temp1(ADDR_CHK1);--should be 01,1
  result2 <= temp2(ADDR_CHK2);--should be 1,01

end Behavioral;