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.

`default_nettype none
module tt_um_sky1 (
    input  wire [7:0] ui_in,
    output wire [7:0] uo_out,
    input  wire [7:0] uio_in,
    output wire [7:0] uio_out,
    output wire [7:0] uio_oe,
    input  wire   ena,
    input  wire   clk,
    input  wire   rst_n  
    
);
//we , instr
    wire [4:0] instr_addr = ui_in[4:0];    
    wire [7:0] instr_in = uio_in[7:0];        // Instruction input (8-bit opcode + 8-bit operand) in 2 PCs or 2 cycles like 8085
    reg [7:0] AC;
    reg [4:0] PC;
    wire we = ui_in[7];
    assign uio_oe = 8'h00;
    assign uio_out = 8'h00;
    assign uo_out = AC;
    reg [7:0] instruction_mem [0:21];
    reg [7:0] B,C;
    reg [1:0] state;
    reg [7:0] opcode;
    reg [7:0] operand;
    reg Zero,Carry,Overflow;

    parameter FETCH = 2'b00, DECODE = 2'b01, EXECUTE = 2'b10, HALT = 2'b11;
    // Instruction Opcodes
    localparam 
        MVI_A   = 8'h01,  // Move Immediate to A
        ADDI    = 8'h02,  // Add Immediate
        SUBI    = 8'h03,  // Sub Immediate
        ANDI    = 8'h04,  // AND Immediate
        ORI     = 8'h05,  // OR Immediate
        XORI    = 8'h06,  // XOR Immediate
        NOTA    = 8'h07,  // NOT A
        SHL     = 8'h08,  // Shift Left A
        SHR     = 8'h09,  // Shift Right A
        HLT    = 8'h0A,  // HALT

        MVI_B   = 8'h0B,  // Move Immediate to B
        MVI_C   = 8'h0C,  // Move Immediate to C
        JMP     = 8'h0D,  // Jump by offset
        INR_A   = 8'h0E,  // Increment A
        DCR_A   = 8'h0F,  // Decrement A
        INR_B   = 8'h10,  // Increment B
        DCR_B   = 8'h11,  // Decrement B
        INR_C   = 8'h12,  // Increment C
        DCR_C   = 8'h13,  // Decrement C

        JNZ     = 8'h14,  // Jump if Zero==0
        JZ      = 8'h15,  // Jump if Zero==1
        JNC     = 8'h16,  // Jump if Carry==0
        JC      = 8'h17,  // Jump if Carry==1
        ADD_B   = 8'h18,  // A = A + B
        ADD_C   = 8'h19,  // A = A + C
        BBC  = 8'h1A,  // B = B + C
        SUB_B   = 8'h1B,  // A = A - B
        SUB_C   = 8'h1C,  // A = A - C  
        JNO = 8'h1D,  // Jump if Overflow==0
        JO = 8'h1E;   // Jump if Overflow==1

    // Wires for ADD, SUB, INR, DCR Based Op
    //wire is_add = (opcode == ADDI)  || (opcode == ADD_B) || (opcode == ADD_C) || (opcode == BBC);
    wire is_sub = (opcode == SUBI)  || (opcode == SUB_B) || (opcode == SUB_C);
    wire is_inr = (opcode == INR_A) || (opcode == INR_B) || (opcode == INR_C);
    wire is_dcr = (opcode == DCR_A) || (opcode == DCR_B) || (opcode == DCR_C);

    // ---------------- ALU input selection ----------------
    wire [7:0] ALU_A =
        (opcode == ADDI || opcode == SUBI) ? AC :
        (opcode == ADD_B || opcode == ADD_C || opcode == SUB_B || opcode == SUB_C) ? AC :
        (opcode == BBC) ? B :
        (opcode == INR_A || opcode == DCR_A) ? AC :
        (opcode == INR_B || opcode == DCR_B) ? B  :
        (opcode == INR_C || opcode == DCR_C) ? C  :
        8'h00;

    wire [7:0] ALU_B =
        (opcode == ADDI || opcode == SUBI) ? operand :
        (opcode == ADD_B || opcode == SUB_B)  ? B :
        (opcode == ADD_C || opcode == SUB_C) ? C :
        (opcode == BBC) ? C :
        (is_inr || is_dcr) ? 8'h01 :     // +1 for INR, -1 via alu_sub for DCR
        8'h00;

    // SUB and DCR enable
    wire ALU_sub = is_sub || is_dcr;

    // ALU Sum Output
    wire [7:0] ALU_sum;
    wire CY,Z,OVF;
    ALU ALU1(
        .A   (ALU_A),
        .B   (ALU_B),
        .sub (ALU_sub),
        .Sum (ALU_sum),
        .Cout(CY),
        .Ovf (OVF),
        .ZERO(Z)
    );


    always @(posedge clk or negedge rst_n) begin
        if (!rst_n) begin
            PC <= 0;
            AC <= 0;
            state <= FETCH;
            opcode <= 0;
            operand <= 0;
            B <= 0;
            C <= 0; 
            Carry    <= 0;
            Overflow <= 0;
            Zero     <= 0;
        end else begin
            if (we) begin
                instruction_mem[instr_addr] <= instr_in;
            end else begin
                case (state)
                    FETCH: begin
                        opcode <= instruction_mem[PC]; // opcode get
                        PC <= PC + 1;
                        state <= DECODE;
                    end

