/* Copyright lowRISC contributors.
Copyright 2018 ETH Zurich and University of Bologna, see also CREDITS.md.
Licensed under the Apache License, Version 2.0, see LICENSE for details.
SPDX-License-Identifier: Apache-2.0 

* Adapted to SERV by @Abdulwadoodd as part of the project under spring '22 LFX Mentorship program */

/* Decodes RISC-V compressed instructions into their RV32i equivalent. */

module serv_compdec 
  (
   input wire i_clk,
   input  wire [31:0] i_instr,
   input  wire i_ack,
   output wire [31:0] o_instr,
   output reg o_iscomp);

  localparam OPCODE_LOAD     = 7'h03;
  localparam OPCODE_OP_IMM   = 7'h13;
  localparam OPCODE_STORE    = 7'h23;
  localparam OPCODE_OP       = 7'h33;
  localparam OPCODE_LUI      = 7'h37;
  localparam OPCODE_BRANCH   = 7'h63;
  localparam OPCODE_JALR     = 7'h67;
  localparam OPCODE_JAL      = 7'h6f;

  reg  [31:0] comp_instr;
  reg  illegal_instr;

  assign o_instr = illegal_instr ? i_instr : comp_instr;

  always @(posedge i_clk) begin
    if(i_ack)
      o_iscomp <= !illegal_instr; 
  end

  always @ (*) begin
    // By default, forward incoming instruction, mark it as legal.
    comp_instr    = i_instr;
    illegal_instr = 1'b0;

    // Check if incoming instruction is compressed.
    case (i_instr[1:0])
      // C0
      2'b00: begin
        case (i_instr[15:14])
          2'b00: begin
            // c.addi4spn -> addi rd', x2, imm
            comp_instr = {2'b0, i_instr[10:7], i_instr[12:11], i_instr[5],
                      i_instr[6], 2'b00, 5'h02, 3'b000, 2'b01, i_instr[4:2], {OPCODE_OP_IMM}};
          end

          2'b01: begin
            // c.lw -> lw rd', imm(rs1')
            comp_instr = {5'b0, i_instr[5], i_instr[12:10], i_instr[6],
                      2'b00, 2'b01, i_instr[9:7], 3'b010, 2'b01, i_instr[4:2], {OPCODE_LOAD}};
          end

          2'b11: begin
            // c.sw -> sw rs2', imm(rs1')
            comp_instr = {5'b0, i_instr[5], i_instr[12], 2'b01, i_instr[4:2],
                      2'b01, i_instr[9:7], 3'b010, i_instr[11:10], i_instr[6],
                      2'b00, {OPCODE_STORE}};
          end

          2'b10: begin
            illegal_instr = 1'b1;
          end

        endcase
      end

      // C1
 
      // Register address checks for RV32E are performed in the regular instruction decoder.
      // If this check fails, an illegal instruction exception is triggered and the controller
      // writes the actual faulting instruction to mtval.
      2'b01: begin
        case (i_instr[15:13])
          3'b000: begin
            // c.addi -> addi rd, rd, nzimm
            // c.nop
            comp_instr = {{6 {i_instr[12]}}, i_instr[12], i_instr[6:2],
                      i_instr[11:7], 3'b0, i_instr[11:7], {OPCODE_OP_IMM}};
          end

          3'b001, 3'b101: begin
            // 001: c.jal -> jal x1, imm
            // 101: c.j   -> jal x0, imm
            comp_instr = {i_instr[12], i_instr[8], i_instr[10:9], i_instr[6],
                      i_instr[7], i_instr[2], i_instr[11], i_instr[5:3],
                      {9 {i_instr[12]}}, 4'b0, ~i_instr[15], {OPCODE_JAL}};
          end

          3'b010: begin
            // c.li -> addi rd, x0, nzimm
            // (c.li hints are translated into an addi hint)
            comp_instr = {{6 {i_instr[12]}}, i_instr[12], i_instr[6:2], 5'b0,
                      3'b0, i_instr[11:7], {OPCODE_OP_IMM}};
          end

          3'b011: begin
            // c.lui -> lui rd, imm
            // (c.lui hints are translated into a lui hint)
            comp_instr = {{15 {i_instr[12]}}, i_instr[6:2], i_instr[11:7], {OPCODE_LUI}};

            if (i_instr[11:7] == 5'h02) begin
              // c.addi16sp -> addi x2, x2, nzimm
              comp_instr = {{3 {i_instr[12]}}, i_instr[4:3], i_instr[5], i_instr[2],
                        i_instr[6], 4'b0, 5'h02, 3'b000, 5'h02, {OPCODE_OP_IMM}};
            end

          end

          3'b100: begin
            case (i_instr[11:10])
              2'b00,
              2'b01: begin
                // 00: c.srli -> srli rd, rd, shamt
                // 01: c.srai -> srai rd, rd, shamt
                // (c.srli/c.srai hints are translated into a srli/srai hint)
                comp_instr = {1'b0, i_instr[10], 5'b0, i_instr[6:2], 2'b01, i_instr[9:7],
                          3'b101, 2'b01, i_instr[9:7], {OPCODE_OP_IMM}};
              end

