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

//        __

//   \\__/ o\    (C) 2017-2018  Robert Finch, Waterloo

//    \  __ /    All rights reserved.

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

//       ||

//

//      FT64_fetchbuf.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/>.    

//

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

//

`include "FT64_config.vh"

`include "FT64_defines.vh"

// FETCH

//

// fetch exactly two instructions from memory into the fetch buffer

// unless either one of the buffers is still full, in which case we

// do nothing (kinda like alpha approach)

// Like to turn this into an independent module at some point.

//

module FT64_fetchbuf(rst, clk4x, clk, fcu_clk,

        cs_i, cyc_i, stb_i, ack_o, we_i, adr_i, dat_i,

        cmpgrp,

        freezePC, thread_en,

        regLR,

    insn0, insn1, phit,

    threadx,

    branchmiss, misspc, branchmiss_thrd, predict_taken0, predict_taken1,

    predict_takenA, predict_takenB, predict_takenC, predict_takenD,

    queued1, queued2, queuedNop,

    pc0, pc1, fetchbuf, fetchbufA_v, fetchbufB_v, fetchbufC_v, fetchbufD_v,

    fetchbufA_instr, fetchbufA_pc,

    fetchbufB_instr, fetchbufB_pc,

    fetchbufC_instr, fetchbufC_pc,

    fetchbufD_instr, fetchbufD_pc,

    fetchbuf0_instr, fetchbuf1_instr, fetchbuf0_insln, fetchbuf1_insln,

    fetchbuf0_thrd, fetchbuf1_thrd,

    fetchbuf0_pc, fetchbuf1_pc,

    fetchbuf0_v, fetchbuf1_v,

    codebuf0, codebuf1,

    btgtA, btgtB, btgtC, btgtD,

    nop_fetchbuf,

    take_branch0, take_branch1,

    stompedRets,

    panic

);

parameter AMSB = `AMSB;

parameter RSTPC = 32'hFFFC0100;

parameter TRUE = 1'b1;

parameter FALSE = 1'b0;

input rst;

input clk4x;

input clk;

input fcu_clk;

input cs_i;

input cyc_i;

input stb_i;

output ack_o;

input we_i;

input [15:0] adr_i;

input [47:0] dat_i;

input [2:0] cmpgrp;

input freezePC;

input thread_en;

input [4:0] regLR;

input [47:0] insn0;

input [47:0] insn1;

input phit;

output threadx;

input branchmiss;

input [AMSB:0] misspc;

input branchmiss_thrd;

output predict_taken0;

output predict_taken1;

input predict_takenA;

input predict_takenB;

input predict_takenC;

input predict_takenD;

input queued1;

input queued2;

input queuedNop;

output reg [AMSB:0] pc0;

output reg [AMSB:0] pc1;

output reg fetchbuf;

output reg fetchbufA_v;

output reg fetchbufB_v;

output reg fetchbufC_v;

output reg fetchbufD_v;

output fetchbuf0_thrd;

output fetchbuf1_thrd;

output reg [47:0] fetchbufA_instr;

output reg [47:0] fetchbufB_instr;

output reg [47:0] fetchbufC_instr;

output reg [47:0] fetchbufD_instr;

output reg [AMSB:0] fetchbufA_pc;

output reg [AMSB:0] fetchbufB_pc;

output reg [AMSB:0] fetchbufC_pc;

output reg [AMSB:0] fetchbufD_pc;

output [47:0] fetchbuf0_instr;

output [47:0] fetchbuf1_instr;

output [AMSB:0] fetchbuf0_pc;

output [AMSB:0] fetchbuf1_pc;

output [2:0] fetchbuf0_insln;

output [2:0] fetchbuf1_insln;

output fetchbuf0_v;

output fetchbuf1_v;

input [47:0] codebuf0;

input [47:0] codebuf1;

input [AMSB:0] btgtA;

input [AMSB:0] btgtB;

input [AMSB:0] btgtC;

input [AMSB:0] btgtD;

input [3:0] nop_fetchbuf;

output take_branch0;

output take_branch1;

input [3:0] stompedRets;

output reg [3:0] panic;

integer n;

