--算邏運算單元：實現的功能有+、-、+1、-1、與、或、非、異或、六種移位運算以及算術比較運算。其
--中定義了臨時變量c_tmp,z_tmp,z1_tmp分別用來表示C,Z符號位，最后賦值輸出。
Library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_arith.all;
use ieee.std_logic_unsigned.all;

entity alu is
	port(
		 reset,clk :	in std_logic;	--時鐘信號，復位信號
		 OP:		    in std_logic_vector(3 downto 0);--操作碼
		 sel:	        in std_logic;--寄存器選擇
		 write:			in std_logic;--寫入寄存器
		 C,Z:			out std_logic;--標志位
		 dinput:        in std_logic_vector(15 downto 0);--運算器的輸入
		 result:        out std_logic_vector(15 downto 0)--運算器的輸出
		);
end alu;

architecture behav of alu is
signal c_tmp :std_logic;		
signal z_tmp :std_logic;		--記錄其它運算的臨時信號
signal z1_tmp :std_logic;		--用于比較時的臨時信號
signal A,B :std_logic_vector(15 downto 0);--定義運算器中的兩個臨時寄存器分別用來存放左右操作數
signal result_t: std_logic_vector(16 downto 0);--中間結果
begin
alu_proc:process(OP,dinput,reset,clk,sel,write)
begin
	if reset='0' then			--復位時，結果清零
		result_t<="00000000000000000";
	elsif (clk'event and clk='1') then 
		if write='1' then
			if sel='0' then
			    A<=dinput;	--時鐘上升沿打入第一個寄存器
			elsif sel='1' then--打入第二個寄存器
				B<=dinput;
			end if;
		elsif write='0' then --當寫信號為低電平時則進行運算	
	    case OP is			--十六種操作
		when "0000" =>
			result_t <= ('0' & A) + ('0' & B);	--加
		when "0001" =>
			result_t <= ('0' & A) +  '1';    --+1
 	    when "0010" =>
			result_t <= ('0' & A) - ('0' & B);  --減
		when "0011" =>
			result_t <= ('0' & A) -  '1';		---1
		when "0100" =>
			result_t <= ('0' & A) and ('0' & B);--與運算
		when "0101" =>
			result_t <= ('0' & A) or ('0' & B); --或運算
		when "0110" =>
			result_t <= not ('0' & B);          --非運算
		when "0111" =>
			result_t <= ('0' & A) xor('0' & B );--異或運算
		when "1000" =>
			result_t <= A(15 downto 0) & A(0);  --算術左移一位
		when "1001" =>
			result_t <= A(15 downto 0) & '0';   --邏輯左移一位
		when "1010" =>
			result_t <= A(15 downto 0) & A(15); --循環左移一位
		when "1011" =>
			result_t <='0'&B;			        --輸出左操作數
		when "1100" =>
			result_t <= "00" & A(15 downto 1);  --邏輯右移一位
			c_tmp <= A(0);
		when "1101" =>
			result_t <= '0' & A(0) & A(15 downto 1);--循環右移一位
			c_tmp <= A(0);
		when "1110" =>
			result_t <= '0' & A(15)& A(15 downto 1);
			c_tmp <= A(0);							--算術右移一位
		when "1111" =>	
			result_t <=	('0' & A) - ('0' & B);
			if (result_t(16)= '1') then		--通過最高位是否溢出來判斷符號位
				c_tmp<='1'; z1_tmp<='0';
			elsif (result_t = "00000000000000000") then
				c_tmp<='0'; z1_tmp<='1';
			else 
				c_tmp<='0'; z1_tmp<='0';
			end if;									--比較運算
		when others => 
			result_t <="XXXXXXXXXXXXXXXXX";
		end case;
		end if;
	end if;
end process;

result <= result_t(15 downto 0);
z_tmp <= (not result_t(15)) and (not result_t(14)) and
		 (not result_t(13)) and (not result_t(12)) and
		 (not result_t(11)) and (not result_t(10)) and
		 (not result_t(9)) and (not result_t(8)) and
		 (not result_t(7)) and (not result_t(6)) and
		 (not result_t(5)) and (not result_t(4)) and
		 (not result_t(3)) and (not result_t(2)) and
		 (not result_t(1)) and (not result_t(0));		--結果是否為全零

c_proc: Process(reset,clk,result_t,OP)
begin
    if reset = '0' then
		C <= '0';
	elsif clk'event and clk = '1' then
	    if (op(3 downto 2)/="11" and op(3 downto 0)/="1011") then  --移位和傳送操作不影響C標志位
			C <= result_t(16);
		else 
			C <= c_tmp;
		end if;
	end if;
end process;

z_proc: process(reset,clk,z_tmp,OP)
begin
   if reset = '0' then
		Z <= '0';
	elsif clk'event and clk = '1' then
		if(OP (3 downto 0)/="1011")then
	    	if(OP (3 downto 2)/="11" )then      --移位和傳送操作不影響Z標志位
				Z <= z_tmp;
			else
				Z <= z1_tmp;
			end if;
        end if;
	end if;
end process;

end behav;