In VHDL … how to count leading zeros of vector?

Question

I\'m working in a VHDL project and I\'m facing a problem to calculate the length of vector. I know there is length attribute of a vector but this not the length I\'m looking

Answer 1

Well, VHDL is not SW, it does not take time to perform an operation like this, it just takes resources from your FPGA.

You can divide your 1024 bits data into 32 bits section and perform an OR between all the bits, this way, you check 32 bits at a time. It is not really necessary since the the for loop would work perfectly fine for what you want to do, just write the code, look for the first 1 in the array and stop the loop and use the loop index number as the pointer to the first 1 in your array. I didn't compile this code, but something like this should work for you:

FirstOne <= 1023;
for i in E'reverse_range loop
  if (E(i) == '1') then
    FirstOne <= i;
    exit;
  end if;
end loop;

It will not be such a big blocks inside an FPGA after all.