//`include "FT64_decode.vh"

function IsBranch;

input [47:0] isn;

casex(isn[`INSTRUCTION_OP])

`Bcc:   IsBranch = TRUE;

`BBc:   IsBranch = TRUE;

`BEQI:  IsBranch = TRUE;

`BCHK:  IsBranch = TRUE;

default: IsBranch = FALSE;

endcase

endfunction

function IsJmp;

input [47:0] isn;

IsJmp = isn[`INSTRUCTION_OP]==`JMP;

endfunction

function IsCall;

input [47:0] isn;

IsCall = isn[`INSTRUCTION_OP]==`CALL;

endfunction

function IsRet;

input [47:0] isn;

IsRet = isn[`INSTRUCTION_OP]==`RET;

endfunction

function IsRTI;

input [47:0] isn;

IsRTI = isn[`INSTRUCTION_OP]==`R2 && isn[`INSTRUCTION_S2]==`RTI;

endfunction

function [2:0] fnInsLength;

input [47:0] ins;

if (ins[`INSTRUCTION_OP]==`CMPRSSD)

        fnInsLength = 3'd2;

else

        case(ins[7:6])

        2'd0:   fnInsLength = 3'd4;

        2'd1:   fnInsLength = 3'd6;

        default:        fnInsLength = 3'd2;

        endcase

endfunction

wire [2:0] fetchbufA_inslen;

wire [2:0] fetchbufB_inslen;

wire [2:0] fetchbufC_inslen;

wire [2:0] fetchbufD_inslen;

FT64_InsLength uilA (fetchbufA_instr, fetchbufA_inslen);

FT64_InsLength uilB (fetchbufB_instr, fetchbufB_inslen);

FT64_InsLength uilC (fetchbufC_instr, fetchbufC_inslen);

FT64_InsLength uilD (fetchbufD_instr, fetchbufD_inslen);

wire [47:0] xinsn0;

wire [47:0] xinsn1;

FT64_iexpander ux1

(

        .cinstr(insn0[15:0]),

        .expand(xinsn0)

);

FT64_iexpander ux2

(

        .cinstr(insn1[15:0]),

        .expand(xinsn1)

);

// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 

// Table of decompressed instructions.

// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 

assign ack_o = cs_i & cyc_i & stb_i;

reg [47:0] DecompressTable [0:2047];

always @(posedge clk)

        if (cs_i & cyc_i & stb_i & we_i)

                DecompressTable[adr_i[12:3]] <= dat_i[47:0];

wire [47:0] expand0 = DecompressTable[{cmpgrp,insn0[15:8]}];

wire [47:0] expand1 = DecompressTable[{cmpgrp,insn1[15:8]}];

// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 

// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 

reg thread;

reg stompedRet;

reg ret0Counted, ret1Counted;

wire [AMSB:0] retpc0, retpc1;

reg did_branchback0;

reg did_branchback1;

assign predict_taken0 = (fetchbuf==1'b0) ? predict_takenA : predict_takenC;

assign predict_taken1 = (fetchbuf==1'b0) ? predict_takenB : predict_takenD;

reg [AMSB:0] branch_pcA;

reg [AMSB:0] branch_pcB;

reg [AMSB:0] branch_pcC;

reg [AMSB:0] branch_pcD;

always @*

case(fetchbufA_instr[`INSTRUCTION_OP])

`RET:           branch_pcA = retpc0;

`JMP,`CALL: branch_pcA = fetchbufA_instr[6] ? {fetchbufA_instr[39:8],1'b0} : {fetchbufA_pc[31:25],fetchbufA_instr[31:8],1'b0};

`R2:            branch_pcA = btgtA;     // RTI

`BRK,`JAL:      branch_pcA = btgtA;

default:        branch_pcA = fetchbufA_pc + {{20{fetchbufA_instr[31]}},fetchbufA_instr[31:21],1'b0} + fetchbufA_inslen;

endcase

always @*

case(fetchbufB_instr[`INSTRUCTION_OP])

