Carry ripple adder

Figure below shows the 8-bit unsigned carry ripple adder. The detailed diagram shows the inputs and outputs signals of the circuit. The a and b are the input vectors to be added, cin is the carry-in bit, s is the sum vector, and cout is the carry-out bit.

LIBRARY ieee;
	USE ieee.std_logic_1164.all;
	ENTITY adder IS
    	  GENERIC (length : INTEGER := 8);
      	  PORT ( a, b: IN STD_LOGIC_VECTOR (length-1 DOWNTO 0);
                cin: IN STD_LOGIC;
                s: OUT STD_LOGIC_VECTOR (length-1 DOWNTO 0);
               cout: OUT STD_LOGIC);
 	END adder;
 	ARCHITECTURE adder OF adder IS
	BEGIN
   		PROCESS (a, b, cin)
      	VARIABLE carry : STD_LOGIC_VECTOR (length DOWNTO 0);
    	BEGIN
    		carry(0) := cin;
      	FOR i IN 0 TO length-1 LOOP
     		s(i) <= a(i) XOR b(i) XOR carry(i);
      	carry(i+1) := (a(i) AND b(i)) OR (a(i) AND
      	carry(i)) OR (b(i) AND carry(i));
      	END LOOP;
     	cout <= carry(length);
    	END PROCESS;
   END adder;


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