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////////////////////////////////////////////////////////////////////////////////

//

// Filename:    pfcache.v

//

// Project:     Zip CPU -- a small, lightweight, RISC CPU soft core

//

// Purpose:     Keeping our CPU fed with instructions, at one per clock and

//              with no stalls.  An unusual feature of this cache is the

//      requirement that the entire cache may be cleared (if necessary).

//

// Creator:     Dan Gisselquist, Ph.D.

//              Gisselquist Technology, LLC

//

////////////////////////////////////////////////////////////////////////////////

//

// Copyright (C) 2015-2016, Gisselquist Technology, LLC

//

// This program is free software (firmware): you can redistribute it and/or

// modify it under the terms of  the GNU General Public License as published

// by the Free Software Foundation, either version 3 of the License, or (at

// your option) any later version.

//

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

// ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or

// FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

// for more details.

//

// License:     GPL, v3, as defined and found on www.gnu.org,

//              http://www.gnu.org/licenses/gpl.html

//

//

////////////////////////////////////////////////////////////////////////////////

//

module  pfcache(i_clk, i_rst, i_new_pc, i_clear_cache,

                        // i_early_branch, i_from_addr,

                        i_stall_n, i_pc, o_i, o_pc, o_v,

                o_wb_cyc, o_wb_stb, o_wb_we, o_wb_addr, o_wb_data,

                        i_wb_ack, i_wb_stall, i_wb_err, i_wb_data,

                        o_illegal);

        parameter       LGCACHELEN = 8, ADDRESS_WIDTH=24,

                        LGLINES=5; // Log of the number of separate cache lines

        localparam      CACHELEN=(1<<LGCACHELEN); // Size of our cache memory

        localparam      CW=LGCACHELEN;  // Short hand for LGCACHELEN

        localparam      PW=LGCACHELEN-LGLINES; // Size of a cache line

        localparam      BUSW = 32;      // Number of data lines on the bus

        localparam      AW=ADDRESS_WIDTH; // Shorthand for ADDRESS_WIDTH

        input                           i_clk, i_rst, i_new_pc;

        input                           i_clear_cache;

        input                           i_stall_n;

        input           [(AW-1):0]       i_pc;

        output  wire    [(BUSW-1):0]     o_i;

        output  wire    [(AW-1):0]       o_pc;

        output  wire                    o_v;

        //

        output  reg             o_wb_cyc, o_wb_stb;

        output  wire            o_wb_we;

        output  reg     [(AW-1):0]       o_wb_addr;

        output  wire    [(BUSW-1):0]     o_wb_data;

        //

        input                           i_wb_ack, i_wb_stall, i_wb_err;

        input           [(BUSW-1):0]     i_wb_data;

        //

        output  reg                     o_illegal;

        // Fixed bus outputs: we read from the bus only, never write.

        // Thus the output data is ... irrelevant and don't care.  We set it

        // to zero just to set it to something.

        assign  o_wb_we = 1'b0;

        assign  o_wb_data = 0;

        wire                    r_v;

        reg     [(BUSW-1):0]     cache   [0:((1<<CW)-1)];

        reg     [(AW-CW-1):0]    tags    [0:((1<<(LGLINES))-1)];

        reg     [((1<<(LGLINES))-1):0]   vmask;

        reg     [(AW-1):0]       lastpc;

        reg     [(CW-1):0]       rdaddr;

        reg     [(AW-1):CW]     tagvalipc, tagvallst;

        wire    [(AW-1):CW]     tagval;

        wire    [(AW-1):PW]     lasttag;

        reg                     illegal_valid;

        reg     [(AW-1):PW]     illegal_cache;

        // initial      o_i = 32'h76_00_00_00;  // A NOOP instruction

        // initial      o_pc = 0;

        reg     [(BUSW-1):0]     r_pc_cache, r_last_cache;

        reg     [(AW-1):0]       r_pc, r_lastpc;

        reg     isrc;

        always @(posedge i_clk)

        begin

                // We don't have the logic to select what to read, we must

                // read both the value at i_pc and lastpc.  cache[i_pc] is

                // the value we return if the cache is good, cacne[lastpc] is

                // the value we return if we've been stalled, weren't valid,

                // or had to wait a clock or two.  (Remember i_pc can't stop

                // changing for a clock, so we need to keep track of the last

                // one from before it stopped.)

