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简单ALU

`timescale 1ns / 1ps

module ALU (
    input [3:0] A,
    input [3:0] B,
    input [2:0] operation,
    output reg [3:0] result,
    output reg cout
);
  always @(*) begin
    cout = 0;
    case (operation)
      3'b000: begin
        result <= A + B;
        // 由于不能直接检测溢出，所以换一种方式检测
        // 如果A + B > 1111 产生进位
        if (A + B < A || A + B < B) cout = 1;
      end
      3'b001: begin
        result <= A - B;
        // 如果A - B < 0 产生借位
        if (A < B) cout = 1;
      end
      3'b010: begin
        result <= B + 1;
        // 1111 + 1 产生进位
        if (B == 4'b1111) cout = 1;
      end
      3'b011: begin
        // 0 - 1 产生借位
        result <= B - 1;
        if (B == 4'b0000) cout = 1;
      end
      3'b100: begin
        result <= ~A;
      end
      3'b101: begin
        result <= A ^ B;
      end
      3'b110: begin
        result <= A & B;
      end
      3'b111: begin
        result <= A | B;
      end
    endcase
  end
endmodule

testbench

`timescale 1ns / 1ps

module ALU_tb;
  reg [3:0] A;
  reg [3:0] B;
  reg [2:0] operation;
  wire [3:0] result;
  wire cout;

  ALU ALU (
      A,
      B,
      operation,
      result,
      cout
  );
  initial begin
    A <= 4'b0001;
    B <= 4'b0010;
    operation <= 000;  // A + B 0011 没有进位

    #10 A <= 4'b1100;
    B <= 4'b1000;
    operation <= 000;  // A + B 0100 产生进位

    #10 A <= 4'b0001;
    B <= 4'b0001;
    operation <= 001;  // A - B 0000

    #10 A <= 4'b0000;
    B <= 4'b0001;
    operation <= 001;  // A - B < 0 1111 产生借位

    #10 A <= 4'b0000;
    B <= 4'b0001;
    operation <= 010;  // B + 1 0010

    #10 A <= 4'b0000;
    B <= 4'b1111;
    operation <= 010;  // B + 1 0000 产生进位

    #10 A <= 4'b0000;
    B <= 4'b0001;
    operation <= 011;  // B - 1 0000

    #10 A <= 4'b0000;
    B <= 4'b0000;
    operation <= 011;  // B - 1 1111 产生借位

    #10 A <= 4'b1001;
    B <= 4'b0001;
    operation <= 100;  //NOT A 0110

    #10 A <= 4'b0001;
    B <= 4'b0010;
    operation <= 101;  //A XOR B 

    #10 A <= 4'b0001;
    B <= 4'b0001;
    operation <= 110;  //A AND B 

    #10 A <= 4'b0001;
    B <= 4'b0000;
    operation <= 110;  //A AND B

    #10 A <= 4'b0001;
    B <= 4'b0000;
    operation <= 111;  //A OR B

    #10 A <= 4'b0000;
    B <= 4'b0000;
    operation <= 111;  //A OR B

    #10 $stop;
  end
endmodule

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