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.

module tt_um_marxkar_jtag(
    input clk,                // TCLK
    input rst_n,              // Active-low reset (used as TRST)
    input ena,                // Always enabled
    input  [7:0] ui_in,       // Input pins: ui_in[0] = TDI, ui_in[1] = TMS, ui_in[2] = TRST
    output [7:0] uo_out,      // Output pins: uo_out[0] = TDO, uo_out[7:1] = debug.
    input  [7:0] uio_in,
    output [7:0] uio_out,
    output [7:0] uio_oe
);

    // Input mapping
    wire tdi = ui_in[0];
    wire tms = ui_in[1];
    wire trst = ~rst_n;  // since rst_n is active-low
    wire tclk = clk;

    wire _unused_ena     = ena;
    wire _unused_ui_in   = |ui_in[7:2];
    wire _unused_uio_in  = |uio_in;

    // Output mapping
    reg tdo;
    assign uo_out[0] = tdo;

    // FSM state variables
    reg [3:0] current_state, next_state;

    // Internal registers
    reg [1:0] instruction, shadow_instruction;
    reg [7:0] id_code = 8'b10101010;
    reg [7:0] shadow_id_code;
    reg bypass;
    reg [1:0] shadow_bsr;
    reg [3:0] bsr;
    wire bsr_wire;

    // Output debug (for optional observation)
    assign uo_out[1] = current_state[0];
    assign uo_out[2] = current_state[1];
    assign uo_out[3] = current_state[2];
    assign uo_out[4] = current_state[3];
    assign uo_out[5] = instruction[0];
    assign uo_out[6] = instruction[1];
    assign uo_out[7] = bypass;

    // Bidirectional pins unused
    assign uio_out = 8'b0;
    assign uio_oe  = 8'b0;

    // FSM state encoding
    localparam [3:0]
        test_logic_reset = 4'b0000,    
        run_idle_test    = 4'b0001,
        select_dr_scan   = 4'b0010,
        capture_dr       = 4'b0011,
        shift_dr         = 4'b0100,
        exit_1_dr        = 4'b0101,
        pause_dr         = 4'b0110,
        exit_2_dr        = 4'b0111,
        update_dr        = 4'b1000,
        select_ir_scan   = 4'b1001,
        capture_ir       = 4'b1010,
        shift_ir         = 4'b1011,
        exit_1_ir        = 4'b1100,
        pause_ir         = 4'b1101,
        exit_2_ir        = 4'b1110,
        update_ir        = 4'b1111;

    // FSM state transition
    always @(posedge tclk or posedge trst) begin
        if (trst)
            current_state <= test_logic_reset;
        else
            current_state <= next_state;
    end

    // Next state logic
    always @(*) begin
        case (current_state)
            test_logic_reset : next_state = (tms) ? test_logic_reset : run_idle_test;
            run_idle_test    : next_state = (tms) ? select_dr_scan   : run_idle_test;
            select_dr_scan   : next_state = (tms) ? select_ir_scan   : capture_dr;
            capture_dr       : next_state = (tms) ? exit_1_dr        : shift_dr;
            shift_dr         : next_state = (tms) ? exit_1_dr        : shift_dr;
            exit_1_dr        : next_state = (tms) ? update_dr        : pause_dr;
            pause_dr         : next_state = (tms) ? exit_2_dr        : pause_dr;
            exit_2_dr        : next_state = (tms) ? update_dr        : shift_dr;
            update_dr        : next_state = (tms) ? select_dr_scan   : run_idle_test;
            select_ir_scan   : next_state = (tms) ? test_logic_reset : capture_ir;
            capture_ir       : next_state = (tms) ? exit_1_ir        : shift_ir;
            shift_ir         : next_state = (tms) ? exit_1_ir        : shift_ir;
            exit_1_ir        : next_state = (tms) ? update_ir        : pause_ir;
            pause_ir         : next_state = (tms) ? exit_2_ir        : pause_ir;
            exit_2_ir        : next_state = (tms) ? update_ir        : shift_ir;
            update_ir        : next_state = (tms) ? select_dr_scan   : run_idle_test;
            default: next_state = test_logic_reset;
        endcase
    end

    // Behavior logic on clock
    always @(posedge tclk or posedge trst) begin
        if (trst) begin
            tdo <= 0;
            bypass <= 0;
            shadow_bsr <= 0;
            shadow_instruction <= 0;
            shadow_id_code <= 0;
            bsr <= 0;
            instruction <= 0;
        end else begin
            bsr[0] <= bsr_wire; // replace bsr_wire logic with shifting dummy wire
            tdo <= 0;
            bypass <= bypass;
            shadow_bsr <= shadow_bsr;
            shadow_instruction <= shadow_instruction;
            shadow_id_code <= shadow_id_code;
            bsr[3:1] <= bsr[3:1];
            instruction <= instruction;
            case (next_state)
                test_logic_reset: begin
                    tdo <= 0;
                    bypass <= 0;
                    shadow_bsr <= 0;
                    shadow_instruction <= 0;
                    shadow_id_code <= 0;
                    bsr <= 0;
                    instruction <= 0;
                end

                run_idle_test: begin
                    if (instruction == 2'b10)
                        tdo <= bsr[0];
                end

                shift_dr, exit_1_dr: begin
                    case (instruction)
                        2'b11: begin tdo <= bypass; bypass <= tdi; end
                        2'b01: begin tdo <= shadow_id_code[0]; shadow_id_code <= {tdi, shadow_id_code[7:1]}; end
                        2'b10: begin shadow_bsr <= {tdi, shadow_bsr[1]}; end
                        default: begin end
                    endcase
                end

                capture_ir: begin
                    shadow_instruction <= instruction;
                end

                shift_ir, exit_1_ir: begin
                    shadow_instruction <= {tdi, shadow_instruction[1]};
                    tdo <= shadow_instruction[0];
                end

                update_ir: begin
                    instruction <= shadow_instruction;
                end

                capture_dr: begin
                    if (instruction == 2'b01)
                        shadow_id_code <= id_code;
                    else if (instruction == 2'b10)
                        shadow_bsr <= bsr[3:2];
                end

                update_dr: begin
                    if (instruction == 2'b10)
                        bsr[3:2] <= shadow_bsr;
                    else if (instruction == 2'b11)
                        tdo <= 0;
                end

                pause_dr : begin
                    if (instruction == 2'b11) 
                        tdo <= 0;
                end

                default : begin
                end
            endcase
        end
    end


d_flip_flop f1 (bsr[3], bsr[2], trst, bsr_wire);
endmodule


module d_flip_flop (
    input wire d,       
    input wire clk,    
    input wire rst,
    output reg q);
// >>> Module Implementation Begin

// <<< Module Implementation End
endmodule




Reference solution:
rtl/generated.v: 
always @(posedge clk or posedge rst)
begin
if ( rst ) 
    q <= 0;
else
    q <= d;
end