--
-- Rcsid[] = "$Id: alu_8bit.vhd,v 2.1 1993/10/06 01:04:36 alex Exp $";
--

-- This component has two 74181s connected to form an 8-bit
-- ALU.  This component also includes the parity1 generator.

entity ALU_8BIT is
port(
    F: out bit_vector(7 downto 0);
    A_eq_B, X, Y, Z, AC, P, CY: out bit;
    S: in bit_vector(3 downto 0);
    A: in bit_vector(7 downto 0);
    B: in bit_vector(7 downto 0);
    Cn, M: in bit
    );
end;

architecture structure of ALU_8BIT is

component sn54181
port(F3, F2, F1, F0: out bit;
     A_EQ_B, X, Cn4, Y: out bit;
     S3, S2, S1, S0: in bit;
     A3, A2, A1, A0: in bit;
     B3, B2, B1, B0: in bit;
     M, Cn: in bit);
end component;

component parity1
port(A, B, C, D, E, F, G, H: in bit;
     EVEN : out bit);
end component;

component buf8
port(BUF_OUT: out bit_vector(7 downto 0);
	 BUF_IN:  in  bit_vector(7 downto 0));
end component;

signal net1, net2, net3, net4, net5, net6: bit;
signal F_buf: bit_vector(7 downto 0);

begin

-- components U1 thru U4 are used to make the connection between the
-- two 4-bit ALUs used as subcomponents (for purposes of carry)
U0 : inv_gate generic map(2,2) port map(AC,net6);
U1 : inv_gate generic map(2,2) port map(net1,Cn);
U2 : and_gate generic map(2,2) port map(net2,net1,net5);
U3 : and_gate generic map(2,2) port map(net3,net4,net5);
U4 : nor_gate generic map(2,2) port map(net6,net2,net3);

-- U5 and U6 are the 4-bit ALUs used to create an 8-bit system
U5 : sn54181 port map(F_buf(3 downto 0),OPEN,net4,OPEN,net5,
                      S(3 downto 0),A(3 downto 0),B(3 downto 0),M,Cn);
U6 : sn54181 port map(F_buf(7 downto 4),A_eq_B,X,CY,Y,
                      S(3 downto 0),A(7 downto 4),B(7 downto 4),M,net6);
U7 : buf8 port map(F(7 downto 0), F_buf(7 downto 0));

-- ZEROCK is the input to the zero flag of the flag register
ZEROCK : nor_gate generic map(2,2) port map(Z,F_buf(7 downto 0));
-- PARITYCK is the input to the parity1 bit of the flag register
PARITYCK : parity1 port map(F_buf(7 downto 0),P);
-- F7 will be used to set the sign.
end structure;

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