VHDL: Latch being created when using input ports - warnings

Question

So I'm working on synthesizing a CPU.
These are the errors:
WARNING:Xst:1710 - FF/Latch <EXS/mem_address_0> (without init value) has a constant value of 0 in block <Top_level_component>. This FF/Latch will be trimmed during the optimization process.
WARNING:Xst:1895 - Due to other FF/Latch trimming, FF/Latch <EXS/mem_address_1> (without init value) has a constant value of 0 in block <Top_level_component>. This FF/Latch will be trimmed during the optimization process.
WARNING:Xst:1895 - Due to other FF/Latch trimming, FF/Latch <EXS/mem_address_2> (without init value) has a constant value of 0 in block <Top_level_component>. This FF/Latch will be trimmed during the optimization process.
WARNING:Xst:1895 - Due to other FF/Latch trimming, FF/Latch <EXS/mem_address_3> (without init value) has a constant value of 0 in block <Top_level_component>. This FF/Latch will be trimmed during the optimization process.
It then cascades onto a bunch of other stuff from here.
This is the part component that is giving me an error:
library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_arith.all;
use ieee.std_logic_unsigned.all;
entity EX_stage is
Port( clk : in STD_LOGIC;
rst : in STD_LOGIC;
mem_opr_in : in STD_LOGIC_VECTOR (1 downto 0);
wb_opr_in : in STD_LOGIC;
out_opr_in : in STD_LOGIC;
alu_mode : in STD_LOGIC_VECTOR (3 downto 0);
in1 : in STD_LOGIC_VECTOR (7 downto 0);
in2 : in STD_LOGIC_VECTOR (7 downto 0);
ra_in : in STD_LOGIC_VECTOR (1 downto 0);
-- The Ports I'm reading from
FW_opcode : in STD_LOGIC_VECTOR (3 downto 0);
FW_ra_IN : in STD_LOGIC_VECTOR (1 downto 0);
FW_rb_IN : in STD_LOGIC_VECTOR (1 downto 0);
-- The port I'm writing to
mem_address : out STD_LOGIC_VECTOR (7 downto 0);
mem_opr_out : out STD_LOGIC_VECTOR (1 downto 0);
wb_opr_out : out STD_LOGIC;
out_opr_out : out STD_LOGIC;
alu_result : out STD_LOGIC_VECTOR (7 downto 0);
alu_mode_out : out STD_LOGIC_VECTOR (3 downto 0);
ra_out : out STD_LOGIC_VECTOR (1 downto 0);
z_flag : out STD_LOGIC;
n_flag : out STD_LOGIC);
end EX_stage;
architecture Behavioral of EX_stage is
begin
process(clk,rst)
variable temp_result: STD_LOGIC_VECTOR (7 downto 0);
begin
-- Not important... I think
end process;
process(clk,rst)
begin
if rst = '1' then
mem_address <= (others => '0');
elsif clk = '1' and clk'event then
mem_address <= FW_opcode & FW_ra_IN & FW_rb_IN;
-- If this is <= x"FF"; then it doesn't give the error
end if;
end process;
end Behavioral;
In the top level component I have:
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;
entity Top_level_component is
Port ( portIN : in STD_LOGIC_VECTOR (7 downto 0);
portOUT : out STD_LOGIC_VECTOR (7 downto 0);
clk : in STD_LOGIC;
rst : in STD_LOGIC);
end Top_level_component;
architecture Behavioral of Top_level_component is
-- Only the offending components shown
component IF_stage
port( clk : in STD_LOGIC;
rst : in STD_LOGIC;
count : in STD_LOGIC_VECTOR (7 downto 0);
-- All three of these get input to ID_stage AND EX_stage
opcode : out STD_LOGIC_VECTOR (3 downto 0);
