Branch_Detector.vhd

LIBRARY IEEE;
USE IEEE.STD_LOGIC_1164.ALL;
USE IEEE.numeric_std.all;
USE IEEE.STD_LOGIC_SIGNED.all;

-- FLAGS
-- Z-N-C
-- also output old pc if stall.
ENTITY BRANCH_DETECTOR IS
 PORT (	BRANCH_DETECTED 		: in std_logic;
 		DH_STALL 				: in std_logic;
		FLAGS_REG_OUTPUT 		: in std_logic_vector(3 downto 0);
 		OPCODE                  : in std_logic_vector(4 downto 0);
 		RDST_VALUE				: in std_logic_vector(15 downto 0);				
 		
 		FLAGS_REG_INPUT			: out std_logic_vector(3 downto 0);
 		BRANCH_TAKEN 			: out std_logic;
 		FLAGS_REG_WRITE     	: out std_logic;
 		DECODING_STAGE_OUTPUT 	: out std_logic_vector(8 downto 0)
       );
END ENTITY BRANCH_DETECTOR;

ARCHITECTURE ARCH OF BRANCH_DETECTOR IS  
	
	-- Opcodes constants.
	CONSTANT CONST_OPCODE_JMP 	: std_logic_vector(4 downto 0):="10100";
	CONSTANT CONST_OPCODE_JZ 	: std_logic_vector(4 downto 0):="10111";
	CONSTANT CONST_OPCODE_JN 	: std_logic_vector(4 downto 0):="11000";
	CONSTANT CONST_OPCODE_JC 	: std_logic_vector(4 downto 0):="11001";

  BEGIN

  -- Input to fetch stage.
  DECODING_STAGE_OUTPUT <= RDST_VALUE(8 downto 0);
  -- Z-N-C
  PROCESS(BRANCH_DETECTED, DH_STALL, FLAGS_REG_OUTPUT, OPCODE, RDST_VALUE)
  BEGIN 
  	
  	-- IF no stall.
  	IF DH_STALL = '0' THEN
	  	--If branch is detected.
	  	IF BRANCH_DETECTED = '1' THEN
	  		
	  		-- Check if JMP
	  		iF OPCODE = CONST_OPCODE_JMP THEN
	  			BRANCH_TAKEN <= '1';
	  			FLAGS_REG_WRITE <= '0';
	  			FLAGS_REG_INPUT <=	FLAGS_REG_OUTPUT;
		  	
		  	-- Check zero flag = 1.
		  	ELSIF OPCODE = CONST_OPCODE_JZ and FLAGS_REG_OUTPUT(0) = '1' THEN
		  		BRANCH_TAKEN <= '1';
		  		FLAGS_REG_WRITE <= '1';
		  		FLAGS_REG_INPUT <= ('0' & FLAGS_REG_OUTPUT(2 downto 0));

	  		-- Check negative flag = 1.
	  		ELSIF OPCODE = CONST_OPCODE_JN and FLAGS_REG_OUTPUT(1) = '1' THEN
	  			BRANCH_TAKEN <= '1';
	  			FLAGS_REG_WRITE <= '1';
	  			FLAGS_REG_INPUT <= (FLAGS_REG_OUTPUT(3) & FLAGS_REG_OUTPUT(2) & '0' & FLAGS_REG_OUTPUT(0));

			-- Check carry flag = 1.
			ELSIF OPCODE = CONST_OPCODE_JC and FLAGS_REG_OUTPUT(2) = '1' THEN
				BRANCH_TAKEN <= '1';
				FLAGS_REG_WRITE <= '1';
				FLAGS_REG_INPUT <= (FLAGS_REG_OUTPUT(3) & '0' & FLAGS_REG_OUTPUT(1 downto 0));
			ELSE 
				BRANCH_TAKEN <='0';
				FLAGS_REG_WRITE <= '0';
				FLAGS_REG_INPUT <= FLAGS_REG_OUTPUT;
		  	END IF;
		  ELSE
  BRANCH_TAKEN <='0';
	FLAGS_REG_WRITE <= '0';
	FLAGS_REG_INPUT <= FLAGS_REG_OUTPUT; 
	  	END IF; 
	ELSE 
	BRANCH_TAKEN <='0';
	FLAGS_REG_WRITE <= '0';
	FLAGS_REG_INPUT <= FLAGS_REG_OUTPUT;

  	END IF;
  END PROCESS;
END ARCH;