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

//

// Filename:    prefetch.v

//

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

//

// Purpose:     This is a very simple instruction fetch approach.  It gets

//              one instruction at a time.  Future versions should pipeline

//              fetches and perhaps even cache results--this doesn't do that.

//              It should, however, be simple enough to get things running.

//

//              The interface is fascinating.  The 'i_pc' input wire is just

//              a suggestion of what to load.  Other wires may be loaded

//              instead. i_pc is what must be output, not necessarily input.

//

//      20150919 -- Added support for the WB error signal.  When reading an

//              instruction results in this signal being raised, the pipefetch

//              module will set an illegal instruction flag to be returned to

//              the CPU together with the instruction.  Hence, the ZipCPU

//              can trap on it if necessary.

//

// Creator:     Dan Gisselquist, Ph.D.

//              Gisselquist Technology, LLC

//

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

//

// Copyright (C) 2015,2017, 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.

//

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

// with this program.  (It's in the $(ROOT)/doc directory.  Run make with no

// target there if the PDF file isn't present.)  If not, see

// <http://www.gnu.org/licenses/> for a copy.

//

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

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

//

//

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

//

//

// Flash requires a minimum of 4 clocks per byte to read, so that would be

// 4*(4bytes/32bit word) = 16 clocks per word read---and that's in pipeline

// mode which this prefetch does not support.  In non--pipelined mode, the

// flash will require (16+6+6)*2 = 56 clocks plus 16 clocks per word read,

// or 72 clocks to fetch one instruction.

module  prefetch(i_clk, i_rst, i_ce, i_stalled_n, i_pc, i_aux,

                        o_i, o_pc, o_aux, o_valid, o_illegal,

                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);

        parameter               ADDRESS_WIDTH=32, AUX_WIDTH = 1, AW=ADDRESS_WIDTH;

        input                           i_clk, i_rst, i_ce, i_stalled_n;

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

        input   [(AUX_WIDTH-1):0]        i_aux;

        output  reg     [31:0]           o_i;

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

        output  reg [(AUX_WIDTH-1):0]    o_aux;

        output  reg                     o_valid, o_illegal;

        // Wishbone outputs

        output  reg                     o_wb_cyc, o_wb_stb;

        output  wire                    o_wb_we;

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

        output  wire    [31:0]           o_wb_data;

        // And return inputs

        input                   i_wb_ack, i_wb_stall, i_wb_err;

        input           [31:0]   i_wb_data;

        assign  o_wb_we = 1'b0;

        assign  o_wb_data = 32'h0000;

        // Let's build it simple and upgrade later: For each instruction

        // we do one bus cycle to get the instruction.  Later we should

        // pipeline this, but for now let's just do one at a time.

        initial o_wb_cyc = 1'b0;

        initial o_wb_stb = 1'b0;

        initial o_wb_addr= 0;

        always @(posedge i_clk)

                if ((i_rst)||(i_wb_ack))

                begin

                        o_wb_cyc <= 1'b0;

                        o_wb_stb <= 1'b0;

                end else if ((i_ce)&&(~o_wb_cyc)) // Initiate a bus cycle

                begin

                        o_wb_cyc <= 1'b1;

                        o_wb_stb <= 1'b1;

                end else if (o_wb_cyc) // Independent of ce

                begin

                        if ((o_wb_cyc)&&(o_wb_stb)&&(~i_wb_stall))

                                o_wb_stb <= 1'b0;

                        if (i_wb_ack)

                                o_wb_cyc <= 1'b0;

                end

        always @(posedge i_clk)

                if (i_rst) // Set the address to guarantee the result is invalid

                        o_wb_addr <= {(AW){1'b1}};

                else if ((i_ce)&&(~o_wb_cyc))

                        o_wb_addr <= i_pc;

        always @(posedge i_clk)

                if ((o_wb_cyc)&&(i_wb_ack))

                        o_aux <= i_aux;

        always @(posedge i_clk)

                if ((o_wb_cyc)&&(i_wb_ack))

                        o_i <= i_wb_data;

        always @(posedge i_clk)

                if ((o_wb_cyc)&&(i_wb_ack))

                        o_pc <= o_wb_addr;

        initial o_valid   = 1'b0;

        initial o_illegal = 1'b0;

        always @(posedge i_clk)

                if ((o_wb_cyc)&&(i_wb_ack))

                begin

                        o_valid <= (i_pc == o_wb_addr)&&(~i_wb_err);

                        o_illegal <= (i_wb_err)&&(i_pc == o_wb_addr);

                end else if (i_stalled_n)

                begin

                        o_valid <= 1'b0;

                        o_illegal <= 1'b0;

                end

endmodule