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+
+-- Copyright (C) 1996 Morgan Kaufmann Publishers, Inc
+
+-- This file is part of VESTs (Vhdl tESTs).
+
+-- VESTs is free software; you can redistribute it and/or modify it
+-- under the terms of the GNU General Public License as published by the
+-- Free Software Foundation; either version 2 of the License, or (at
+-- your option) any later version.
+
+-- VESTs is distributed in the hope that it will be useful, but WITHOUT
+-- ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+-- FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
+-- for more details.
+
+-- You should have received a copy of the GNU General Public License
+-- along with VESTs; if not, write to the Free Software Foundation,
+-- Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+
+-- ---------------------------------------------------------------------
+--
+-- $Id: ch_06_pas-b.vhd,v 1.2 2001-10-26 16:29:34 paw Exp $
+-- $Revision: 1.2 $
+--
+-- ---------------------------------------------------------------------
+
+architecture behavioral of product_adder_subtracter is
+begin
+
+ behavior : process (a, b) is
+
+ constant Tpd_in_out : time := 3 ns;
+ variable op2 : std_ulogic_vector(b'range);
+ variable carry_in : std_ulogic;
+ variable carry_out : std_ulogic;
+
+ begin
+ carry_out := To_X01(mode);
+ if To_X01(mode) = '1' then
+ op2 := not b;
+ else
+ op2 := b;
+ end if;
+ for index in 0 to 31 loop
+ carry_in := carry_out; -- of previous bit
+ s(index) <= a(index) xor op2(index) xor carry_in after Tpd_in_out;
+ carry_out := (a(index) and op2(index))
+ or (carry_in and (a(index) xor op2(index)));
+ end loop;
+ s(32) <= a(31) xor op2(31) xor carry_out after Tpd_in_out;
+ end process behavior;
+
+end architecture behavioral;