`RET:           branch_pcB = retpc1;

`JMP,`CALL: branch_pcB = fetchbufB_instr[6] ? {fetchbufB_instr[39:8],1'b0} : {fetchbufB_pc[31:25],fetchbufB_instr[31:8],1'b0};

`R2:            branch_pcB = btgtB;     // RTI

`BRK,`JAL:      branch_pcB = btgtB;

default:        branch_pcB = fetchbufB_pc + {{20{fetchbufB_instr[31]}},fetchbufB_instr[31:21],1'b0} + fetchbufB_inslen;

endcase

always @*

case(fetchbufC_instr[`INSTRUCTION_OP])

`RET:           branch_pcC = retpc0;

`JMP,`CALL: branch_pcC = fetchbufC_instr[6] ? {fetchbufC_instr[39:8],1'b0} : {fetchbufC_pc[31:25],fetchbufC_instr[31:8],1'b0};

`R2:            branch_pcC = btgtC;     // RTI

`BRK,`JAL:      branch_pcC = btgtC;

default:        branch_pcC = fetchbufC_pc + {{20{fetchbufC_instr[31]}},fetchbufC_instr[31:21],1'b0} + fetchbufC_inslen;

endcase

always @*

case(fetchbufD_instr[`INSTRUCTION_OP])

`RET:           branch_pcD = retpc1;

`JMP,`CALL: branch_pcD = fetchbufD_instr[6] ? {fetchbufD_instr[39:8],1'b0} : {fetchbufD_pc[31:25],fetchbufD_instr[31:8],1'b0};

`R2:            branch_pcD = btgtD;     // RTI

`BRK,`JAL:      branch_pcD = btgtD;

default:        branch_pcD = fetchbufD_pc + {{20{fetchbufD_instr[31]}},fetchbufD_instr[31:21],1'b0} + fetchbufD_inslen;

endcase

wire take_branchA = ({fetchbufA_v, IsBranch(fetchbufA_instr), predict_takenA}  == {`VAL, `TRUE, `TRUE}) ||

                        ((IsRet(fetchbufA_instr)||IsJmp(fetchbufA_instr)||IsCall(fetchbufA_instr)||

                        IsRTI(fetchbufA_instr)|| fetchbufA_instr[`INSTRUCTION_OP]==`BRK || fetchbufA_instr[`INSTRUCTION_OP]==`JAL) &&

                        fetchbufA_v);

wire take_branchB = ({fetchbufB_v, IsBranch(fetchbufB_instr), predict_takenB}  == {`VAL, `TRUE, `TRUE}) ||

                        ((IsRet(fetchbufB_instr)|IsJmp(fetchbufB_instr)|IsCall(fetchbufB_instr) ||

                        IsRTI(fetchbufB_instr)|| fetchbufB_instr[`INSTRUCTION_OP]==`BRK || fetchbufB_instr[`INSTRUCTION_OP]==`JAL) &&

                        fetchbufB_v);

wire take_branchC = ({fetchbufC_v, IsBranch(fetchbufC_instr), predict_takenC}  == {`VAL, `TRUE, `TRUE}) ||

                        ((IsRet(fetchbufC_instr)|IsJmp(fetchbufC_instr)|IsCall(fetchbufC_instr) ||

                        IsRTI(fetchbufC_instr)|| fetchbufC_instr[`INSTRUCTION_OP]==`BRK || fetchbufC_instr[`INSTRUCTION_OP]==`JAL) &&

                        fetchbufC_v);

wire take_branchD = ({fetchbufD_v, IsBranch(fetchbufD_instr), predict_takenD}  == {`VAL, `TRUE, `TRUE}) ||

                        ((IsRet(fetchbufD_instr)|IsJmp(fetchbufD_instr)|IsCall(fetchbufD_instr) ||

                        IsRTI(fetchbufD_instr)|| fetchbufD_instr[`INSTRUCTION_OP]==`BRK || fetchbufD_instr[`INSTRUCTION_OP]==`JAL) &&

