how do I take the absolute value of a std_logic_vector? in VHDL

Question

I can't figure out how to take the absolute value of two std_logic_vector(31 downto 0);

here is an example of the code:

library ieee;
use ieee.std_logic_1164.all;        
use ieee.numeric_std.all;       -- for the signed, unsigned types and arithmetic ops
use ieee.std_logic_arith.all;
use ieee.std_logic_unsigned.all;
...
...
port (  
    X: in std_logic_vector(31 downto 0); 
    Y: in std_logic_vector(31 downto 0); 
    F: out std_logic_vector(31 downto 0) 
  );

..
..
..
process(X,Y)
 begin
 F <= abs(X-Y)     --this doesnt work

How would you compute the absolute value of a binary value using pencil and paper? Just duplicate that algorithm in VHDL. — user1619508
@JoeHass that's the correct answer if you're learning, or if you have to something not built-in, but using the built-in abs on the standard signed type will often synthesize to better code in many FPGA and ASIC vendor's synthesizers which special case some built-in math operations. — wjl

wjl wjl · Accepted Answer · 2014-02-21T17:47:48

Ditch the non-standard library includes and use the standard signed type which has a built-in abs function:

library ieee;
use ieee.std_logic_1164.all;        
use ieee.numeric_std.all; -- this is the standard package where signed is defined
-- never use non-standard ieee.std_logic_arith and ieee.std_logic_unsigned

...

port (  
  X: in  std_logic_vector(31 downto 0); 
  Y: in  std_logic_vector(31 downto 0); 
  F: out std_logic_vector(31 downto 0) 
);

...

process(X,Y) is
begin
  F <= std_logic_vector(abs(signed(X)-signed(Y)));
end process;

That last line has a lot of [probably needless] converting between std_logic_vector and signed, so you might prefer this interface instead if it makes sense with the rest of your design:

port (  
  X: in  signed(31 downto 0); 
  Y: in  signed(31 downto 0); 
  F: out signed(31 downto 0) 
);

Then the last line is just:

 F <= abs(X-Y);

how do I take the absolute value of a std_logic_vector? in VHDL

1 Answers