`timescale 1ns / 1ps
`default_nettype none

// -----------------------------------------------------------------------------
// ID 级组合译码器。
//
// 与原始 Logisim 控制器一致：操作码取标准 RISC-V 的 IR[6:2] 五位字段，
// 真值表中以十六进制形式存储该字段。
// 所有输出在无匹配时保持"无操作/不写回"的默认值。
// -----------------------------------------------------------------------------
module cpu21_riscv_decoder (
    input wire [31:0] ir_i,

    output reg  [ 4:0] rs1_idx_o,
    output reg  [ 4:0] rs2_idx_o,
    output reg  [ 4:0] rd_o,
    output reg  [31:0] imm_o,
    output reg  [ 3:0] alu_op_o,
    output reg  [ 2:0] wb_sel_o,
    output wire [11:0] csr_addr_o,
    output reg         uses_rs1_o,
    output reg         uses_rs2_o,
    output reg         reg_write_o,
    output reg         mem_to_reg_o,
    output reg         mem_write_o,
    output reg         mem_byte_o,
    output reg         alu_src_o,
    output reg         branch_o,
    output reg         beq_o,
    output reg         bne_o,
    output reg         blt_o,
    output reg         bltu_o,
    output reg         jal_o,
    output reg         jalr_o,
    output reg         ecall_o,
    output reg         uret_o,
    output reg         csr_set_o,
    output reg         csr_clear_o,
    output reg         csr_write_o
);
  // CPU21 自定义操作码（取自所提供的真值表）。
  localparam OP_LOAD = 5'h00;
  localparam OP_R = 5'h0c;
  localparam OP_I = 5'h04;
  localparam OP_STORE = 5'h08;
  localparam OP_JALR = 5'h19;
  localparam OP_BRANCH = 5'h18;
  localparam OP_JAL = 5'h1b;
  localparam OP_SYS = 5'h1c;

  localparam ALU_SLL = 4'd0;
  localparam ALU_SRA = 4'd1;
  localparam ALU_SRL = 4'd2;
  localparam ALU_MUL = 4'd3;
  localparam ALU_DIVU = 4'd4;
  localparam ALU_ADD = 4'd5;
  localparam ALU_SUB = 4'd6;
  localparam ALU_AND = 4'd7;
  localparam ALU_OR = 4'd8;
  localparam ALU_XOR = 4'd9;
  localparam ALU_SLT = 4'd11;
  localparam ALU_SLTU = 4'd12;

  wire [4:0] opcode = ir_i[6:2];
  wire [2:0] funct3 = ir_i[14:12];
  wire [6:0] funct7 = ir_i[31:25];

  // CSR 地址字段在译码时直接旁路输出（供 EX 级读取 CSR）。
  assign csr_addr_o = ir_i[31:20];

  // 默认控制信号：全部为"无操作/不写回"，再由下面的 case 覆盖。
  always @* begin
    rs1_idx_o    = ir_i[19:15];
    rs2_idx_o    = ir_i[24:20];
    rd_o         = ir_i[11:7];
    imm_o        = {{20{ir_i[31]}}, ir_i[31:20]};
    alu_op_o     = ALU_ADD;
    wb_sel_o     = 3'd0;
    uses_rs1_o   = 1'b0;
    uses_rs2_o   = 1'b0;
    reg_write_o  = 1'b0;
    mem_to_reg_o = 1'b0;
    mem_write_o  = 1'b0;
    mem_byte_o   = 1'b0;
    alu_src_o    = 1'b0;
    branch_o     = 1'b0;
    beq_o        = 1'b0;
    bne_o        = 1'b0;
    blt_o        = 1'b0;
    bltu_o       = 1'b0;
    jal_o        = 1'b0;
    jalr_o       = 1'b0;
    ecall_o      = 1'b0;
    uret_o       = 1'b0;
    csr_set_o    = 1'b0;
    csr_clear_o  = 1'b0;
    csr_write_o  = 1'b0;

    case (opcode)
      // R 型运算：由 funct3/funct7 决定具体操作。
      OP_R: begin
        uses_rs1_o  = 1'b1;
        uses_rs2_o  = 1'b1;
        reg_write_o = 1'b1;
        // 电路中额外的 REMU 控制对应标准 R 型的 funct7=1/funct3=111
        // 形式；此时 ALU 的 result2 即为余数。
        if ((funct7 == 7'b0000001) && (funct3 == 3'b111)) begin
          alu_op_o = ALU_DIVU;
          wb_sel_o = 3'd3;
        end else if ((funct7 == 7'b0000001) && (funct3 == 3'b000)) begin
          alu_op_o = ALU_MUL;
        end else begin
          // 标准 R 型 funct3 译码；add/sub 与 sra/srl 由 funct7[5] 区分。
          case (funct3)
            3'b000:  alu_op_o = (funct7[5] ? ALU_SUB : ALU_ADD);
            3'b001:  alu_op_o = ALU_SLL;
            3'b010:  alu_op_o = ALU_SLT;
            3'b011:  alu_op_o = ALU_SLTU;
            3'b100:  alu_op_o = ALU_XOR;
            3'b101:  alu_op_o = (funct7[5] ? ALU_SRA : ALU_SRL);
            3'b110:  alu_op_o = ALU_OR;
            3'b111:  alu_op_o = ALU_AND;
            default: reg_write_o = 1'b0;
          endcase
        end
      end

