Is there any cure for this..i intend to use Xilinx only.

 Real Numbers are not synthesizable . Consider  using fix point format
.

Which kind log do you want to implement ? Log2, for instance easy to
implement for integer.

function Log2(input : integer) return integer is
    variable n    : integer;
    variable logn : integer;
  begin
    n := 1;
    for i in 0 to 31 loop
      logn := i;
      exit when (n >= input);
      n    := n * 2;
    end loop;
    return logn;
  end;

"charsi" <gupta_an...@nospam.rediffmail.coooom> a écrit dans le message de
news: 40074f5037b6cee8f57fca3db2ee0__BEGIN_MASK_n#9g02mG7!__...__END_MASK_i?a63jfAD$z__@localhost.talkaboutprogramming.com...

I think that the function you really want is in the new "numeric_std"
package.

Download this file:
http://www.eda.org/vhdl-200x/vhdl-200x-ft/packages/additions_11.tar.Z

Compile the file "numeric_std_additions.vhd"  (this are just the new
functions, not the whole thing)

Use this function:

signal XXX : UNSIGNED (16 downto 0);
signal log_base_2 : integer;

log_base_2 := find_msb (XXX, '1');