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/* A 4-bit multiplier, following the algorithm from

 * Pg. 179, Figure 3.7. */

module MUL4 (mul1, mul2, result, clk);

    input[0:3] mul1, mul2;

    output[0:7] result;

    input clk;

    wire[0:3] w_alu_out;

    wire[0:3] w_multiplicand;

    wire[0:7] w_product;

    wire[0:1] w_alu_op;

    wire w_write_op, w_shift_op;

    reg[0:3] count;

    MUL4_MULTIPLICAND m_mcand (mul1, w_multiplicand, clk);

    MUL4_PRODUCT m_prod (w_alu_out, mul2, w_shift_op, w_write_op, w_product, clk);

    MUL4_CONTROL m_ctrl (w_product[7], w_alu_op, w_write_op, w_shift_op, clk);

    ALU4 m_alu (w_product[0:3], w_multiplicand, cin, w_alu_op, cout, w_alu_out);

    always @(mul1 or mul2) begin

        count = 'b0000;

    end

    always @(clk) begin

        if (count == 4) begin

            $display ("set the result");

            force result = w_product;

        end

        count = count + 1;

    end

endmodule

module MUL4_2 (mul1, mul2, out);

    input[0:3] mul1, mul2;

    output[0:7] out;

    initial begin

        clk = 'b0;

    end

    always begin

        clk = ~clk;

    end

endmodule

module MUL4_CONTROL (prod_in, alu_op, write_op, shift_op, clk);

    input prod_in, clk;

    output[0:1] alu_op;

    output write_op, shift_op;

    reg[0:1] a_op;

    reg w_op, s_op;

    initial begin

        a_op = 'b00;

        w_op = 'b0;

        s_op = 'b0;

    end

    always @(clk) begin

        if (prod_in == 'b1)

            begin

                a_op = 'b10; // ADD

                w_op = 'b1;  // STORE

                s_op = 'b1;  // SHIFT

            end

        else

            begin

                a_op = 'b00; // AND, unnecessary

                w_op = 'b0;  // DO NOT WRITE

                s_op = 'b1;  // SHIFT

            end

    end //always

    assign alu_op = a_op;

    assign write_op = w_op;

    assign shift_op = s_op;

endmodule

module MUL4_MULTIPLICAND (in, out, clk);

    input[0:3] in;

    output[0:3] out;

    input clk;

    reg[0:3] value;

    always @(in) begin

        $display ("multiplicand changed");

        value = in;

    end

    assign out = value;

endmodule

module MUL4_PRODUCT (left4, right4, shift_op, write_op, out, clk);

    input[0:3] left4, right4;

    input shift_op, write_op, clk;

    output[0:7] out;

    reg[0:7] value;

    always @(right4) begin

        value[0:3] = 'b0000;

        value[4:7] = right4;

        $display ("right4 changed");

    end

    always @(clk) begin

        $display ("value=%b", value);

        if (shift_op == 1) begin

            $display ("shift 1");

            if (write_op == 1) begin

                $display ("write 1");

                value[0:3] = left4[0:3];

            end

            value = value >> 1;

        end

    end

    assign out = value;

endmodule

module testme;

    reg[0:3] a, b;

    wire[0:7] prod;

    initial begin

        $monitor ("time=%0d a=%b b=%b prod=%b", $time, a, b, prod);

    end

    initial begin

           clk = 'b0;

           a = 'b0000; b = 'b0000;

        #4 a = 'b0011; b = 'b1111;

        #4

        $finish;

    end

    reg clk;

    // Clock Generator

    always begin

        #1 clk = ~clk;

    end

    MUL4 m4 (a, b, prod, clk);

endmodule