Define operation code

Operations	Control code
Addition (ADD)	0 0 0
Subtract (SUB)	0 0 1
Logical OR (OR)	0 1 0
Logical AND (AND)	0 1 1
Logical NOT (NOT)	1 0 0
Arithmetic shift right (ASR)	1 0 1
Arithmetic shift left (ASL)	1 1 0
Nibble swap (SWP)	1 1 1

Define label of the operation code by "`define".
By this define, the operation code is described with the defined label as distinct from bit pattern.
These defines are described in "aluop.v" file.

  `define IADD 3'b000
  `define ISUB 3'b001
     ...
     ...

`timescale 1ns/1ps
`include "alu_op.v"

module alu ( ... );
  ...
endmodule

Next, Describe operations .