ISA

step4.v

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+/**
+ * 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  rst_i,      // reset button
+    output [4:0] led, // system LEDs
+    input  RXD,        // UART receive
+    output TXD         // UART transmit
+);
+
+   wire clk_i;    // 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_i) 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 led = isSYSTEM ? 31 : {PC[0],isALUreg,isALUimm,isStore,isLoad};
+   
+`ifdef BENCH   
+   always @(posedge clk_i) 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(rst_i),
+     .clk(clk_i),
+     .resetn(resetn)
+   );
+   
+   assign TXD  = 1'b0; // not used for now   
+endmodule
+