-- (C) 2005 Altera Corporation. All rights reserved. The design examples are 
-- being provided on an "as-is" basis and as an accommodation and therefore 
-- all warranties, representations or guarantees of any kind (whether express,
-- implied or statutory) including, without limitation, warranties of 
-- merchantability, non-infringement, or fitness for a particular purpose, 
-- are specifically disclaimed.

LIBRARY ieee ;
USE ieee.std_logic_1164.all ;

-- Top-level entity
ENTITY addersubtractor2 IS
	GENERIC ( n : INTEGER := 16 ) ;
	PORT (	A, B 	: IN STD_LOGIC_VECTOR(n-1 DOWNTO 0) ;
			Clock, Reset, Sel, AddSub : IN STD_LOGIC ;
			Z 	: BUFFER	STD_LOGIC_VECTOR(n-1 DOWNTO 0) ;
			Overflow: OUT STD_LOGIC ) ;
END addersubtractor2 ;

ARCHITECTURE Behavior OF addersubtractor2 IS
	SIGNAL G, M, Areg, Breg, Zreg : STD_LOGIC_VECTOR(n-1 DOWNTO 0) ;
	SIGNAL SelR, AddSubR, over_flow : STD_LOGIC ;
	COMPONENT mux2to1
		GENERIC ( k : INTEGER := 8 ) ;
		PORT (	V, W 	: IN 	STD_LOGIC_VECTOR(k-1 DOWNTO 0) ;
				Selm 	: IN	STD_LOGIC ;
				F 	: OUT	STD_LOGIC_VECTOR(k-1 DOWNTO 0) ) ;
	END COMPONENT ;
	COMPONENT megaddsub
		PORT ( add_sub: IN  STD_LOGIC ;
			 dataa, datab   : IN  STD_LOGIC_VECTOR(15 DOWNTO 0) ;
			 result 	: OUT STD_LOGIC_VECTOR(15 DOWNTO 0) ;
			 overflow	: OUT STD_LOGIC ) ;
	END COMPONENT ;
BEGIN
-- Define flip-flops and registers
	PROCESS ( Reset, Clock )
	BEGIN
		IF Reset = '1' THEN
			Areg <= (OTHERS => '0'); Breg <= (OTHERS => '0'); 
			Zreg <= (OTHERS => '0'); SelR <= '0'; AddSubR <= '0'; Overflow <= '0';
		ELSIF Clock'EVENT AND Clock = '1' THEN
			Areg <= A; Breg <= B; Zreg <= M;
			SelR <= Sel; AddSubR <= NOT AddSub; Overflow <= over_flow;
		END IF ;
	END PROCESS ;

-- Define combinational logic circuit
	nbit_addsub: megaddsub 
		PORT MAP (AddSubR, G, Breg, M, over_flow);
	multiplexer: mux2to1 
		GENERIC MAP ( k => n )
		PORT MAP ( Areg, Z, SelR, G ) ;
	Z <= Zreg ;
END Behavior;

-- k-bit 2-to-1 multiplexer
LIBRARY ieee ;
USE ieee.std_logic_1164.all ;

ENTITY mux2to1 IS
GENERIC ( k : INTEGER := 8 ) ;
	PORT ( 	V, W 	: IN		STD_LOGIC_VECTOR(k-1 DOWNTO 0) ;
			Selm 	: IN		STD_LOGIC ;
			F 	: OUT 	STD_LOGIC_VECTOR(k-1 DOWNTO 0) ) ;
END mux2to1 ;

ARCHITECTURE Behavior OF mux2to1 IS
BEGIN
	PROCESS ( V, W, Selm )
	BEGIN
		IF Selm = '0' THEN
			F <= V ;
		ELSE
			F <= W ;
		END IF ;
	END PROCESS ;
END Behavior ;

-- 16-bit adder/subtractor LPM created by the MegaWizard
LIBRARY ieee;
USE ieee.std_logic_1164.all;

LIBRARY lpm;
USE lpm.lpm_components.all;

ENTITY megaddsub IS
	PORT
	(
		add_sub		: IN STD_LOGIC ;
		dataa		: IN STD_LOGIC_VECTOR (15 DOWNTO 0);
		datab		: IN STD_LOGIC_VECTOR (15 DOWNTO 0);
		result		: OUT STD_LOGIC_VECTOR (15 DOWNTO 0);
		overflow		: OUT STD_LOGIC 
	);
END megaddsub;

ARCHITECTURE SYN OF megaddsub IS

	SIGNAL sub_wire0	: STD_LOGIC ;
	SIGNAL sub_wire1	: STD_LOGIC_VECTOR (15 DOWNTO 0);

	COMPONENT lpm_add_sub
	GENERIC (
		lpm_width		: NATURAL;
		lpm_direction	: STRING;
		lpm_type		: STRING;
		lpm_hint		: STRING
	);
	PORT (
			dataa	: IN STD_LOGIC_VECTOR (15 DOWNTO 0);
			add_sub	: IN STD_LOGIC ;
			datab	: IN STD_LOGIC_VECTOR (15 DOWNTO 0);
			overflow	: OUT STD_LOGIC ;
			result	: OUT STD_LOGIC_VECTOR (15 DOWNTO 0)
	);
	END COMPONENT;

BEGIN
	overflow    <= sub_wire0;
	result    <= sub_wire1(15 DOWNTO 0);

	lpm_add_sub_component : lpm_add_sub
	GENERIC MAP (
		lpm_width => 16,
		lpm_direction => "UNUSED",
		lpm_type => "LPM_ADD_SUB",
		lpm_hint => "ONE_INPUT_IS_CONSTANT=NO,CIN_USED=NO"
	)
	PORT MAP (
		dataa => dataa,
		add_sub => add_sub,
		datab => datab,
		overflow => sub_wire0,
		result => sub_wire1
	);
END SYN;