在 VHDL 中表示整数的位数

问题描述

我必须转换一个整数才能找到表示该整数需要多少位.假设整数值为 22.我知道表示这个整数需要 5 位.VHDL 中是否有任何属性可以执行此操作?重要提示:结果也应该是一个整数，代表位数.

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.

推荐答案

没有 VHDL 属性或函数，但您可以创建一个函数，如:

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;

然而，有时 ceil_log2 函数也很有用，因为它根据内存映射中的条目给出所需的地址位数，并且 ceil_log2(val) = bits_req(val -1).

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).

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