                //

                // Here we keep track of which answer we want/need

                isrc <= ((r_v)&&(i_stall_n))||(i_new_pc);

                // Here we read both, and select which was write using isrc

                // on the next clock.

                r_pc_cache <= cache[i_pc[(CW-1):0]];

                r_last_cache <= cache[lastpc[(CW-1):0]];

                r_pc <= i_pc;

                r_lastpc <= lastpc;

        end

        assign  o_pc = (isrc) ? r_pc : r_lastpc;

        assign  o_i  = (isrc) ? r_pc_cache : r_last_cache;

        reg     tagsrc;

        always @(posedge i_clk)

                // It may be possible to recover a clock once the cache line

                // has been filled, but our prior attempt to do so has lead

                // to a race condition, so we keep this logic simple.

                if (((r_v)&&(i_stall_n))||(i_clear_cache)||(i_new_pc))

                        tagsrc <= 1'b1;

                else

                        tagsrc <= 1'b0;

        initial tagvalipc = 0;

        always @(posedge i_clk)

                tagvalipc <= tags[i_pc[(CW-1):PW]];

        initial tagvallst = 0;

        always @(posedge i_clk)

                tagvallst <= tags[lastpc[(CW-1):PW]];

        assign  tagval = (tagsrc)?tagvalipc : tagvallst;

        // i_pc will only increment when everything else isn't stalled, thus

        // we can set it without worrying about that.   Doing this enables

        // us to work in spite of stalls.  For example, if the next address

        // isn't valid, but the decoder is stalled, get the next address

        // anyway.

        initial lastpc = 0;

        always @(posedge i_clk)

                if (((r_v)&&(i_stall_n))||(i_clear_cache)||(i_new_pc))

                        lastpc <= i_pc;

        assign  lasttag = lastpc[(AW-1):PW];

        wire    w_v_from_pc, w_v_from_last;

        assign  w_v_from_pc = ((i_pc[(AW-1):PW] == lasttag)

                                &&(tagvalipc == i_pc[(AW-1):CW])

                                &&(vmask[i_pc[(CW-1):PW]]));

        assign  w_v_from_last = (

                                //(lastpc[(AW-1):PW] == lasttag)&&

                                (tagval == lastpc[(AW-1):CW])

                                &&(vmask[lastpc[(CW-1):PW]]));

        reg     [1:0]    delay;

        initial delay = 2'h3;

        reg     rvsrc;

        always @(posedge i_clk)

                if ((i_rst)||(i_clear_cache)||(i_new_pc)||((r_v)&&(i_stall_n)))

                begin

                        // r_v <= r_v_from_pc;

                        rvsrc <= 1'b1;

                        delay <= 2'h2;

                end else if (~r_v) begin // Otherwise, r_v was true and we were

                        // stalled, hence only if ~r_v

                        rvsrc <= 1'b0;

                        if (o_wb_cyc)

                                delay <= 2'h2;

                        else if (delay != 0)

                                delay <= delay + 2'b11; // i.e. delay -= 1;

                end

        reg     r_v_from_pc, r_v_from_last;

        always @(posedge i_clk)

                r_v_from_pc <= w_v_from_pc;

        always @(posedge i_clk)

                r_v_from_last <= w_v_from_last;

        assign  r_v = ((rvsrc)?(r_v_from_pc):(r_v_from_last));

        assign  o_v = (((rvsrc)?(r_v_from_pc):(r_v_from_last))

                                ||((o_illegal)&&(~o_wb_cyc)))

                        &&(~i_new_pc)&&(~i_rst);

        reg     last_ack;

        initial last_ack = 1'b0;

        always @(posedge i_clk)

                last_ack <= (o_wb_cyc)&&(

                                (rdaddr[(PW-1):1]=={(PW-1){1'b1}})

                                &&((rdaddr[0])||(i_wb_ack)));

        reg     needload;

        initial needload = 1'b0;

        always @(posedge i_clk)

                needload <= ((~r_v)&&(delay==0)

                        &&((tagvallst != lastpc[(AW-1):CW])

                                ||(~vmask[lastpc[(CW-1):PW]]))

                        &&((~illegal_valid)

                                ||(lastpc[(AW-1):PW] != illegal_cache)));

        reg     last_addr;

        initial last_addr = 1'b0;

        always @(posedge i_clk)

                last_addr <= (o_wb_cyc)&&(o_wb_addr[(PW-1):1] == {(PW-1){1'b1}})

