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//=============================================================================

//        __

//   \\__/ o\    (C) 2013-2018  Robert Finch, Waterloo

//    \  __ /    All rights reserved.

//     \/_//     robfinch<remove>@finitron.ca

//       ||

//  

//      FT64_BranchPredictor.v

//

//  

// This source file 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.                                      

//                                                                          

// This source file 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 General Public License for more details.                             

//                                                                          

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

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

//                                                                          

//

//=============================================================================

//

module FT64_BranchPredictor(rst, clk, en,

    xisBranch0, xisBranch1,

    pcA, pcB, pcC, pcD, xpc0, xpc1, takb0, takb1,

    predict_takenA, predict_takenB, predict_takenC, predict_takenD);

parameter DBW=32;

input rst;

input clk;

input en;

input xisBranch0;

input xisBranch1;

input [DBW-1:0] pcA;

input [DBW-1:0] pcB;

input [DBW-1:0] pcC;

input [DBW-1:0] pcD;

input [DBW-1:0] xpc0;

input [DBW-1:0] xpc1;

input takb0;

input takb1;

output predict_takenA;

output predict_takenB;

output predict_takenC;

output predict_takenD;

integer n;

reg [31:0] pcs [0:31];

reg [31:0] pc;

reg takb;

reg [4:0] pcshead,pcstail;

reg wrhist;

reg [2:0] gbl_branch_hist;

reg [1:0] branch_history_table [511:0];

// For simulation only, initialize the history table to zeros.

// In the real world we don't care.

initial begin

    gbl_branch_hist = 3'b000;

        for (n = 0; n < 512; n = n + 1)

                branch_history_table[n] = 3;

end

wire [8:0] bht_wa = {pc[8:2],gbl_branch_hist[2:1]};              // write address

wire [8:0] bht_raA = {pcA[8:2],gbl_branch_hist[2:1]};    // read address (IF stage)

wire [8:0] bht_raB = {pcB[8:2],gbl_branch_hist[2:1]};    // read address (IF stage)

wire [8:0] bht_raC = {pcC[8:2],gbl_branch_hist[2:1]};    // read address (IF stage)

wire [8:0] bht_raD = {pcD[8:2],gbl_branch_hist[2:1]};    // read address (IF stage)

wire [1:0] bht_xbits = branch_history_table[bht_wa];

wire [1:0] bht_ibitsA = branch_history_table[bht_raA];

wire [1:0] bht_ibitsB = branch_history_table[bht_raB];

wire [1:0] bht_ibitsC = branch_history_table[bht_raC];

wire [1:0] bht_ibitsD = branch_history_table[bht_raD];

assign predict_takenA = (bht_ibitsA==2'd0 || bht_ibitsA==2'd1) && en;

assign predict_takenB = (bht_ibitsB==2'd0 || bht_ibitsB==2'd1) && en;

assign predict_takenC = (bht_ibitsC==2'd0 || bht_ibitsC==2'd1) && en;

assign predict_takenD = (bht_ibitsD==2'd0 || bht_ibitsD==2'd1) && en;

always @(posedge clk)

if (rst)

        pcstail <= 5'd0;

else begin

        if (xisBranch0 & xisBranch1) begin

                pcs[pcstail] <= {xpc0[31:1],takb0};

                pcs[pcstail+1] <= {xpc1[31:1],takb1};

                pcstail <= pcstail + 5'd2;

        end

        else if (xisBranch0) begin

                pcs[pcstail] <= {xpc0[31:1],takb0};

                pcstail <= pcstail + 5'd1;

        end

        else if (xisBranch1) begin

                pcs[pcstail] <= {xpc1[31:1],takb1};

                pcstail <= pcstail + 5'd1;

        end

end

always @(posedge clk)

if (rst)

        pcshead <= 5'd0;

else begin

        wrhist <= 1'b0;

        if (pcshead != pcstail) begin

                pc <= pcs[pcshead];

                takb <= pcs[pcshead][0];

                wrhist <= 1'b1;

                pcshead <= pcshead + 5'd1;

        end

end

// Two bit saturating counter

// If taking a branch in commit0 then a following branch

// in commit1 is never encountered. So only update for

// commit1 if commit0 is not taken.

reg [1:0] xbits_new;

always @*

if (takb & wrhist) begin

        if (bht_xbits != 2'd1)

                xbits_new <= bht_xbits + 2'd1;

        else

                xbits_new <= bht_xbits;

end

else begin

        if (bht_xbits != 2'd2)

                xbits_new <= bht_xbits - {1'b0,wrhist};

        else

                xbits_new <= bht_xbits;

end

always @(posedge clk)

if (rst)

        gbl_branch_hist <= 3'b000;

else begin

    if (en) begin

        if (wrhist) begin

            gbl_branch_hist <= {gbl_branch_hist[1:0],takb};

            branch_history_table[bht_wa] <= xbits_new;

        end

        end

end

endmodule