                        fetchbufD_v);

assign take_branch0 = fetchbuf==1'b0 ? take_branchA : take_branchC;

assign take_branch1 = fetchbuf==1'b0 ? take_branchB : take_branchD;

wire take_branch = take_branch0 || take_branch1;

/*

always @*

begin

        pc0 <= thread_en ? (fetchbuf ? pc0b : pc0a) : pc0a;

        pc1 <= thread_en ? (fetchbuf ? pc1b : pc1a) : pc1a;

end

*/

assign threadx = fetchbuf;

`ifdef FCU_ENH

FT64_RSB #(AMSB) ursb1

(

        .rst(rst),

        .clk(fcu_clk),

        .regLR(regLR),

        .queued1(queued1),

        .queued2(queued2),

        .fetchbuf0_v(fetchbuf0_v),

        .fetchbuf0_pc(fetchbuf0_pc),

        .fetchbuf0_instr(fetchbuf0_instr),

        .fetchbuf1_v(fetchbuf1_v),

        .fetchbuf1_pc(fetchbuf1_pc),

        .fetchbuf1_instr(fetchbuf1_instr),

        .stompedRets(stompedRets),

        .stompedRet(stompedRet),

        .pc(retpc0)

);

FT64_RSB #(AMSB) ursb2

(

        .rst(rst),

        .clk(fcu_clk),

        .regLR(regLR),

        .queued1(queued1),

        .queued2(1'b0),

        .fetchbuf0_v(fetchbuf1_v),

        .fetchbuf0_pc(fetchbuf1_pc),

        .fetchbuf0_instr(fetchbuf1_instr),

        .fetchbuf1_v(1'b0),

        .fetchbuf1_pc(32'h00000000),

        .fetchbuf1_instr(`NOP_INSN),

        .stompedRets(stompedRets[3:1]),

        .stompedRet(stompedRet),

        .pc(retpc1)

);

`else

assign retpc0 = RSTPC;

assign retpc1 = RSTPC;

`endif

wire peclk, neclk;

edge_det ued1 (.rst(rst), .clk(clk4x), .ce(1'b1), .i(clk), .pe(peclk), .ne(neclk), .ee());

always @(posedge clk)

if (rst) begin

        pc0 <= RSTPC;

  pc1 <= RSTPC;

        fetchbufA_v <= 0;

        fetchbufB_v <= 0;

        fetchbufC_v <= 0;

        fetchbufD_v <= 0;

        fetchbuf <= 0;

        panic <= `PANIC_NONE;

end

else begin

        did_branchback0 <= take_branch0;

        did_branchback1 <= take_branch1;

        stompedRet = FALSE;

        begin

        // On a branch miss with threading enabled all fectch buffers are

        // invalidated even though the data in the fetch buffer would be valid

        // for the thread that isn't in a branchmiss state. This is done to

        // keep things simple. For the thread that doesn't miss the current

        // data for the fetch buffer needs to be retrieved again, so the pc

        // for that thread is assigned the current fetchbuf pc.

        // For the thread that misses the pc is simply assigned the misspc.

        if (branchmiss) begin

                $display("***********");

                $display("Branch miss");

                $display("***********");

                if (branchmiss_thrd) begin

                        pc0 <= fetchbuf0_pc;

`ifdef SUPPORT_SMT

                        pc1 <= misspc;

`endif

                end

                else begin

                        pc0 <= misspc;

`ifdef SUPPORT_SMT

                        pc1 <= fetchbuf1_pc;

`endif

                end

                fetchbufA_v <= `INV;

                fetchbufB_v <= `INV;

                fetchbufC_v <= `INV;

                fetchbufD_v <= `INV;

                fetchbuf <= 1'b0;

             $display("********************");

             $display("********************");

             $display("********************");

             $display("Branch miss");

             $display("misspc=%h", misspc);

             $display("********************");

             $display("********************");

             $display("********************");

        end

        // Some of the testing for valid branch conditions has been removed. In real

        // hardware it isn't needed, and just increases the size of the core. It's

        // assumed that the hardware is working.

