next_pc.v

`timescale 1ns / 1ps

// in stage ID

module next_pc(
  input [31:0] old, // IF/ID.pc
  input [31:0] instru, // IF/ID.instruction
    // [15-0] used for sign-extention
    // [25-0] used for shift-left-2
  input Jump, // ID.control
  input Branch,
  input Bne,
  input zero, // it now depends on the RG. comparator.
  output reg [31:0] next,
  output reg c_if_flush // IF.Flush
);

  // no connection with Hazard-detection

  reg [31:0] sign_ext;
  reg [31:0] old_alter; // pc+4
  reg [31:0] jump; // jump addr.
  reg zero_alter;

  initial begin
    next = 32'b0;
    // $display("NEXT: 0x%H",next);
  end

  always @(old) begin
    old_alter = old + 4;
  end

  always @(zero,Bne) begin
    zero_alter = zero;
    if (Bne == 1) begin
      zero_alter = ! zero_alter;
    end
    // $display("zero: %d | zero_alter: %d",zero,zero_alter);
  end

  always @(instru) begin
    // jump-shift-left
    jump = {4'b0,instru[25:0],2'b0};

    // sign-extension
    // $display("original instru: 0x%H",instru);
    if (instru[15] == 1'b0) begin
      // $display("next_pc:: positive addr_dev");
      sign_ext = {16'b0,instru[15:0]};
    end else begin
      // $display("next_pc:: negative addr_dev");
      sign_ext = {{16{1'b1}},instru[15:0]};
    end
    // $display("sign_ext: 0x%H",sign_ext);
    sign_ext = {sign_ext[29:0],2'b0}; // shift left
    // $display("sign_ext: 0x%H",sign_ext);
  end

  always @(instru or old_alter or jump) begin
    jump = {old_alter[31:28],jump[27:0]};
  end
  
  always @(*) begin
    // assign next program counter value
    if (Jump == 1) begin
      // $display("Taking jump");
      next = jump;
      c_if_flush = 1;
    end else begin
      if (Branch == 1 & zero_alter == 1) begin
        // $display("Taking branch");
        next = old_alter + sign_ext;
        c_if_flush = 1;
      end else begin
        // $display("Normal proceeding");
        next = old_alter;
        c_if_flush = 0;
      end
    end
  end

endmodule