01.txt

,2006altera大赛－基于软核Nios的宽谱正弦信号发生器设计：摘要：本设计运用了基于 Nios II 嵌入式处理器的 SOPC 技

library ieee;
use ieee.std_logic_1164.all;
entity 74_sl_165 is 
port ( clk,c0: in std_logic;
           md: in std_logic_vector(2 downto 0);
           d : in std_logic_vector(7 downto 0);
           qb: out std_logic_vector(7 downto 0);
           cn: out std_logic);
end entity;
architecture behav of 74_sl_165 is
signal reg: std_logic_vector(7 downto 0);
signal cy : std_logic;
begin
  process( clk,md,c0)
    begin
      if clk'event and clk='1' then
	case md is
	when "001"=> reg(0)<=c0;
   		reg(7 downto 1) <=reg (6 downto 0); cy<=reg(7);
	when "010"=>reg(0)<=reg(7);
		reg(7 downto 1)<=reg (6 downto 0);
	when "011"=>reg(7)<=reg(0);
		reg(6 downto 0)<= reg(7 downto 1);
	when "100"=>reg(7)<=c0;
		reg(6 downto 0)<=reg(7 downto 1);cy<=reg(0);
	when "101"=>reg(7 downto 0)<= d(7 downto 0);
	when others=> reg<=reg; cy<=cy;
end case;
end if;	
end process;
	qb(7 downto 0)<=reg(7 downto 0); cn<=cy;
end behav;
几点不明白的地方：

1.MD, D,CN这三个端口作用，如果MD是移位控制字那怎么是3位的，不都是2位吗。

2.cy<=reg(7)和cy<=reg(0)这两句的作用是什么呀？

3.这是个什么样寄存器，感觉和4个D触发器组成的寄存器不太一样。