        // The risk is an error will occur during simulation and go missed.

        else if (take_branch) begin

            // update the fetchbuf valid bits as well as fetchbuf itself

            // ... this must be based on which things are backwards branches, how many things

            // will get enqueued (0, 1, or 2), and how old the instructions are

            if (fetchbuf == 1'b0) case ({fetchbufA_v, fetchbufB_v, fetchbufC_v, fetchbufD_v})

                4'b0000 : ;     // do nothing

//              4'b0001 : panic <= `PANIC_INVALIDFBSTATE;

//              4'b0010 : panic <= `PANIC_INVALIDFBSTATE;

//              4'b0011 : panic <= `PANIC_INVALIDFBSTATE;       // this looks like it might be screwy fetchbuf logic

                // because the first instruction has been enqueued, 

                // we must have noted this in the previous cycle.

                // therefore, pc0 and pc1 have to have been set appropriately ... so do a regular fetch

                // this looks like the following:

                //   cycle 0 - fetched a INSTR+BEQ, with fbB holding a branchback

                //   cycle 1 - enqueued fbA, stomped on fbB, stalled fetch + updated pc0/pc1

                //   cycle 2 - where we are now ... fetch the two instructions & update fetchbufB_v appropriately

                4'b0100 :

                    begin

                            FetchCD();

                             fetchbufB_v <= !(queued1|queuedNop);       // if it can be queued, it will

                              fetchbuf <= fetchbuf + (queued1|queuedNop);

                        end

                // Can occur with threading enabled

                4'b0101:

                        fetchbufB_v <= !(queued1|queuedNop);

//              4'b0101 : panic <= `PANIC_INVALIDFBSTATE;

//              4'b0110 : panic <= `PANIC_INVALIDFBSTATE;

                // this looks like the following:

                //   cycle 0 - fetched an INSTR+BEQ, with fbB holding a branchback

                //   cycle 1 - enqueued fbA, but not fbB, recognized branchback in fbB, stalled fetch + updated pc0/pc1

                //   cycle 2 - still could not enqueue fbB, but fetched from backwards target

                //   cycle 3 - where we are now ... update fetchbufB_v appropriately

                //

                // however -- if there are backwards branches in the latter two slots, it is more complex.

                // simple solution: leave it alone and wait until we are through with the first two slots.

                4'b0111 :

                        begin

                            fetchbufB_v <= !(queued1|queuedNop);        // if it can be queued, it will

                                fetchbuf <= fetchbuf + (queued1|queuedNop);

                        end

                // this looks like the following:

                //   cycle 0 - fetched a BEQ+INSTR, with fbA holding a branchback

                //   cycle 1 - stomped on fbB, but could not enqueue fbA, stalled fetch + updated pc0/pc1

                //   cycle 2 - where we are now ... fetch the two instructions & update fetchbufA_v appropriately

                4'b1000 :

                        begin

                            FetchCD();

                             fetchbufA_v <= !(queued1|queuedNop);       // if it can be queued, it will

                              fetchbuf <= fetchbuf + (queued1|queuedNop);

                        end

//              4'b1001 : panic <= `PANIC_INVALIDFBSTATE;

//              4'b1010 : panic <= `PANIC_INVALIDFBSTATE;

                // this looks like the following:

                //   cycle 0 - fetched a BEQ+INSTR, with fbA holding a branchback

                //   cycle 1 - stomped on fbB, but could not enqueue fbA, stalled fetch + updated pc0/pc1

                //   cycle 2 - still could not enqueue fbA, but fetched from backwards target

                //   cycle 3 - where we are now ... set fetchbufA_v appropriately

                //

                // however -- if there are backwards branches in the latter two slots, it is more complex.

                // simple solution: leave it alone and wait until we are through with the first two slots.

