Number of bits to represent an integer in VHDL

Question

I have to convert an integer to find how many bits are required to represent that integer. Let say, integer value is 22. I know 5 bits are required to represent this integer. Is there any attribute in VHDL to do this? Important: The result should also be an integer, which should represent number of bits.

Answer 1

There is no VHDL attribute or function, but you can create a function like:

-- Returns number of bits required to represent val in binary vector
function bits_req(val : natural) return natural is
  variable res_v    : natural;  -- Result
  variable remain_v : natural;  -- Remainder used in iteration
begin
  res_v := 0;
  remain_v := val;
  while remain_v > 0 loop  -- Iteration for each bit required
    res_v := res_v + 1;
    remain_v := remain_v / 2;
  end loop;
  return res_v;
end function;

However, sometimes a ceil_log2 function is also useful, since that gives the number of required address bits based on entries in a memory map, and ceil_log2(val) = bits_req(val - 1).