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`include "define.h"

//Feb.25.2005 Verilog2001 Style

//Jan.20.2005 implict event list

//Jun.14.2004 Initial Version

//Jul.4.2004 sensibity list bug fix

//Jul.5.2004 less area version

//Apr.14.2005  Stratix 2 workaround Quartus 4.1/4.2 

module substruct (input [31:0] a,b,

                                                                  output [31:0] c);

assign c=a+~b+1;//NG Quartus 4.1/4.2 a-b

endmodule

module alu (input [31:0] a,b,

                     output reg [31:0] alu_out,

                     input [3:0] alu_func);

                                wire [31:0] c;

parameter   [3:0] alu_nop                 =4'b0000,

                  alu_add                 =4'b0001,

                  alu_sub                 =4'b0010,

                  alu_less_than_unsigned  =4'b0101, //Jul.5.2004

                  alu_less_than_signed    =4'b0100, //Jul.5.2004

                  alu_OR                  =4'b0011,

                  alu_AND                 =4'b0110,

                  alu_XOR                 =4'b0111,

                  alu_NOR =4'b1000;

        reg [32:0] sum;

        always @* begin //

                case (alu_func)

                        alu_nop       : alu_out=32'h0000;

                        alu_add        : alu_out=a+b;

                        alu_sub        : alu_out=c;//Apr.14.2005 NG a-b Quartus 4.1/4.2

                        alu_OR         : alu_out=a | b;

                        alu_AND        : alu_out=a & b;

                        alu_XOR        : alu_out=a ^ b;

                        alu_NOR        : alu_out=~(a | b);

                        alu_less_than_unsigned : alu_out=a < b;//Jun.29.2004

                        alu_less_than_signed: begin

                                                 sum={a[31],a}+~{b[31],b}+33'h0_0000_0001;//Apr.14.2005 1'b1;//Important 33'h0_0000_000 :a-b                                                                      $signed(a) > $signed(b);

                                                 alu_out={31'h0000_0000,sum[32]};//{31'h0000_0000,sum[32]}; 

                                               end

                        default : alu_out=32'h0000_0000;

                endcase

        end

                substruct sub(a,b,c);

endmodule