Continuous assignment verilog

Question

-This code is written in verilog using Modelsim 10.2d.The errors below indicate there is some problem with {cout,l3} assignment.

module alu(a,b,bin,cin,op,cout,res);
input [31:0] a,b;
input [1:0] op;
input bin,cin;
reg [31:0] l1,l2,l3;
output cout;
output [31:0] res;

assign l1 = a & b;
assign l2 = a | b;

initial
if(bin == 1'b0)
  assign {cout,l3} = a + b + cin;
else
  assign {cout,l3} = a - b + cin;

mux4to1(l1,l2,l3,op,res);
endmodule

Error-
v(14): LHS in procedural continuous assignment may not be a net: cout.
v(16): LHS in procedural continuous assignment may not be a net: cout.

Answer 1

wire can not be assigned inside an initial or always block. You should change the type to reg.

The initial block will only be run once at the start of the simulation, not continually evaluated, therefore you should use always instead.

//...
output reg cout;
//...

always @* begin
  if(bin == 1'b0) begin
    {cout,l3} = a + b + cin;
  end
  else begin
    {cout,l3} = a - b + cin;
  end
end