                4'b1011 :

                        begin

                             fetchbufA_v <=!(queued1|queuedNop);        // if it can be queued, it will

                              fetchbuf <= fetchbuf + (queued1|queuedNop);

                        end

                // if fbB has the branchback, can't immediately tell which of the following scenarios it is:

                //   cycle 0 - fetched a pair of instructions, one or both of which is a branchback

                //   cycle 1 - where we are now.  stomp, enqueue, and update pc0/pc1

                // or

                //   cycle 0 - fetched a INSTR+BEQ, with fbB holding a branchback

                //   cycle 1 - could not enqueue fbA or fbB, stalled fetch + updated pc0/pc1

                //   cycle 2 - where we are now ... fetch the two instructions & update fetchbufX_v appropriately

                // if fbA has the branchback, then it is scenario 1.

                // if fbB has it: if pc0 == fbB_pc, then it is the former scenario, else it is the latter

                4'b1100:

                        if (thread_en) begin

                                if (take_branchA && take_branchB) begin

                                        pc0 <= branch_pcA;

                                        pc1 <= branch_pcB;

                                        fetchbufA_v <= !(queued1|queuedNop);    // if it can be queued, it will

                                        fetchbufB_v <= !(queued2|queuedNop);    // if it can be queued, it will

                                        if ((queued2|queuedNop))   fetchbuf <= 1'b1;

                                end

                                else if (take_branchA) begin

                                        pc0 <= branch_pcA;

                                        fetchbufA_v <= !(queued1|queuedNop);    // if it can be queued, it will

                                        fetchbufB_v <= !(queued2|queuedNop);    // if it can be queued, it will

                                        if ((queued2|queuedNop))   fetchbuf <= 1'b1;

                                end

                                else if (take_branchB) begin

                                        pc1 <= branch_pcB;

                                        fetchbufA_v <= !(queued1|queuedNop);    // if it can be queued, it will

                                        fetchbufB_v <= !(queued2|queuedNop);    // if it can be queued, it will

                                        if ((queued2|queuedNop))   fetchbuf <= 1'b1;

                                end

                        end

                        else begin

                                if (take_branchA) begin

                                        pc0 <= branch_pcA;

                                        fetchbufA_v <= !(queued1|queuedNop);    // if it can be queued, it will

                                        fetchbufB_v <= `INV;

                                        if ((queued1|queuedNop))   fetchbuf <= 1'b1;

                                end

                                else begin

                                        if (did_branchback0) begin

                                                FetchCD();

                                                fetchbufA_v <= !(queued1|queuedNop);    // if it can be queued, it will

                                                fetchbufB_v <= !(queued2|queuedNop);    // if it can be queued, it will

                                                fetchbuf <= fetchbuf + ((queued2|queuedNop));

                                        end

                                        else begin

                                                pc0 <= branch_pcB;

                                                fetchbufA_v <= !(queued1|queuedNop);    // if it can be queued, it will

                                                fetchbufB_v <= !(queued2|queuedNop);    // if it can be queued, it will

                                                if ((queued2|queuedNop))   fetchbuf <= 1'b1;

                                        end

                    end

                  end

//              4'b1101 : panic <= `PANIC_INVALIDFBSTATE;

//              4'b1110 : panic <= `PANIC_INVALIDFBSTATE;

                // this looks like the following:

                //   cycle 0 - fetched an INSTR+BEQ, with fbB holding a branchback

                //   cycle 1 - enqueued neither fbA nor fbB, recognized branchback in fbB, stalled fetch + updated pc0/pc1

                //   cycle 2 - still could not enqueue fbB, but fetched from backwards target

                //   cycle 3 - where we are now ... update fetchbufX_v appropriately

                //

                // however -- if there are backwards branches in the latter two slots, it is more complex.

                // simple solution: leave it alone and wait until we are through with the first two slots.

