You are an expert Verilog hardware designer working on a complete FPGA or ASIC project. You are given the full source code of a Verilog project. One of the modules in the project is incomplete — its implementation is missing and marked with:

// >>> Module Implementation Begin

// <<< Module Implementation End

Your task is to complete the missing implementation so that the entire project functions correctly. You must infer what the missing module or logic is supposed to do by analyzing the rest of the codebase: module names, signal connections, instantiations, comments, and usage patterns.

Follow these rules:

1. Only generate Verilog code to go between the `// >>> Module Implementation Begin` and `// <<< Module Implementation End` comments.
2. Use Verilog-2001 syntax. Ensure the code is synthesizable.
3. Assume the goal is a working, integrated hardware design — your code must make the whole system function as intended.
4. Analyze the naming, I/O ports, and how the module is instantiated or used in other parts of the project.
5. Implement correct behavior based on design context. Do not guess randomly — reason it out.
6. Avoid vendor-specific features unless explicitly required.
7. Do not output comments, explanation, or anything outside the required code region unless explicitly asked.

Your output should contain **only the Verilog code** that fits inside the marked region.

// Wrapper for GCD module

module tt_um_gcd_stephan (
    input  wire [7:0] ui_in,    // Dedicated inputs
    input  wire [7:0] uio_in,   // IOs: Input path
    output wire [7:0] uo_out,   // Dedicated outputs
    output wire [7:0] uio_out,  // IOs: Output path
    output wire [7:0] uio_oe,   // IOs: Enable path (active high: 0=input, 1=output)
    input  wire       ena,      // always 1 when the design is powered, so you can ignore it
    input  wire       clk,      // clock
    input  wire       rst_n     // reset_n - low to reset
    );

    wire [15:0] AB;
    wire [15:0] C;     
    wire req_w;
    wire ack_w;
    wire rst;
    wire zero_bit_wire = 1'b0;

    wire [8:0] unused_bits = C[15:7];

    assign AB = {zero_bit_wire, uio_in[6:0], ui_in[7:0]};
    assign req_w = uio_in[7];

    assign uo_out[6:0] = C[6:0];
    assign uo_out[7] = ack_w;
    
    assign uio_oe = 8'b0; //{zero_bit_wire, zero_bit_wire, zero_bit_wire, zero_bit_wire, zero_bit_wire, zero_bit_wire, zero_bit_wire, zero_bit_wire};
    assign uio_out = 8'b0; //{zero_bit_wire, zero_bit_wire, zero_bit_wire, zero_bit_wire, zero_bit_wire, zero_bit_wire, zero_bit_wire, zero_bit_wire};

    assign rst = ~rst_n;

    wire _unused = &{ena, 1'b0};

    gcd_module gcd_module_inst (
        .reset(rst),
        .clk(clk),
        .req(req_w),
        .AB(AB),
        .ack(ack_w),
        .C(C)
    );
endmodule

// Lab 3 exercise
// Greatest common divisor (GCD) computation

`default_nettype none

module gcd_module
    (
        input wire        reset,
        input wire        clk,

        input wire        req,
        input wire [15:0] AB,

        output wire        ack,
        output wire [15:0] C
    );

    wire        ABorALU_int, LDA_int, LDB_int, N_int, Z_int;
    wire [1:0]  FN_int;

    fsm fsm_comp (
        .clk(clk),
        .reset(reset),
        
        .req(req),
        .ack(ack),

        .ABorALU(ABorALU_int),
        .LDA(LDA_int),
        .LDB(LDB_int),
        .N(N_int),
        .Z(Z_int),

        .FN(FN_int)
    );

    datapath datapath_comp (
        .clk(clk),
        .reset(reset),

        .AB(AB),
        .C(C),

        .ABorALU(ABorALU_int),
        .LDA(LDA_int),
        .LDB(LDB_int),

        .N(N_int),
        .Z(Z_int),
        
        .FN(FN_int)
    );

endmodule

`default_nettype none

// Simple ALU twos complement
// FN selects function:
// fn = 00 : C = A - B
// fn = 01 : C = B - A
// fn = 10 : C = A
// fn = 11 : C = B
// The ALU sets the two flags "Z" and "N" which indicates if the result was zero or negative.
module alu #(parameter W = 16)
    (
        input wire  [W-1:0] A,
        input wire  [W-1:0] B,
        input wire  [1:0] fn,
        input wire  rst,
        output reg [W-1:0] C,
        output reg Z,
        output reg N
    );

    always @(*) begin
        if (rst == 1) begin
            C = 0;
        end else begin
        case (fn)
            2'b00: C = A - B;
            2'b01: C = B - A;
            2'b10: C = A;
            2'b11: C = B;
            default: C = 0;
        endcase
        end
    end;

    always @(*) begin
        if (rst) begin
            Z = 0;
        end
        else if (C != 0) begin
            Z = 0;
        end
        else begin
            Z = 1;
        end 
    end;

    always @(*) begin
        if (rst) begin
            N = 0;
        end
        else if (C[W-1] == 1) begin
            N = 1;
        end
        else begin
            N = 0;
        end
    end

endmodule


`default_nettype none

// Buffer components.
module buff #(parameter N = 16)
    (
        input   wire [N-1:0] data_in,
        output  wire [N-1:0] data_out
    );

