How to read data from rom_type in VHDL?

Question

How can I read data from rom_type?

entity my_rom is
 port(
  addr: in std_logic_vector(3 downto 0);
  data: out std_logic_vector(0 to 7)
 );
end my_rom;
architecture a of my_rom is

 type rom_type is array (0 to 7) of std_logic_vector(0 to 7);
 constant R1_ROM: rom_type :=
 (
  -- data
 );
begin
 data <= R1_rom(conv_integer(addr));
end a;

Answer 1

You are using conv_integer, which is not part of raw VHDL... it's in a library. However, you don't want to use it - it's from a non-standard library.

Instead use ieee.numeric_std.all; is what you need before your entity. Then use to_integer(unsigned(addr)) to index the ROM. Better still, pass the address in as an unsigned vector, or even directly as an integer.

Try to get out of the habit of using std_logic_vector (which is just a bag of bits) to represent numbers, and use the well-defined numerical types. Or use Verilog, which doesn't care :) Myself, I prefer VHDL's strong-typing to keep me from daft foot-shooting...