                                &&((~i_wb_stall)|(o_wb_addr[0]));

        initial o_wb_cyc  = 1'b0;

        initial o_wb_stb  = 1'b0;

        initial o_wb_addr = {(AW){1'b0}};

        initial rdaddr    = 0;

        always @(posedge i_clk)

                if ((i_rst)||(i_clear_cache))

                begin

                        o_wb_cyc <= 1'b0;

                        o_wb_stb <= 1'b0;

                end else if (o_wb_cyc)

                begin

                        if (i_wb_err)

                                o_wb_stb <= 1'b0;

                        else if ((o_wb_stb)&&(~i_wb_stall)&&(last_addr))

                                o_wb_stb <= 1'b0;

                        if (((i_wb_ack)&&(last_ack))||(i_wb_err))

                                o_wb_cyc <= 1'b0;

                        // else if (rdaddr[(PW-1):1] == {(PW-1){1'b1}})

                        //      tags[lastpc[(CW-1):PW]] <= lastpc[(AW-1):CW];

                end else if (needload)

                begin

                        o_wb_cyc  <= 1'b1;

                        o_wb_stb  <= 1'b1;

                end

        always @(posedge i_clk)

                if (o_wb_cyc) // &&(i_wb_ack)

                        tags[o_wb_addr[(CW-1):PW]] <= o_wb_addr[(AW-1):CW];

        always @(posedge i_clk)

                if ((o_wb_cyc)&&(i_wb_ack))

                        rdaddr <= rdaddr + 1;

                else if (~o_wb_cyc)

                        rdaddr <= { lastpc[(CW-1):PW], {(PW){1'b0}} };

        always @(posedge i_clk)

                if ((o_wb_stb)&&(~i_wb_stall)&&(~last_addr))

                        o_wb_addr[(PW-1):0] <= o_wb_addr[(PW-1):0]+1;

                else if (~o_wb_cyc)

                        o_wb_addr <= { lastpc[(AW-1):PW], {(PW){1'b0}} };

        // Can't initialize an array, so leave cache uninitialized

        // We'll also never get an ack without sys being active, so skip

        // that check.  Or rather, let's just use o_wb_cyc instead.  This

        // will work because multiple writes to the same address, ending with

        // a valid write, aren't a problem.

        always @(posedge i_clk)

                if (o_wb_cyc) // &&(i_wb_ack)

                        cache[rdaddr] <= i_wb_data;

        // VMask ... is a section loaded?

        // Note "svmask".  It's purpose is to delay the vmask setting by one

        // clock, so that we can insure the right value of the cache is loaded

        // before declaring that the cache line is valid.  Without this, the

        // cache line would get read, and the instruction would read from the

        // last cache line.

        reg     svmask;

        initial vmask = 0;

        initial svmask = 1'b0;

        reg     [(LGLINES-1):0]  saddr;

        always @(posedge i_clk)

                if ((i_rst)||(i_clear_cache))

                begin

                        vmask <= 0;

                        svmask<= 1'b0;

                end

                else begin

                        svmask <= ((o_wb_cyc)&&(i_wb_ack)&&(last_ack));

                        if (svmask)

                                vmask[saddr] <= 1'b1;

                        if ((~o_wb_cyc)&&(needload))

                                vmask[lastpc[(CW-1):PW]] <= 1'b0;

                end

        always @(posedge i_clk)

                if ((o_wb_cyc)&&(i_wb_ack))

                        saddr <= rdaddr[(CW-1):PW];

        initial illegal_cache = 0;

        initial illegal_valid = 0;

        always @(posedge i_clk)

                if ((i_rst)||(i_clear_cache))

                begin

                        illegal_cache <= 0;

                        illegal_valid <= 0;

                end else if ((o_wb_cyc)&&(i_wb_err))

                begin

                        illegal_cache <= o_wb_addr[(AW-1):PW];

                        illegal_valid <= 1'b1;

                end

        initial o_illegal = 1'b0;

        always @(posedge i_clk)

                if ((i_rst)||(i_clear_cache)||(o_wb_cyc))

                        o_illegal <= 1'b0;

                else

                        o_illegal <= (illegal_valid)

                                &&(illegal_cache == i_pc[(AW-1):PW]);

endmodule