cvw/wally-pipelined/src/ifu/twoBitPredictor.sv

///////////////////////////////////////////
// twoBitPredictor.sv
//
// Written: Ross Thomposn
// Email: ross1728@gmail.com
// Created: February 14, 2021
// Modified: 
//
// Purpose: 2 bit saturating counter predictor with parameterized table depth.
// 
// A component of the Wally configurable RISC-V project.
// 
// Copyright (C) 2021 Harvey Mudd College & Oklahoma State University
//
// Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation
// files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, 
// modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software 
// is furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES 
// OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS 
// BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT 
// OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
///////////////////////////////////////////

`include "wally-config.vh"

module twoBitPredictor
  #(parameter int Depth = 10
    )
  (input logic clk,
   input logic reset,
   input logic [`XLEN-1:0] LookUpPC,
   output logic [1:0] 	   Prediction,
   // update
   input logic [`XLEN-1:0] UpdatePC,
   input logic 		   UpdateEN,
   input logic [1:0] 	   UpdatePrediction
   );

  logic [Depth-1:0] 	   LookUpPCIndex, UpdatePCIndex;
  logic [1:0] 		   PredictionMemory;
  

  // hashing function for indexing the PC
  // We have Depth bits to index, but XLEN bits as the input.
  // bit 0 is always 0, bit 1 is 0 if using 4 byte instructions, but is not always 0 if
  // using compressed instructions.  XOR bit 1 with the MSB of index.
  assign UpdatePCIndex = {UpdatePC[Depth+1] ^ UpdatePC[1], UpdatePC[Depth:2]};
  assign LookUpPCIndex = {LookUpPC[Depth+1] ^ LookUpPC[1], LookUpPC[Depth:2]};  


  SRAM2P1R1W #(Depth, 2) memory(.clk(clk),
				.reset(reset),
				.RA1(LookUpPCIndex),
				.RD1(PredictionMemory),
				.REN1(1'b1),
				.WA1(UpdatePCIndex),
				.WD1(UpdatePrediction),
				.WEN1(UpdateEN),
				.BitWEN1(2'b11));

  // need to forward when updating to the same address as reading.
  assign Prediction = (UpdatePC == LookUpPC) ? UpdatePrediction : PredictionMemory;
  
endmodule