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-- EMACS settings: -*- tab-width: 2; indent-tabs-mode: t -*-
-- vim: tabstop=2:shiftwidth=2:noexpandtab
-- kate: tab-width 2; replace-tabs off; indent-width 2;
--
-- =============================================================================
-- Description: Prefix AND computation: y(i) <= '1' when x(i downto 0) = (i downto 0 => '1') else '0'
-- This implementation uses carry chains for wider implementations.
--
-- Authors: Thomas B. Preusser
-- =============================================================================
-- Copyright 2007-2014 Technische Universität Dresden - Germany
-- Chair for VLSI-Design, Diagnostics and Architecture
--
-- Licensed under the Apache License, Version 2.0 (the "License");
-- you may not use this file except in compliance with the License.
-- You may obtain a copy of the License at
--
-- http://www.apache.org/licenses/LICENSE-2.0
--
-- Unless required by applicable law or agreed to in writing, software
-- distributed under the License is distributed on an "AS IS" BASIS,
-- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-- See the License for the specific language governing permissions and
-- limitations under the License.
-- =============================================================================
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
library unisim;
entity arith_prefix_and is
generic (
N : positive
);
port (
x : in std_logic_vector(N-1 downto 0);
y : out std_logic_vector(N-1 downto 0)
);
end arith_prefix_and;
architecture rtl of arith_prefix_and is
type T_VENDOR is (VENDOR_XILINX, VENDOR_ALTERA);
constant VENDOR : T_VENDOR := VENDOR_XILINX;
begin
y(0) <= x(0);
gen1: if N > 1 generate
signal p : unsigned(N-1 downto 1);
begin
p(1) <= x(0) and x(1);
gen2: if N > 2 generate
p(N-1 downto 2) <= unsigned(x(N-1 downto 2));
-- Generic Carry Chain through Addition
genGeneric: if VENDOR /= VENDOR_XILINX generate
signal s : std_logic_vector(N downto 1);
begin
s <= std_logic_vector(('0' & p) + 1);
y(N-1 downto 2) <= s(N downto 3) xor ('0' & x(N-1 downto 3));
end generate genGeneric;
-- Direct Carry Chain by MUXCY Instantiation
genXilinx: if VENDOR = VENDOR_XILINX generate
-- component MUXCY
-- port (
-- S : in std_logic;
-- DI : in std_logic;
-- CI : in std_logic;
-- O : out std_logic
-- );
-- end component;
signal c : std_logic_vector(N-1 downto 0);
begin
c(0) <= '1';
genChain: for i in 1 to N-1 generate
mux : entity unisim.MUXCY
port map (
S => p(i),
DI => '0',
CI => c(i-1),
O => c(i)
);
end generate genChain;
y(N-1 downto 2) <= c(N-1 downto 2);
end generate genXilinx;
end generate gen2;
y(1) <= p(1);
end generate gen1;
end rtl;
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