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-- ------------------------------------------------------------------------

-- Copyright (C) 2004 Arif Endro Nugroho

-- All rights reserved.

-- 

-- Redistribution and use in source and binary forms, with or without

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-- 1. Redistributions of source code must retain the above copyright

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-- 

-- THIS SOFTWARE IS PROVIDED BY ARIF ENDRO NUGROHO "AS IS" AND ANY EXPRESS

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-- ------------------------------------------------------------------------

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

entity sub_12bit is

   port (

      addend_12bit           : in  bit_vector (11 downto 0);

      subtrahend_12bit       : in  bit_vector (11 downto 0);

      subtractor12_output    : out bit_vector (11 downto 0)

      );

end sub_12bit;

architecture structural of sub_12bit is

   component fulladder

      port (

      addend        : in   bit;

      augend        : in   bit;

      carry_in      : in   bit;

      sum           : out  bit;

      carry         : out  bit

      );

   end component;

-- internal signal

signal c00 : bit;

signal c01 : bit;

signal c02 : bit;

signal c03 : bit;

signal c04 : bit;

signal c05 : bit;

signal c06 : bit;

signal c07 : bit;

signal c08 : bit;

signal c09 : bit;

signal c10 : bit;

signal c11 : bit;

signal c12 : bit;

signal augend_12bit    : bit_vector (11 downto 0);

signal adder12_output  : bit_vector (11 downto 0);

begin

c00               <= '1'; -- add one to get 2's complement

-- first complement

augend_12bit (11) <= not (subtrahend_12bit (11));

augend_12bit (10) <= not (subtrahend_12bit (10));

augend_12bit (09) <= not (subtrahend_12bit (09));

augend_12bit (08) <= not (subtrahend_12bit (08));

augend_12bit (07) <= not (subtrahend_12bit (07));

augend_12bit (06) <= not (subtrahend_12bit (06));

augend_12bit (05) <= not (subtrahend_12bit (05));

augend_12bit (04) <= not (subtrahend_12bit (04));

augend_12bit (03) <= not (subtrahend_12bit (03));

augend_12bit (02) <= not (subtrahend_12bit (02));

augend_12bit (01) <= not (subtrahend_12bit (01));

augend_12bit (00) <= not (subtrahend_12bit (00));

subtractor12_output <= adder12_output;

fa11 : fulladder

   port map (

      addend     => addend_12bit(11),

      augend     => augend_12bit(11),

      carry_in   => c11,

      sum        => adder12_output(11),

      carry      => c12

      );

fa10 : fulladder

   port map (

      addend     => addend_12bit(10),

      augend     => augend_12bit(10),

      carry_in   => c10,

      sum        => adder12_output(10),

      carry      => c11

      );

fa09 : fulladder

   port map (

      addend     => addend_12bit(09),

      augend     => augend_12bit(09),

      carry_in   => c09,

      sum        => adder12_output(09),

      carry      => c10

      );

fa08 : fulladder

   port map (

      addend     => addend_12bit(08),

      augend     => augend_12bit(08),

      carry_in   => c08,

      sum        => adder12_output(08),

      carry      => c09

      );

fa07 : fulladder

   port map (

      addend     => addend_12bit(07),

      augend     => augend_12bit(07),

      carry_in   => c07,

      sum        => adder12_output(07),

      carry      => c08

      );

fa06 : fulladder

   port map (

      addend     => addend_12bit(06),

      augend     => augend_12bit(06),

      carry_in   => c06,

      sum        => adder12_output(06),

      carry      => c07

      );

fa05 : fulladder

   port map (

      addend     => addend_12bit(05),

      augend     => augend_12bit(05),

      carry_in   => c05,

      sum        => adder12_output(05),

      carry      => c06

      );

fa04 : fulladder

   port map (

      addend     => addend_12bit(04),

      augend     => augend_12bit(04),

      carry_in   => c04,

      sum        => adder12_output(04),

      carry      => c05

      );

fa03 : fulladder

   port map (

      addend     => addend_12bit(03),

      augend     => augend_12bit(03),

      carry_in   => c03,

      sum        => adder12_output(03),

      carry      => c04

      );

fa02 : fulladder

   port map (

      addend     => addend_12bit(02),

      augend     => augend_12bit(02),

      carry_in   => c02,

      sum        => adder12_output(02),

      carry      => c03

      );

fa01 : fulladder

   port map (

      addend     => addend_12bit(01),

      augend     => augend_12bit(01),

      carry_in   => c01,

      sum        => adder12_output(01),

      carry      => c02

      );

fa00 : fulladder

   port map (

      addend     => addend_12bit(00),

      augend     => augend_12bit(00),

      carry_in   => c00,

      sum        => adder12_output(00),

      carry      => c01

      );

end structural;