reg_a : out STD_LOGIC_VECTOR (1 downto 0);
reg_b : out STD_LOGIC_VECTOR (1 downto 0));
end component;
component ID_stage is
port( clk : in STD_LOGIC;
port_IN : in STD_LOGIC_VECTOR (7 downto 0);
opcode : in STD_LOGIC_VECTOR (3 downto 0);
ra_IN : in STD_LOGIC_VECTOR (1 downto 0);
rb_IN : in STD_LOGIC_VECTOR (1 downto 0);
zflag : in STD_LOGIC;
nflag : in STD_LOGIC;
RD1_IN : in STD_LOGIC_VECTOR (7 downto 0);
RD2_IN : in STD_LOGIC_VECTOR (7 downto 0);
count : in STD_LOGIC_VECTOR (7 downto 0);
previous_opcode : in STD_LOGIC_VECTOR (3 downto 0);
MEM_opr : out STD_LOGIC_VECTOR (1 downto 0);
WB_opr : out STD_LOGIC;
OUT_opr : out STD_LOGIC;
ALU_mode : out STD_LOGIC_VECTOR (3 downto 0);
RD1_OUT : out STD_LOGIC_VECTOR (7 downto 0);
RD2_OUT : out STD_LOGIC_VECTOR (7 downto 0);
ra_OUT : out STD_LOGIC_VECTOR (1 downto 0);
rb_OUT : out STD_LOGIC_VECTOR (1 downto 0);
new_count : out STD_LOGIC_VECTOR (7 downto 0);
set_pc : out STD_LOGIC);
end component;
component EX_stage is
port( clk : in STD_LOGIC;
rst : in STD_LOGIC;
mem_opr_in : in STD_LOGIC_VECTOR (1 downto 0);
wb_opr_in : in STD_LOGIC;
out_opr_in : in STD_LOGIC;
alu_mode : in STD_LOGIC_VECTOR (3 downto 0);
in1 : in STD_LOGIC_VECTOR (7 downto 0);
in2 : in STD_LOGIC_VECTOR (7 downto 0);
ra_in : in STD_LOGIC_VECTOR (1 downto 0);
-- The Offending ports: This is where the signals go in
FW_opcode : in STD_LOGIC_VECTOR (3 downto 0);
FW_ra_IN : in STD_LOGIC_VECTOR (1 downto 0);
FW_rb_IN : in STD_LOGIC_VECTOR (1 downto 0);
mem_address : out STD_LOGIC_VECTOR (7 downto 0);
mem_opr_out : out STD_LOGIC_VECTOR (1 downto 0);
wb_opr_out : out STD_LOGIC;
out_opr_out : out STD_LOGIC;
alu_result : out STD_LOGIC_VECTOR (7 downto 0);
alu_mode_out : out STD_LOGIC_VECTOR (3 downto 0);
ra_out : out STD_LOGIC_VECTOR (1 downto 0);
z_flag : out STD_LOGIC;
n_flag : out STD_LOGIC);
end component;
-- Signals used
signal set_pc : STD_LOGIC := '0';
signal new_count : STD_LOGIC_VECTOR (7 downto 0) := (others => '0');
signal count : STD_LOGIC_VECTOR (7 downto 0) := (others => '0');
signal IF_opcode : STD_LOGIC_VECTOR (3 downto 0) := (others => '0');
signal IF_reg_a : STD_LOGIC_VECTOR (1 downto 0) := (others => '0');
signal IF_reg_b : STD_LOGIC_VECTOR (1 downto 0) := (others => '0');
signal rd_data1 : std_logic_vector(7 downto 0) := (others => '0');
signal rd_data2 : std_logic_vector(7 downto 0) := (others => '0');
signal ID_opcode : STD_LOGIC_VECTOR (3 downto 0) := (others => '0');
signal ID_reg_a : STD_LOGIC_VECTOR (1 downto 0) := (others => '0');
signal ID_reg_b : STD_LOGIC_VECTOR (1 downto 0) := (others => '0');
signal ID_data1 : std_logic_vector(7 downto 0) := (others => '0');
signal ID_data2 : std_logic_vector(7 downto 0) := (others => '0');
signal ID_mem_opr : STD_LOGIC_VECTOR (1 downto 0) := (others => '0');
signal ID_wb_opr : STD_LOGIC := '0';
signal ID_out_opr : STD_LOGIC := '0';
signal EX_data1 : std_logic_vector(7 downto 0) := (others => '0');
signal EX_data2 : std_logic_vector(7 downto 0) := (others => '0');
signal EX_reg_a : STD_LOGIC_VECTOR (1 downto 0) := (others => '0');
signal EX_results : STD_LOGIC_VECTOR (7 downto 0) := (others => '0');
