-- (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 module
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, H, M, Areg, Breg, Zreg, AddSubR_n : STD_LOGIC_VECTOR(n-1 DOWNTO 0) ;
	SIGNAL SelR, AddSubR, carryout, 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 adderk
		GENERIC ( k : INTEGER := 8 ) ;
		PORT ( carryin: IN  STD_LOGIC ;
			 X, Y   : IN  STD_LOGIC_VECTOR(k-1 DOWNTO 0) ;
			 S 	  : OUT STD_LOGIC_VECTOR(k-1 DOWNTO 0) ;
			 carryout : 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 <= AddSub; Overflow <= over_flow;
		END IF ;
	END PROCESS ;

-- Define combinational logic circuit
	nbit_adder: adderk
		GENERIC MAP ( k => n )
		PORT MAP ( AddSubR, G, H, M, carryout ) ;
	multiplexer: mux2to1 
		GENERIC MAP ( k => n )
		PORT MAP ( Areg, Z, SelR, G ) ;
	AddSubR_n <= (OTHERS => AddSubR) ;
	H <= Breg XOR AddSubR_n ;
	over_flow <= carryout XOR G(n-1) XOR H(n-1) XOR M(n-1) ;
	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 ;

-- k-bit adder
LIBRARY ieee ;
USE ieee.std_logic_1164.all ;
USE ieee.std_logic_signed.all ;

ENTITY adderk IS
	GENERIC ( k : INTEGER := 8 ) ;
	PORT ( 	carryin : IN 	STD_LOGIC ;
			X, Y 	: IN 	STD_LOGIC_VECTOR(k-1 DOWNTO 0) ;
			S 	: OUT 	STD_LOGIC_VECTOR(k-1 DOWNTO 0) ;
			carryout 	: OUT		STD_LOGIC ) ;
END adderk ;

ARCHITECTURE Behavior OF adderk IS
	SIGNAL Sum : STD_LOGIC_VECTOR(k DOWNTO 0) ;
BEGIN
	Sum <= ('0' & X) + ('0' & Y) + carryin ;
	S <= Sum(k-1 DOWNTO 0) ;
	carryout <= Sum(k) ;
END Behavior ;