/**
 * Step 4: Creating a RISC-V processor
 *         The instruction decoder
 * central LED blinks, other LEDs show instr type.
 * DONE*
 */

`default_nettype none
`include "clockworks.v"

module SOC (
    input  CLK,        // system clock 
    input  RESET,      // reset button
    output [4:0] LEDS, // system LEDs
    input  RXD,        // UART receive
    output TXD         // UART transmit
);

   wire clk;    // internal clock
   wire resetn; // internal reset signal, goes low on reset
   
   reg [31:0] MEM [0:255]; 
   reg [31:0] PC;       // program counter
   reg [31:0] instr;    // current instruction
   
   initial begin
      PC = 0;
      // add x0, x0, x0
      //                   rs2   rs1  add  rd   ALUREG
      instr = 32'b0000000_00000_00000_000_00000_0110011;
      // add x1, x0, x0
      //                    rs2   rs1  add  rd  ALUREG
      MEM[0] = 32'b0000000_00000_00000_000_00001_0110011;
      // addi x1, x1, 1
      //             imm         rs1  add  rd   ALUIMM
      MEM[1] = 32'b000000000001_00001_000_00001_0010011;
      // addi x1, x1, 1
      //             imm         rs1  add  rd   ALUIMM
      MEM[2] = 32'b000000000001_00001_000_00001_0010011;
      // addi x1, x1, 1
      //             imm         rs1  add  rd   ALUIMM
      MEM[3] = 32'b000000000001_00001_000_00001_0010011;
      // addi x1, x1, 1
      //             imm         rs1  add  rd   ALUIMM
      MEM[4] = 32'b000000000001_00001_000_00001_0010011;
      // lw x2,0(x1)
      //             imm         rs1   w   rd   LOAD
      MEM[5] = 32'b000000000000_00001_010_00010_0000011;
      // sw x2,0(x1)
      //             imm   rs2   rs1   w   imm  STORE
      MEM[6] = 32'b000000_00010_00001_010_00000_0100011;
      
      // ebreak
      //                                        SYSTEM
      MEM[7] = 32'b000000000001_00000_000_00000_1110011;
      
   end


   // See the table P. 105 in RISC-V manual
   
   // The 10 RISC-V instructions
   wire isALUreg  =  (instr[6:0] == 7'b0110011); // rd <- rs1 OP rs2   
   wire isALUimm  =  (instr[6:0] == 7'b0010011); // rd <- rs1 OP Iimm
   wire isBranch  =  (instr[6:0] == 7'b1100011); // if(rs1 OP rs2) PC<-PC+Bimm
   wire isJALR    =  (instr[6:0] == 7'b1100111); // rd <- PC+4; PC<-rs1+Iimm
   wire isJAL     =  (instr[6:0] == 7'b1101111); // rd <- PC+4; PC<-PC+Jimm
   wire isAUIPC   =  (instr[6:0] == 7'b0010111); // rd <- PC + Uimm
   wire isLUI     =  (instr[6:0] == 7'b0110111); // rd <- Uimm   
   wire isLoad    =  (instr[6:0] == 7'b0000011); // rd <- mem[rs1+Iimm]
   wire isStore   =  (instr[6:0] == 7'b0100011); // mem[rs1+Simm] <- rs2
   wire isSYSTEM  =  (instr[6:0] == 7'b1110011); // special

   // The 5 immediate formats
   wire [31:0] Uimm={    instr[31],   instr[30:12], {12{1'b0}}};
   wire [31:0] Iimm={{21{instr[31]}}, instr[30:20]};
   wire [31:0] Simm={{21{instr[31]}}, instr[30:25],instr[11:7]};
   wire [31:0] Bimm={{20{instr[31]}}, instr[7],instr[30:25],instr[11:8],1'b0};
   wire [31:0] Jimm={{12{instr[31]}}, instr[19:12],instr[20],instr[30:21],1'b0};

   // Source and destination registers
   wire [4:0] rs1Id = instr[19:15];
   wire [4:0] rs2Id = instr[24:20];
   wire [4:0] rdId  = instr[11:7];

   // function codes
   wire [2:0] funct3 = instr[14:12];
   wire [6:0] funct7 = instr[31:25];
   
   always @(posedge clk) begin
      if(!resetn) begin
	 PC <= 0;
	 instr <= 32'b0000000_00000_00000_000_00000_0110011; // NOP
      end else if(!isSYSTEM) begin
	 instr <= MEM[PC];
	 PC <= PC+1;
      end
`ifdef BENCH      
      if(isSYSTEM) $finish();
`endif      
   end

   assign LEDS = isSYSTEM ? 31 : {PC[0],isALUreg,isALUimm,isStore,isLoad};
   
`ifdef BENCH   
   always @(posedge clk) begin
      $display("PC=%0d",PC);
      case (1'b1)
	isALUreg: $display(
	      "ALUreg rd=%d rs1=%d rs2=%d funct3=%b",
              rdId, rs1Id, rs2Id, funct3
        );
	isALUimm: $display(
	       "ALUimm rd=%d rs1=%d imm=%0d funct3=%b",
               rdId, rs1Id, Iimm, funct3
        );
	isBranch: $display("BRANCH");
	isJAL:    $display("JAL");
	isJALR:   $display("JALR");
	isAUIPC:  $display("AUIPC");
	isLUI:    $display("LUI");	
	isLoad:   $display("LOAD");
	isStore:  $display("STORE");
	isSYSTEM: $display("SYSTEM");
      endcase 
   end
`endif

   // Gearbox and reset circuitry.
   Clockworks #(
     .SLOW(21) // Divide clock frequency by 2^21
   )CW(
     .CLK(CLK),
     .RESET(RESET),
     .clk(clk),
     .resetn(resetn)
   );
   
   assign TXD  = 1'b0; // not used for now   
endmodule