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[/] [zipcpu/] [trunk/] [rtl/] [core/] [pipefetch.v] - Blame information for rev 38

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1 2 dgisselq
////////////////////////////////////////////////////////////////////////////////
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//
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// Filename:    pipefetch.v
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//
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// Project:     Zip CPU -- a small, lightweight, RISC CPU soft core
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//
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// Purpose:     Keeping our CPU fed with instructions, at one per clock and
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//              with no stalls, can be quite a chore.  Worse, the Wishbone
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//              takes a couple of cycles just to read one instruction from
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//              the bus.  However, if we use pipeline accesses to the Wishbone
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//              bus, then we can read more and faster.  Further, if we cache
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//              these results so that we have them before we need them, then
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//              we have a chance of keeping our CPU from stalling.  Those are
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//              the purposes of this instruction fetch module: 1) Pipeline
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//              wishbone accesses, and 2) an instruction cache.
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//
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//      20150919 -- Fixed a nasty race condition whereby the pipefetch routine
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//              would produce either the same instruction twice, or skip
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//              an instruction.  This condition was dependent on the CPU stall
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//              condition, and would only take place if the pipeline wasn't 
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//              completely full throughout the stall.
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//
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//              Interface support was also added for trapping on illegal
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//              instructions (i.e., instruction fetches that cause bus errors),
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//              however the internal interface has not caught up to supporting
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//              these exceptions yet.
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//
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// Creator:     Dan Gisselquist, Ph.D.
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//              Gisselquist Tecnology, LLC
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//
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////////////////////////////////////////////////////////////////////////////////
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//
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// Copyright (C) 2015, Gisselquist Technology, LLC
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//
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// This program is free software (firmware): you can redistribute it and/or
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// modify it under the terms of  the GNU General Public License as published
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// by the Free Software Foundation, either version 3 of the License, or (at
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// your option) any later version.
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//
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// This program is distributed in the hope that it will be useful, but WITHOUT
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// ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or
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// FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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// for more details.
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//
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// License:     GPL, v3, as defined and found on www.gnu.org,
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//              http://www.gnu.org/licenses/gpl.html
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//
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//
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////////////////////////////////////////////////////////////////////////////////
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//
51 18 dgisselq
module  pipefetch(i_clk, i_rst, i_new_pc, i_clear_cache, i_stall_n, i_pc,
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                        o_i, o_pc, o_v,
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                o_wb_cyc, o_wb_stb, o_wb_we, o_wb_addr, o_wb_data,
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                        i_wb_ack, i_wb_stall, i_wb_err, i_wb_data, i_wb_request,
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                        o_illegal);
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        parameter       RESET_ADDRESS=32'h0010_0000,
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                        LGCACHELEN = 6, CACHELEN=(1<<LGCACHELEN),
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                        BUSW=32;
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        input                           i_clk, i_rst, i_new_pc,
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                                        i_clear_cache, i_stall_n;
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        input           [(BUSW-1):0]     i_pc;
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        output  reg     [(BUSW-1):0]     o_i;
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        output  reg     [(BUSW-1):0]     o_pc;
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        output  wire                    o_v;
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        //
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        output  reg             o_wb_cyc, o_wb_stb;
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        output  wire            o_wb_we;
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        output  reg     [(BUSW-1):0]     o_wb_addr;
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        output  wire    [(BUSW-1):0]     o_wb_data;
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        //
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        input                   i_wb_ack, i_wb_stall, i_wb_err;
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        input           [(BUSW-1):0]     i_wb_data;
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        //
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        // Is the (data) memory unit also requesting access to the bus?
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        input                           i_wb_request;
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        output  wire                    o_illegal;
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78 36 dgisselq
        assign  o_illegal = 1'b0;
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        // Fixed bus outputs: we read from the bus only, never write.
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        // Thus the output data is ... irrelevant and don't care.  We set it
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        // to zero just to set it to something.
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        assign  o_wb_we = 1'b0;
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        assign  o_wb_data = 0;
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        reg     [(BUSW-1):0]             r_cache_base;
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        reg     [(LGCACHELEN):0] r_nvalid, r_acks_waiting;
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        reg     [(BUSW-1):0]             cache[0:(CACHELEN-1)];
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        reg     [(LGCACHELEN-1):0]       r_cache_offset;
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        reg                     r_addr_set;
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        reg     [(BUSW-1):0]     r_addr;
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        wire    [(BUSW-1):0]     bus_nvalid;
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        assign  bus_nvalid = { {(BUSW-LGCACHELEN-1){1'b0}}, r_nvalid };
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        // What are some of the conditions for which we need to restart the
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        // cache?
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        wire    w_pc_out_of_bounds;
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        assign  w_pc_out_of_bounds = ((i_new_pc)&&((r_nvalid == 0)
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                                        ||(i_pc < r_cache_base)
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                                        ||(i_pc >= r_cache_base + CACHELEN)));
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        wire    w_ran_off_end_of_cache;
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        assign  w_ran_off_end_of_cache =((r_addr_set)&&((r_addr < r_cache_base)
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                                        ||(r_addr >= r_cache_base + CACHELEN)));
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        wire    w_running_out_of_cache;
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        assign  w_running_out_of_cache = (r_addr_set)
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                        &&(r_addr >= r_cache_base + (1<<(LGCACHELEN-2))
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                                                + (1<<(LGCACHELEN-1)));
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        initial r_cache_base = RESET_ADDRESS;
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        always @(posedge i_clk)
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        begin
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                if ((i_rst)||(i_clear_cache))
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                begin
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                        o_wb_cyc <= 1'b0;
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                        o_wb_stb <= 1'b0;
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                        // r_cache_base <= RESET_ADDRESS;
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                // end else if ((~o_wb_cyc)&&(i_new_pc)&&(r_nvalid != 0)
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                //              &&(i_pc >= r_cache_base)
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                //              &&(i_pc < r_cache_base + bus_nvalid))