signal EX_alu_mode : STD_LOGIC_VECTOR (3 downto 0) := (others => '0');
signal mem_address : STD_LOGIC_VECTOR (7 downto 0) := (others => '0');
signal EX_mem_opr : STD_LOGIC_VECTOR (1 downto 0) := (others => '0');
signal EX_wb_opr : STD_LOGIC := '0';
signal EX_out_opr : STD_LOGIC := '0';
signal z_flag : std_logic := '0';
signal n_flag : std_logic := '0';
begin
IFS: IF_stage PORT MAP(
clk => clk,
rst => rst,
count => count,
opcode => IF_opcode,
reg_a => IF_reg_a,
reg_b => IF_reg_b);
IDS: ID_stage port map(
clk => clk,
port_IN => portIN,
opcode => IF_opcode,
ra_IN => IF_reg_a,
rb_IN => IF_reg_b,
zflag => z_flag,
nflag => n_flag,
RD1_IN => rd_data1,
RD2_IN => rd_data2,
count => count,
previous_opcode => EX_alu_mode,
MEM_opr => ID_mem_opr,
WB_opr => ID_wb_opr,
OUT_opr => ID_out_opr,
ALU_mode => ID_opcode,
RD1_OUT => ID_data1,
RD2_OUT => ID_data2,
ra_OUT => ID_reg_a,
rb_OUT => ID_reg_b,
new_count => new_count,
set_pc => set_pc);
EXS: EX_stage port map(
clk => clk,
rst => rst,
mem_opr_in => ID_mem_opr,
wb_opr_in => ID_wb_opr,
out_opr_in => ID_out_opr,
alu_mode => ID_opcode,
in1 => EX_data1,
in2 => EX_data2,
ra_in => ID_reg_a,
FW_opcode => IF_opcode,
FW_ra_IN => IF_reg_a,
FW_rb_IN => IF_reg_b,
mem_address => mem_address,
mem_opr_out => EX_mem_opr,
wb_opr_out => EX_wb_opr,
out_opr_out => EX_out_opr,
alu_result => EX_results,
alu_mode_out => EX_alu_mode,
ra_out => EX_reg_a,
z_flag => z_flag,
n_flag => n_flag);
end Behavioral;
So in the EX_stage, when using FW_opcode, FW_ra_IN, or FW_rb_IN, I get the errors. I Don't get it. Any Ideas? Is it that I'm trying to merge the inputs into an output?

Answer 1

That is not an error. It is a warning saying that the synthesizer has detected that the signal will always be 0, and that it can thus be optimized out.
Also, it has nothing to do with a latch being created as your title says (notice that the wording is FF/Latch, which just says that the signal EXS/mem_address_0 is either a flip-flop or a latch).
This is not necessarily a bad thing, unless you know that they should also be able to take on other values.
In this case, I'd say it seems as if your FW_ra_IN and FW_rb_IN signals are always 0. If they should be able to take on other values, then make sure that they are actually sourced from the correct place.
The reason why you don't get the warnings when replacing the assignment with x"FF" is that you then have the signal being all 0 when under reset, and all 1 when not - and none of the bits are thus constant.
Also have a look at VHDL synthesis warning FF/Latch has a constant value of 0

Answer 2

first of all, what you posted are warnings and not errors... should therefore be nonblocking.
however, the warnings tell you, that mem_address(3..0) has values that are always '0'. this means, that FW_ra_IN and FW_rb_IN is always '0'. these signals come from your IF_stage block (IF_reg_a and IF_reg_b). check, whether you update these signals within IF_stage or not.
another thing is, that mem_address is not used within top_level block. it might therefore be removed by the synthesizer during optimization.