`timescale 1ns / 1ps
//////////////////////////////////////////////////////////////////////////////////
// Company:
// Engineer:
//
// Create Date:
// Design Name:
// Module Name: calculator
// Project Name:
// Target Devices:
// Tool versions:
// Description:
//
// Dependencies:
//
// Revision:
// Revision 0.01 - File Created
// Additional Comments:
//
//////////////////////////////////////////////////////////////////////////////////
module calculator (input [3:0] opCode,
               input signed [15:0] in1,
               input signed [15:0] in2,
               output reg signed [15:0] result,
               output reg overflow);

reg signed [18:0] prod;

always @(opCode or in1 or in2)
   begin
   case (opCode)
         'b0000:
            begin
            result = in1 + in2;
            if ((in1[15] == in2[15]) && (in2[15] != result[15]))
               begin
               overflow = 'b1;
               end
            else
               begin
               overflow = 'b0;
               end
            end
         'b0001:
            begin
            result = in1 - in2;
            if ((in1[15] != in2[15]) && (in2[15] == result[15]))
               begin
               overflow = 'b1;
               end
            else
               begin
               overflow = 'b0;
               end
            end
         'b0010:
            begin
            prod = in1 * 5;
            result = prod[15:0];
            if (((in1[15] == 'b1) && (prod[18:15] != 'b1111)) || ((in1[15] == 'b0) && (prod[18:15] != 'b0000)))
               begin
               overflow

1. A an external memory unit (shown in the block diagram below) can...