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                // begin
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                        // The new instruction is in our cache, do nothing
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                        // with the bus here.
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                end else if ((o_wb_cyc)&&(w_pc_out_of_bounds))
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                begin
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                        // We need to abandon our bus action to start over in
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                        // a new region, setting up a new cache.  This may
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                        // happen mid cycle while waiting for a result.  By
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                        // dropping o_wb_cyc, we state that we are no longer
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                        // interested in that result--whatever it might be.
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                        o_wb_cyc <= 1'b0;
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                        o_wb_stb <= 1'b0;
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                end else if ((~o_wb_cyc)&&(~r_nvalid[LGCACHELEN])&&(~i_wb_request)&&(r_addr_set))
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                begin
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                        // Restart a bus cycle that was interrupted when the
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                        // data section wanted access to our bus.
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                        o_wb_cyc <= 1'b1;
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                        o_wb_stb <= 1'b1;
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                        // o_wb_addr <= r_cache_base + bus_nvalid;
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                end else if ((~o_wb_cyc)&&(
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                                (w_pc_out_of_bounds)||(w_ran_off_end_of_cache)))
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                begin
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                        // Start a bus transaction
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                        o_wb_cyc <= 1'b1;
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                        o_wb_stb <= 1'b1;
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                        // o_wb_addr <= (i_new_pc) ? i_pc : r_addr;
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                        // r_nvalid <= 0;
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                        // r_cache_base <= (i_new_pc) ? i_pc : r_addr;
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                        // r_cache_offset <= 0;
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                end else if ((~o_wb_cyc)&&(w_running_out_of_cache))
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                begin
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                        // If we're using the last quarter of the cache, then
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                        // let's start a bus transaction to extend the cache.
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                        o_wb_cyc <= 1'b1;
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                        o_wb_stb <= 1'b1;
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                        // o_wb_addr <= r_cache_base + (1<<(LGCACHELEN));
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                        // r_nvalid <= r_nvalid - (1<<(LGCACHELEN-2));
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                        // r_cache_base <= r_cache_base + (1<<(LGCACHELEN-2));
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                        // r_cache_offset <= r_cache_offset + (1<<(LGCACHELEN-2));
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                end else if (o_wb_cyc)
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                begin
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                        // This handles everything ... but the case where
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                        // while reading we need to extend our cache.
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                        if ((o_wb_stb)&&(~i_wb_stall))
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                        begin
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                                // o_wb_addr <= o_wb_addr + 1;
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                                if ((o_wb_addr - r_cache_base >= CACHELEN-1)
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                                        ||(i_wb_request))
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                                        o_wb_stb <= 1'b0;
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                        end
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                        if (i_wb_ack)
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                        begin
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                                // r_nvalid <= r_nvalid + 1;
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                                if ((r_acks_waiting == 1)&&(~o_wb_stb))
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                                        o_wb_cyc <= 1'b0;
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                        end else if ((r_acks_waiting == 0)&&(~o_wb_stb))
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                                o_wb_cyc <= 1'b0;
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                end
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        end
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183 36 dgisselq
        initial r_nvalid = 0;
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        always @(posedge i_clk)
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                if ((i_rst)||(i_clear_cache)) // Required, so we can reload memoy and then reset
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                        r_nvalid <= 0;
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                else if ((~o_wb_cyc)&&(
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                                (w_pc_out_of_bounds)||(w_ran_off_end_of_cache)))
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                        r_nvalid <= 0;
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                else if ((~o_wb_cyc)&&(w_running_out_of_cache))
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                        r_nvalid <= r_nvalid - (1<<(LGCACHELEN-2));
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                else if ((o_wb_cyc)&&(i_wb_ack))
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                        r_nvalid <= r_nvalid+1;
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        always @(posedge i_clk)
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                if (i_clear_cache)
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                        r_cache_base <= i_pc;
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                else if ((~o_wb_cyc)&&(
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                                (w_pc_out_of_bounds)
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                                ||(w_ran_off_end_of_cache)))
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                        r_cache_base <= (i_new_pc) ? i_pc : r_addr;
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                else if ((~o_wb_cyc)&&(w_running_out_of_cache))
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                        r_cache_base <= r_cache_base + (1<<(LGCACHELEN-2));
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        always @(posedge i_clk)
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                if (i_clear_cache)
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                        r_cache_offset <= 0;
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                else if ((~o_wb_cyc)&&(
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                                (w_pc_out_of_bounds)
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                                ||(w_ran_off_end_of_cache)))
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                        r_cache_offset <= 0;
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                else if ((~o_wb_cyc)&&(w_running_out_of_cache))
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                        r_cache_offset <= r_cache_offset + (1<<(LGCACHELEN-2));
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        always @(posedge i_clk)
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                if (i_clear_cache)
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                        o_wb_addr <= i_pc;
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                else if ((o_wb_cyc)&&(w_pc_out_of_bounds))
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                begin
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                        if (i_wb_ack)
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                                o_wb_addr <= r_cache_base + bus_nvalid+1;
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                        else
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                                o_wb_addr <= r_cache_base + bus_nvalid;
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                end else if ((~o_wb_cyc)&&((w_pc_out_of_bounds)
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                                        ||(w_ran_off_end_of_cache)))
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                        o_wb_addr <= (i_new_pc) ? i_pc : r_addr;
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                else if ((o_wb_cyc)&&(o_wb_stb)&&(~i_wb_stall))
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                        o_wb_addr <= o_wb_addr + 1;
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        initial r_acks_waiting = 0;
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        always @(posedge i_clk)
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                if (~o_wb_cyc)
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                        r_acks_waiting <= 0;
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                else if ((o_wb_cyc)&&(o_wb_stb)&&(~i_wb_stall)&&(~i_wb_ack))
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                        r_acks_waiting <= r_acks_waiting + 1;
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                else if ((o_wb_cyc)&&(i_wb_ack)&&((~o_wb_stb)||(i_wb_stall)))
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                        r_acks_waiting <= r_acks_waiting - 1;
238 3 dgisselq
 
