VHDL - '+' operator Usage

library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;

entity Vedic4m is
port(a : in unsigned(3 downto 0);
b : in unsigned(3 downto 0);
prod : out unsigned(7 downto 0));
end Vedic4m;

architecture behv of Vedic4m is

signal p0 : unsigned(1 downto 0);
signal p1, p6 : unsigned(1 downto 0);
signal p2, p3, p4, p5 : unsigned(2 downto 0);

begin
p0 <= a(0) + b(0);
p1 <= (a(1) and b(0)) + (a(0) and b(1)) + p0(1);
p2 <= (a(2) and b(0)) + (a(1) and b(1)) + (a(0) and b(2)) + p1(1);
p3 <= (a(3) and b(0)) + (a(2) and b(1)) + (a(1) and b(2)) + (a(0) and b(3)) + p2(2 downto 1);
p4 <= (a(3) and b(1)) + (a(2) and b(2)) + (a(1) and b(3)) + p3(2 downto 1);
p5 <= (a(3) and b(2)) + (a(2) and b(3)) + p4(2 downto 1);
p6 <= (a(3) and b(3)) + p5(2 downto 1);

prod <= p6 & p5 & p4 & p3 & p2 & p1 & p0;
end behv;

this is my code............

I want to use the + operator to add the Bits ans Words as shown

The error says

# ERROR: C:/Modeltech_ae/examples/Vedic4(20): No feasible entries for infix op: "+"
# ERROR: C:/Modeltech_ae/examples/Vedic4(20): Type error resolving infix expression.
# ERROR: C:/Modeltech_ae/examples/Vedic4(21): No feasible entries for infix op: "+"
# ERROR: C:/Modeltech_ae/examples/Vedic4(21): Bad expression.

Can't I use the + operator this way??

Please help

deena102

[joris]

The problem is that a(0) or p0(1) are of type std_logic, and adding std_logic with something else is not allowed.

You can get around that using these helper functions:

function"+"(x,y : std_logic) return unsigned is
  begin
    return unsigned'('0' & x) + unsigned'('0' & y);
  end;
  
  function"+"(x : unsigned; y : std_logic) return unsigned is
  begin
    return x + unsigned'('0' & y);
  end;

  function"+"(x : std_logic; y : unsigned) return unsigned is
  begin
    return unsigned'('0' & x) + y;
  end;

Oh, and then you end up with one more error that you need to ensure that the prod calculation doesn't thave the right number of bits. An untested attempt at it: (it compiles, but functionality is untested)

prod <= p6(1 downto 0) & p5(0) & p4(0) & p3(0) & p2(0) & p1(0) & p0(0);

joris