                4'b1111 :

                        begin

                             fetchbufA_v <= !(queued1|queuedNop);       // if it can be queued, it will

                             fetchbufB_v <= !(queued2|queuedNop);       // if it can be queued, it will

                              fetchbuf <= fetchbuf + (queued2|queuedNop);

                        end

        default:    ;

            endcase

            else case ({fetchbufC_v, fetchbufD_v, fetchbufA_v, fetchbufB_v})

                4'b0000 : ; // do nothing

//              4'b0001 : panic <= `PANIC_INVALIDFBSTATE;

//              4'b0010 : panic <= `PANIC_INVALIDFBSTATE;

//              4'b0011 : panic <= `PANIC_INVALIDFBSTATE;       // this looks like it might be screwy fetchbuf logic

                // because the first instruction has been enqueued, 

                // we must have noted this in the previous cycle.

                // therefore, pc0 and pc1 have to have been set appropriately ... so do a regular fetch

                // this looks like the following:

                //   cycle 0 - fetched a INSTR+BEQ, with fbD holding a branchback

                //   cycle 1 - enqueued fbC, stomped on fbD, stalled fetch + updated pc0/pc1

                //   cycle 2 - where we are now ... fetch the two instructions & update fetchbufB_v appropriately

                4'b0100 :

                        begin

                            FetchAB();

                             fetchbufD_v <= !(queued1|queuedNop);       // if it can be queued, it will

                              fetchbuf <= fetchbuf + (queued1|queuedNop);

                        end

                4'b0101:

                        fetchbufD_v <= !(queued1|queuedNop);

//              4'b0101 : panic <= `PANIC_INVALIDFBSTATE;

//              4'b0110 : panic <= `PANIC_INVALIDFBSTATE;

                // this looks like the following:

                //   cycle 0 - fetched an INSTR+BEQ, with fbD holding a branchback

                //   cycle 1 - enqueued fbC, but not fbD, recognized branchback in fbD, stalled fetch + updated pc0/pc1

                //   cycle 2 - still could not enqueue fbD, but fetched from backwards target

                //   cycle 3 - where we are now ... update fetchbufD_v appropriately

                //

                // however -- if there are backwards branches in the latter two slots, it is more complex.

                // simple solution: leave it alone and wait until we are through with the first two slots.

                4'b0111 :

                        begin

                             fetchbufD_v <= !(queued1|queuedNop);       // if it can be queued, it will

                              fetchbuf <= fetchbuf + (queued1|queuedNop);

                        end

                // this looks like the following:

                //   cycle 0 - fetched a BEQ+INSTR, with fbC holding a branchback

                //   cycle 1 - stomped on fbD, but could not enqueue fbC, stalled fetch + updated pc0/pc1

                //   cycle 2 - where we are now ... fetch the two instructions & update fetchbufC_v appropriately

                4'b1000 :

                        begin

                            FetchAB();

                             fetchbufC_v <= !(queued1|queuedNop);       // if it can be queued, it will

                              fetchbuf <= fetchbuf + (queued1|queuedNop);

                        end

//              4'b1001 : panic <= `PANIC_INVALIDFBSTATE;

//              4'b1010 : panic <= `PANIC_INVALIDFBSTATE;

                // this looks like the following:

                //   cycle 0 - fetched a BEQ+INSTR, with fbC holding a branchback

                //   cycle 1 - stomped on fbD, but could not enqueue fbC, stalled fetch + updated pc0/pc1

                //   cycle 2 - still could not enqueue fbC, but fetched from backwards target

                //   cycle 3 - where we are now ... set fetchbufC_v appropriately

                //

                // however -- if there are backwards branches in the latter two slots, it is more complex.

                // simple solution: leave it alone and wait until we are through with the first two slots.

                4'b1011 :

                        begin

                             fetchbufC_v <= !(queued1|queuedNop);       // if it can be queued, it will

                              fetchbuf <= fetchbuf + (queued1|queuedNop);

                        end

                // if fbD has the branchback, can't immediately tell which of the following scenarios it is:

                //   cycle 0 - fetched a pair of instructions, one or both of which is a branchback

                //   cycle 1 - where we are now.  stomp, enqueue, and update pc0/pc1

                // or

                //   cycle 0 - fetched a INSTR+BEQ, with fbD holding a branchback

                //   cycle 1 - could not enqueue fbC or fbD, stalled fetch + updated pc0/pc1