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// $Id: aeMB_bpcu.v,v 1.4 2007-11-14 22:14:34 sybreon Exp $

//

// AEMB BRANCH PROGRAMME COUNTER UNIT

// 

// Copyright (C) 2004-2007 Shawn Tan Ser Ngiap <shawn.tan@aeste.net>

//  

// This file is part of AEMB.

//

// AEMB is free software: you can redistribute it and/or modify it

// under the terms of the GNU Lesser General Public License as

// published by the Free Software Foundation, either version 3 of the

// License, or (at your option) any later version.

//

// AEMB is distributed in the hope that it will be useful, but WITHOUT

// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY

// or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General

// Public License for more details.

//

// You should have received a copy of the GNU Lesser General Public

// License along with AEMB. If not, see <http://www.gnu.org/licenses/>.

//

// $Log: not supported by cvs2svn $

// Revision 1.3  2007/11/10 16:39:38  sybreon

// Upgraded license to LGPLv3.

// Significant performance optimisations.

//

// Revision 1.2  2007/11/02 19:20:58  sybreon

// Added better (beta) interrupt support.

// Changed MSR_IE to disabled at reset as per MB docs.

//

// Revision 1.1  2007/11/02 03:25:39  sybreon

// New EDK 3.2 compatible design with optional barrel-shifter and multiplier.

// Fixed various minor data hazard bugs.

// Code compatible with -O0/1/2/3/s generated code.

//

`timescale  1ns/1ps

module aeMB_bpcu (/*AUTOARG*/

   // Outputs

   iwb_adr_o, rPC, rPCLNK, rBRA, rDLY,

   // Inputs

   rMXALT, rOPC, rRD, rRA, rRESULT, rDWBDI, rREGA, gclk, grst, gena

   );

   parameter IW = 24;

   // INST WISHBONE

   output [IW-1:2] iwb_adr_o;

   // INTERNAL

   output [31:2]   rPC, rPCLNK;

   output          rBRA;

   output          rDLY;

   //output [1:0]    rATOM;

   //output [1:0]    xATOM;

   input [1:0]      rMXALT;

   input [5:0]      rOPC;

   input [4:0]      rRD, rRA;

   input [31:0]    rRESULT; // ALU

   input [31:0]    rDWBDI; // RAM

   input [31:0]    rREGA;

   //input [1:0]           rXCE;   

   // SYSTEM

   input           gclk, grst, gena;

   // --- BRANCH CONTROL --------------------------------------------

   // Controls the branch and delay flags

   wire            fRTD = (rOPC == 6'o55);

   wire            fBCC = (rOPC == 6'o47) | (rOPC == 6'o57);

   wire            fBRU = (rOPC == 6'o46) | (rOPC == 6'o56);

   wire [31:0]      wREGA;

   assign          wREGA = (rMXALT == 2'o2) ? rDWBDI :

                           (rMXALT == 2'o1) ? rRESULT :

                           rREGA;

   wire            wBEQ = (wREGA == 32'd0);

   wire            wBNE = ~wBEQ;

   wire            wBLT = wREGA[31];

   wire            wBLE = wBLT | wBEQ;

   wire            wBGE = ~wBLT;

   wire            wBGT = ~wBLE;

   reg             xXCC;

   always @(/*AUTOSENSE*/rRD or wBEQ or wBGE or wBGT or wBLE or wBLT

            or wBNE)

     case (rRD[2:0])

       3'o0: xXCC <= wBEQ;

       3'o1: xXCC <= wBNE;

       3'o2: xXCC <= wBLT;

       3'o3: xXCC <= wBLE;

       3'o4: xXCC <= wBGT;

       3'o5: xXCC <= wBGE;

       default: xXCC <= 1'bX;

     endcase // case (rRD[2:0])

   reg             rBRA, xBRA;

   reg             rDLY, xDLY;

   wire            fSKIP = rBRA & !rDLY;

   always @(/*AUTOSENSE*/fBCC or fBRU or fRTD or rBRA or rRA or rRD

            or xXCC)

     //if (rBRA | |rXCE) begin

     if (rBRA) begin

        /*AUTORESET*/

        // Beginning of autoreset for uninitialized flops

        xBRA <= 1'h0;

        xDLY <= 1'h0;

        // End of automatics

     end else begin

        xDLY <= (fBRU & rRA[4]) | (fBCC & rRD[4]) | fRTD;

        xBRA <= (fRTD | fBRU) ? 1'b1 :

                (fBCC) ? xXCC :

                1'b0;

     end

   // --- PC PIPELINE ------------------------------------------------

   // PC and related changes

   reg [31:2]      rIPC, xIPC;

   reg [31:2]      rPC, xPC;

   reg [31:2]      rPCLNK, xPCLNK;

   assign          iwb_adr_o = rIPC[IW-1:2];

   always @(/*AUTOSENSE*/rBRA or rIPC or rPC or rRESULT) begin

      //xPCLNK <= (^rATOM) ? rPC : rPC;

      xPCLNK <= rPC;

      //xPC <= (^rATOM) ? rIPC : rRESULT[31:2]; 

      xPC <= rIPC;

      //xIPC <= (rBRA) ? rRESULT[31:2] : (rIPC + 1);

      /*

     case (rXCE)

       2'o1: xIPC <= 30'h2;

       2'o2: xIPC <= 30'h4;

       2'o3: xIPC <= 30'h6;

       default: xIPC <= (rBRA) ? rRESULT[31:2] : (rIPC + 1);

     endcase // case (rXCE)

       */

      xIPC <= (rBRA) ? rRESULT[31:2] : (rIPC + 1);

   end

   // --- ATOMIC CONTROL ---------------------------------------------

   // This is used to indicate 'safe' instruction borders.

   wire         wIMM = (rOPC == 6'o54) & !fSKIP;

   wire         wRTD = (rOPC == 6'o55) & !fSKIP;

   wire         wBCC = xXCC & ((rOPC == 6'o47) | (rOPC == 6'o57)) & !fSKIP;

   wire         wBRU = ((rOPC == 6'o46) | (rOPC == 6'o56)) & !fSKIP;

   wire         fATOM = ~(wIMM | wRTD | wBCC | wBRU | rBRA);

   reg [1:0]     rATOM, xATOM;

   always @(/*AUTOSENSE*/fATOM or rATOM)

     xATOM <= {rATOM[0], (rATOM[0] ^ fATOM)};

   // --- SYNC PIPELINE ----------------------------------------------

   always @(posedge gclk)

     if (grst) begin

        /*AUTORESET*/

        // Beginning of autoreset for uninitialized flops

        rATOM <= 2'h0;

        rBRA <= 1'h0;

        rDLY <= 1'h0;

        rIPC <= 30'h0;

        rPC <= 30'h0;

        rPCLNK <= 30'h0;

        // End of automatics

     end else if (gena) begin

        rIPC <= #1 xIPC;

        rBRA <= #1 xBRA;

        rPC <= #1 xPC;

        rPCLNK <= #1 xPCLNK;

        rDLY <= #1 xDLY;

        rATOM <= #1 xATOM;

     end

endmodule // aeMB_bpcu