I am trying to make a simple MACC to work, but it does unexpected things.

The multiplication is not working. 00001 * 00001 outputs 00000

```
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;
entity macc is
Port ( clk : in STD_LOGIC;
rst : in STD_LOGIC;
en : in STD_LOGIC;
A : in STD_LOGIC_VECTOR (4 downto 0);
B : in STD_LOGIC_VECTOR (4 downto 0);
P : out STD_LOGIC_VECTOR (8 downto 0));
end macc;
architecture Behavioral of macc is
signal product : STD_LOGIC_VECTOR (8 downto 0);
signal acc_in : STD_LOGIC_VECTOR (8 downto 0);
signal acc_out : STD_LOGIC_VECTOR (8 downto 0);
begin
product <= A*B;
acc_in <= acc_out + product;
acc: process is
begin
wait until rising_edge(clk);
if (rst = '1') then
acc_out <= (others => '0');
elsif (en = '1') then
acc_out <= acc_in;
end if;
end process acc;
P <= acc_out;
end Behavioral;
```

## Best Answer

If you multiply 2 5-bit numbers (

`A`

and`B`

are both`std_logic_vector(4 downto 0)`

) don't you need 10 bits (not 9) to store it in (so`P`

should be`std_logic_vector(9 downto 0)`

? (31*31 = 961: needs 10 bits)But also - don't use

`std_logic_arith`

/`_unsigned`

. Use`ieee.numeric_std`

and then use the`unsigned`

data type.