239
        always @(posedge i_clk)
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                if ((o_wb_cyc)&&(i_wb_ack))
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                        cache[r_nvalid[(LGCACHELEN-1):0]+r_cache_offset]
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                                        <= i_wb_data;
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244 2 dgisselq
        initial r_addr_set = 1'b0;
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        always @(posedge i_clk)
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                if ((i_rst)||(i_clear_cache))
247 2 dgisselq
                        r_addr_set <= 1'b0;
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                else if (i_new_pc)
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                        r_addr_set <= 1'b1;
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251
        // Now, read from the cache
252
        wire    w_cv;   // Cache valid, address is in the cache
253
        reg     r_cv;
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        assign  w_cv = ((r_nvalid != 0)&&(r_addr>=r_cache_base)
255
                        &&(r_addr-r_cache_base < bus_nvalid));
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        always @(posedge i_clk)
257 36 dgisselq
                r_cv <= (~i_new_pc)&&((w_cv)||((~i_stall_n)&&(r_cv)));
258 2 dgisselq
        assign  o_v = (r_cv)&&(~i_new_pc);
259
 
260
        always @(posedge i_clk)
261
                if (i_new_pc)
262
                        r_addr <= i_pc;
263 36 dgisselq
                else if ( ((i_stall_n)&&(w_cv)) || ((~i_stall_n)&&(w_cv)&&(r_addr == o_pc)) )
264 2 dgisselq
                        r_addr <= r_addr + 1;
265
 
266
        wire    [(LGCACHELEN-1):0]       c_rdaddr, c_cache_base;
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        assign  c_cache_base   = r_cache_base[(LGCACHELEN-1):0];
268 3 dgisselq
        assign  c_rdaddr = r_addr[(LGCACHELEN-1):0]-c_cache_base+r_cache_offset;
269 2 dgisselq
        always @(posedge i_clk)
270 36 dgisselq
                if ((~o_v)||((i_stall_n)&&(o_v)))
271 2 dgisselq
                        o_i <= cache[c_rdaddr];
272
        always @(posedge i_clk)
273 36 dgisselq
                if ((~o_v)||((i_stall_n)&&(o_v)))
274 2 dgisselq
                        o_pc <= r_addr;
275
 
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endmodule

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