    assign data_out = data_in;

endmodule

`default_nettype none

module datapath #(parameter W = 16)
    (
    input wire clk,
    input wire reset,

    input  wire [15:0] AB,
    output wire [15:0] C,

    input wire ABorALU,
    input wire LDA,
    input wire LDB,

    output wire N,
    output wire Z,

    input wire [1:0] FN
    );

    wire [W-1:0] C_int, RegA_int, RegB_int;
    wire [W:0] Y; 

    wire unused_bit = Y[W:W];

    buff #(.N(W)) output_buffer (
        .data_in(C_int), 
        .data_out(C)
    );

    mux #(.N(W)) input_mux (
        .data_in1(Y[W-1:0]),
        .data_in2(AB),
        .s(ABorALU),
        .data_out(C_int)
    );

    regi #(.N(W)) regA (
        .clk(clk),
        .en(LDA),
        .rst(reset),
        .data_in(C_int),
        .data_out(RegA_int)
    );

    regi #(.N(W)) regB (
        .clk(clk),
        .en(LDB),
        .rst(reset),
        .data_in(C_int),
        .data_out(RegB_int)
    );

    alu #(.W(W+1)) alu_comp (
        .A({1'b0, RegA_int}),
        .B({1'b0, RegB_int}),
        .fn(FN),
        .rst(reset),
        .C(Y),
        .Z(Z),
        .N(N)
    );
endmodule

`default_nettype none

module fsm (
    input wire clk,
    input wire reset,

    input  wire req,
    output reg ack,

    output reg ABorALU,
    output reg LDA,
    output reg LDB,

    input  wire N,
    input  wire Z,

    output reg [1:0] FN
    );
// >>> Module Implementation Begin

// <<< Module Implementation End
endmodule



`default_nettype none

// 2 to 1 multiplexor

module mux #(parameter N = 16)
    (
        input   wire [N-1:0] data_in1,
        input   wire [N-1:0] data_in2,
        input   wire         s,
        output  wire [N-1:0] data_out
    );

    assign data_out = s ? data_in2 : data_in1;

endmodule

`default_nettype none

// Pos edge reg with en
module regi #(parameter N = 16)
    (
        input   wire         clk,
        input   wire         en,    
        input   wire         rst,
        input   wire [N-1:0] data_in,
        output  reg [N-1:0] data_out
    );

    always @(posedge clk or posedge rst)
    begin
        if (rst == 1) begin
            data_out <= 0;
        end
        else if (en == 1) begin
            data_out <= data_in;
        end
        else begin
            data_out <= data_out;
        end
    end
endmodule


Reference solution:
rtl/generated.v: 
    reg [2:0] state, next_state;

    parameter ready_a   = 0,
              load_a    = 1,
              ready_b   = 2,
              load_b    = 3,
              calc      = 4,
              calc_done = 5,
              sub_ba    = 6,
              sub_ab    = 7;

    // Registers for the state machine
    always @(posedge clk or posedge reset)
    begin
        if (reset == 1)
            state <= 3'b000;
        else
            state <= next_state;
    end

    // State machine logic
    always @(*) 
    begin
        case (state) 
        ready_a: begin
            FN         = 2'b10;
            LDA        = 0;
            LDB        = 0;
            ABorALU    = 0;
            ack        = 0;
            next_state = ready_a;

            if (req == 1)
                next_state = load_a;
        end

        load_a: begin
            FN          = 2'b10;
            ABorALU     = 1;
            LDA         = 1;
            LDB         = 0;
            ack         = 1;
            next_state  = load_a;

            if (req == 1'b0)
                next_state = ready_b;
        end

        ready_b: begin
            FN          = 2'b10;
            ABorALU     = 0;
            LDA         = 0;
            LDB         = 0;
            ack         = 0;
            next_state  = ready_b;

            if (req == 1'b1)
                next_state = load_b;
        end

        load_b: begin
            FN          = 2'b11;
            ABorALU     = 1;
            LDA         = 0;
            LDB         = 1;
            ack         = 0;
            next_state = load_b;

            if (req == 1'b1)
                next_state = calc;
        end

        calc: begin
            FN          = 2'b00;
            ABorALU     = 0;
            LDA         = 0;
            LDB         = 0;
            ack         = 0;
            next_state  = calc;

            if (Z == 1)
                next_state = calc_done;
            else if (N == 1)
                next_state = sub_ba;
            else
                next_state = sub_ab;
        end

        calc_done: begin
            FN          = 2'b10;
            ABorALU     = 0;
            LDA         = 0;
            LDB         = 0;
            ack         = 1;
            next_state  = calc_done;

            if (req == 0)
                next_state = ready_a;
        end

        sub_ba: begin
            FN          = 2'b01;
            ABorALU     = 0;
            LDA         = 0;
            LDB         = 1;
            ack         = 0;
            
            next_state  = calc;
        end

        sub_ab: begin
            FN          = 2'b00;
            ABorALU     = 0;
            LDA         = 1;
            LDB         = 0;
            ack         = 0;

            next_state = calc;
        end

        default: begin
            next_state = ready_a;
            FN         = 2'b00;
            LDA        = 0;
            LDB        = 0;
            ABorALU    = 0;
            ack        = 0;
        end

        endcase
    end