              2'b10: begin
                // c.andi -> andi rd, rd, imm
                comp_instr = {{6 {i_instr[12]}}, i_instr[12], i_instr[6:2], 2'b01, i_instr[9:7],
                          3'b111, 2'b01, i_instr[9:7], {OPCODE_OP_IMM}};
              end

              2'b11: begin
                case (i_instr[6:5])
                  2'b00: begin
                    // c.sub -> sub rd', rd', rs2'
                    comp_instr = {2'b01, 5'b0, 2'b01, i_instr[4:2], 2'b01, i_instr[9:7],
                                  3'b000, 2'b01, i_instr[9:7], {OPCODE_OP}};
                  end

                  2'b01: begin
                    // c.xor -> xor rd', rd', rs2'
                    comp_instr = {7'b0, 2'b01, i_instr[4:2], 2'b01, i_instr[9:7], 3'b100,
                              2'b01, i_instr[9:7], {OPCODE_OP}};
                  end

                  2'b10: begin
                    // c.or  -> or  rd', rd', rs2'
                    comp_instr = {7'b0, 2'b01, i_instr[4:2], 2'b01, i_instr[9:7], 3'b110,
                              2'b01, i_instr[9:7], {OPCODE_OP}};
                  end

                  2'b11: begin
                    // c.and -> and rd', rd', rs2'
                    comp_instr = {7'b0, 2'b01, i_instr[4:2], 2'b01, i_instr[9:7], 3'b111,
                              2'b01, i_instr[9:7], {OPCODE_OP}};
                  end
                endcase
              end
            endcase
          end

          3'b110, 3'b111: begin
            // 0: c.beqz -> beq rs1', x0, imm
            // 1: c.bnez -> bne rs1', x0, imm
            comp_instr = {{4 {i_instr[12]}}, i_instr[6:5], i_instr[2], 5'b0, 2'b01,
                      i_instr[9:7], 2'b00, i_instr[13], i_instr[11:10], i_instr[4:3],
                      i_instr[12], {OPCODE_BRANCH}};
          end
        endcase
      end

      // C2

      // Register address checks for RV32E are performed in the regular instruction decoder.
      // If this check fails, an illegal instruction exception is triggered and the controller
      // writes the actual faulting instruction to mtval.
      2'b10: begin
        case (i_instr[15:14])
          2'b00: begin
            // c.slli -> slli rd, rd, shamt
            // (c.ssli hints are translated into a slli hint)
            comp_instr = {7'b0, i_instr[6:2], i_instr[11:7], 3'b001, i_instr[11:7], {OPCODE_OP_IMM}};
          end

          2'b01: begin
            // c.lwsp -> lw rd, imm(x2)
            comp_instr = {4'b0, i_instr[3:2], i_instr[12], i_instr[6:4], 2'b00, 5'h02,
                      3'b010, i_instr[11:7], OPCODE_LOAD};
          end

          2'b10: begin
            if (i_instr[12] == 1'b0) begin
              if (i_instr[6:2] != 5'b0) begin
                // c.mv -> add rd/rs1, x0, rs2
                // (c.mv hints are translated into an add hint)
                comp_instr = {7'b0, i_instr[6:2], 5'b0, 3'b0, i_instr[11:7], {OPCODE_OP}};
              end else begin
                // c.jr -> jalr x0, rd/rs1, 0
                comp_instr = {12'b0, i_instr[11:7], 3'b0, 5'b0, {OPCODE_JALR}};
              end
            end else begin
              if (i_instr[6:2] != 5'b0) begin
                // c.add -> add rd, rd, rs2
                // (c.add hints are translated into an add hint)
                comp_instr = {7'b0, i_instr[6:2], i_instr[11:7], 3'b0, i_instr[11:7], {OPCODE_OP}};
              end else begin
                if (i_instr[11:7] == 5'b0) begin
                  // c.ebreak -> ebreak
                  comp_instr = {32'h00_10_00_73};
                end else begin
                  // c.jalr -> jalr x1, rs1, 0
                  comp_instr = {12'b0, i_instr[11:7], 3'b000, 5'b00001, {OPCODE_JALR}};
                end
              end
            end
          end

          2'b11: begin
            // c.swsp -> sw rs2, imm(x2)
            comp_instr = {4'b0, i_instr[8:7], i_instr[12], i_instr[6:2], 5'h02, 3'b010,
                      i_instr[11:9], 2'b00, {OPCODE_STORE}};
          end
        endcase
      end

      // Incoming instruction is not compressed.
      2'b11: illegal_instr = 1'b1;

    endcase
  end
  
  endmodule