      // I 型运算（addi/slli/slti/xori/srai/srli/ori/andi）。
      OP_I: begin
        uses_rs1_o  = 1'b1;
        alu_src_o   = 1'b1;
        reg_write_o = 1'b1;
        case (funct3)
          3'b000: alu_op_o = ALU_ADD;  // addi：立即数加
          3'b001: alu_op_o = ALU_SLL;  // slli：立即数逻辑左移
          3'b010: alu_op_o = ALU_SLT;  // slti：有符号小于置 1
          3'b100: alu_op_o = ALU_XOR;  // xori：立即数异或
          3'b101:
          alu_op_o = (funct7[5] ? ALU_SRA : ALU_SRL);  // srai/srli：立即数算术/逻辑右移
          3'b110: alu_op_o = ALU_OR;  // ori：立即数或
          3'b111: alu_op_o = ALU_AND;  // andi：立即数与
          default: reg_write_o = 1'b0;
        endcase
      end

      // 加载指令，当前仅支持 lw（funct3=010）。
      OP_LOAD: begin
        if (funct3 == 3'b010) begin
          uses_rs1_o   = 1'b1;
          alu_src_o    = 1'b1;
          alu_op_o     = ALU_ADD;
          mem_to_reg_o = 1'b1;
          reg_write_o  = 1'b1;
          wb_sel_o     = 3'd1;
        end
      end

      // 存储指令：sw（funct3=010）与 sb（funct3=000）。
      OP_STORE: begin
        if ((funct3 == 3'b010) || (funct3 == 3'b000)) begin
          uses_rs1_o  = 1'b1;
          uses_rs2_o  = 1'b1;
          alu_src_o   = 1'b1;
          alu_op_o    = ALU_ADD;
          mem_write_o = 1'b1;
          mem_byte_o  = (funct3 == 3'b000);  // sb：字节存储
          imm_o       = {{20{ir_i[31]}}, ir_i[31:25], ir_i[11:7]};
        end
      end

      // 条件分支：beq（000）/bne（001）/blt（100，有符号小于）/bltu（110，无符号小于）。
      OP_BRANCH: begin
        if ((funct3 == 3'b000) || (funct3 == 3'b001) ||
            (funct3 == 3'b100) || (funct3 == 3'b110)) begin
          uses_rs1_o = 1'b1;
          uses_rs2_o = 1'b1;
          branch_o   = 1'b1;
          beq_o      = (funct3 == 3'b000);
          bne_o      = (funct3 == 3'b001);
          blt_o      = (funct3 == 3'b100);
          bltu_o     = (funct3 == 3'b110);
          // 真值表：beq/bne 用减法比较，blt 用 ALU_SLT，bltu 用 ALU_SLTU。
          alu_op_o   = blt_o ? ALU_SLT : (bltu_o ? ALU_SLTU : ALU_SUB);
          imm_o      = {{19{ir_i[31]}}, ir_i[31], ir_i[7], ir_i[30:25], ir_i[11:8], 1'b0};
        end
      end

      // 无条件跳转并链接：jal（写回 PC+4）。
      OP_JAL: begin
        jal_o       = 1'b1;
        reg_write_o = 1'b1;
        wb_sel_o    = 3'd2;
        imm_o       = {{11{ir_i[31]}}, ir_i[31], ir_i[19:12], ir_i[20], ir_i[30:21], 1'b0};
      end

      // 寄存器间接跳转并链接：jalr。
      OP_JALR: begin
        if (funct3 == 3'b000) begin
          jalr_o      = 1'b1;
          uses_rs1_o  = 1'b1;
          alu_src_o   = 1'b1;
          alu_op_o    = ALU_ADD;
          reg_write_o = 1'b1;
          wb_sel_o    = 3'd2;
        end
      end

      // 系统指令：ecall / uret / CSR 读写。
      OP_SYS: begin
        // 电路用 IR[21] 区分 URET 与 ecall。
        if (funct3 == 3'b000) begin
          if (ir_i[21]) begin
            uret_o = 1'b1;
          end else begin
            ecall_o    = 1'b1;
            // 按文档说明，ecall 读取 a7(rs17) 与 a0(rs10)。
            uses_rs1_o = 1'b1;
            uses_rs2_o = 1'b1;
            rs1_idx_o  = 5'd17;
            rs2_idx_o  = 5'd10;
          end
        end else if (funct3 == 3'b001) begin
          csr_write_o = 1'b1;  // CSRRW：写 CSR，并把旧值写回 rd
          uses_rs1_o  = 1'b1;
          alu_src_o   = 1'b1;
          imm_o       = {27'b0, ir_i[19:15]};
          reg_write_o = (rd_o != 5'd0);
          wb_sel_o    = 3'd4;
        end else if (funct3 == 3'b110) begin
          csr_set_o   = 1'b1;  // CSRRSI：置位 CSR 中的指定位
          imm_o       = {27'b0, ir_i[19:15]};
          reg_write_o = (rd_o != 5'd0);
          wb_sel_o    = 3'd4;
        end else if (funct3 == 3'b111) begin
          csr_clear_o = 1'b1;  // CSRRCI：清除 CSR 中的指定位
          imm_o       = {27'b0, ir_i[19:15]};
          reg_write_o = (rd_o != 5'd0);
          wb_sel_o    = 3'd4;
        end
      end

      default: begin
      end
    endcase
  end
endmodule

`default_nettype wire