                    DECODE: begin
                        if((opcode == NOTA)||(opcode == SHL)||(opcode == SHR)||(opcode == HLT)||(opcode == INR_A)
                        ||(opcode == DCR_A)||(opcode == INR_B)||(opcode == DCR_B)||(opcode == INR_C)||(opcode == DCR_C)
                        ||(opcode == ADD_B)||(opcode == ADD_C)||(opcode == BBC)||(opcode == SUB_B)||(opcode == SUB_C)) begin
                            state <= EXECUTE;
                        end
                        else begin  // Immediate
                        operand <= instruction_mem[PC];  // operand get
                        PC <= PC + 1;
                        state <= EXECUTE;
                        end
                    end

                    EXECUTE: begin
                        case (opcode)
                            MVI_A:  AC <= operand;
                            ADDI:   begin AC <= ALU_sum; Carry <= CY ; Zero <= Z; Overflow <= OVF; end
                            SUBI:   begin AC <= ALU_sum; Carry <= CY ; Zero <= Z; Overflow <= OVF; end
                            ANDI:   AC <= AC & operand;
                            ORI:    AC <= AC | operand;
                            XORI:   AC <= AC ^ operand;
                            NOTA:   AC <= ~AC;
                            SHL:    AC <= AC << 1;
                            SHR:    AC <= AC >> 1;
                            HLT:   state <= HALT;

                            MVI_B:  B <= operand;
                            MVI_C:  C <= operand;
                            JMP:    PC <= PC + operand[4:0];
                            INR_A:  begin AC <= ALU_sum; Carry <= CY ; Zero <= Z; Overflow <= OVF; end
                            DCR_A:  begin AC <= ALU_sum; Carry <= CY ; Zero <= Z; Overflow <= OVF; end
                            INR_B:  begin B  <= ALU_sum; Carry <= CY ; Zero <= Z; Overflow <= OVF; end
                            DCR_B:  begin B  <= ALU_sum; Carry <= CY ; Zero <= Z; Overflow <= OVF; end
                            INR_C:  begin C  <= ALU_sum; Carry <= CY ; Zero <= Z; Overflow <= OVF; end
                            DCR_C:  begin C  <= ALU_sum; Carry <= CY ; Zero <= Z; Overflow <= OVF; end
                            JNZ: if(Zero == 1'b0) begin PC <= PC + operand[4:0]; end // JNZ addr
                            JZ: if(Zero == 1'b1) begin PC <= PC + operand[4:0]; end // JZ addr
                            JNC: if(Carry == 1'b0) begin PC <= PC + operand[4:0]; end // JNC addr
                            JC: if(Carry == 1'b1) begin PC <= PC + operand[4:0]; end // JC addr
                            ADD_B:  begin AC <= ALU_sum; Carry <= CY ; Zero <= Z; Overflow <= OVF; end
                            ADD_C:  begin AC <= ALU_sum; Carry <= CY ; Zero <= Z; Overflow <= OVF; end
                            BBC:    begin B  <= ALU_sum; Carry <= CY ; Zero <= Z; Overflow <= OVF; end
                            SUB_B:  begin AC <= ALU_sum; Carry <= CY ; Zero <= Z; Overflow <= OVF; end
                            SUB_C:  begin AC <= ALU_sum; Carry <= CY ; Zero <= Z; Overflow <= OVF; end
                            JNO: if(Overflow == 1'b0) begin PC <= PC + operand[4:0]; end // JNO addr
                            JO: if(Overflow == 1'b1) begin PC <= PC + operand[4:0]; end // JO addr
                            

                            
                            default: state <= HALT;
                        endcase
                        if (opcode != 8'h0A)
                            state <= FETCH;
                    end

                    HALT: begin
                        state <= HALT;
                    end
                endcase
            end
        end
    end
    wire _unused = &{ena,ui_in[6:5]};
endmodule


module ALU (
    input wire [7:0] A,      
    input wire [7:0] B,
    input  wire    sub,     // 0 = Add, 1 = Subtract
    output [7:0] Sum, 
    output       Cout,    // Carry flag
    output       Ovf,     // Overflow flag
    output       ZERO
);
// >>> Module Implementation Begin

// <<< Module Implementation End
endmodule




Reference solution:
rtl/generated.v: 
    wire [7:0] B_in;
    wire Cin;
    wire I_Carry;

    // If subtracting, invert B and set Cin=1
    assign B_in = sub ? ~B : B;
    assign Cin  = sub ? 1'b1 : 1'b0;

    // Perform addition
    assign {I_Carry, Sum} = A + B_in + {7'b0000000,Cin};
    assign Cout = sub ? ~(I_Carry) : (I_Carry);

    // Signed overflow detection
    assign Ovf = (A[7] == B_in[7]) && (Sum[7] != A[7]);
    assign ZERO = (Sum == 8'h0);