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

//                                                              //

//  Fetch - Instantiates the fetch stage sub-modules of         //

//  the Amber 25 Core                                           //

//                                                              //

//  This file is part of the Amber project                      //

//  http://www.opencores.org/project,amber                      //

//                                                              //

//  Description                                                 //

//  Instantiates the Cache and Wishbone I/F                     //

//  Also contains a little bit of logic to decode memory        //

//  accesses to decide if they are cached or not                //

//                                                              //

//  Author(s):                                                  //

//      - Conor Santifort, csantifort.amber@gmail.com           //

//                                                              //

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

//                                                              //

// Copyright (C) 2011 Authors and OPENCORES.ORG                 //

//                                                              //

// This source file may be used and distributed without         //

// restriction provided that this copyright statement is not    //

// removed from the file and that any derivative work contains  //

// the original copyright notice and the associated disclaimer. //

//                                                              //

// 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 2.1 of the License, or (at your option) any   //

// later version.                                               //

//                                                              //

// This source 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 Lesser General Public License for more //

// details.                                                     //

//                                                              //

// You should have received a copy of the GNU Lesser General    //

// Public License along with this source; if not, download it   //

// from http://www.opencores.org/lgpl.shtml                     //

//                                                              //

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

module a25_fetch

(

input                       i_clk,

input                       i_mem_stall,

input                       i_conflict,         // Decode stage stall pipeline because of an instruction conflict

output                      o_fetch_stall,      // when this is asserted all registers 

                                                // in decode and exec stages are frozen

input                       i_system_rdy,       // External system can stall core with this signal

input       [31:0]          i_iaddress,

input                       i_iaddress_valid,

input       [31:0]          i_iaddress_nxt,     // un-registered version of address to the cache rams

output      [31:0]          o_fetch_instruction,

input                       i_cache_enable,     // cache enable

input                       i_cache_flush,      // cache flush

input       [31:0]          i_cacheable_area,   // each bit corresponds to 2MB address space

output                      o_wb_req,

output                      o_wb_qword,         // High for a quad-word fetch request

output      [31:0]          o_wb_address,

input       [31:0]          i_wb_read_data,

input                       i_wb_ready

);

`include "memory_configuration.v"

wire                        core_stall;

wire                        cache_stall;

wire    [31:0]              cache_read_data;

wire                        sel_cache;

wire                        uncached_instruction_read;

wire                        address_cachable;

wire                        icache_wb_req;

wire                        wait_wb;

reg                         wb_req_r = 'd0;

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

// Memory Decode

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

assign address_cachable  = in_cachable_mem( i_iaddress ) && i_cacheable_area[i_iaddress[25:21]];

assign sel_cache         = address_cachable && i_iaddress_valid && i_cache_enable;

// Don't start wishbone transfers when the cache is stalling the core

// The cache stalls the core during its initialization sequence

assign uncached_instruction_read = !sel_cache && i_iaddress_valid && !(cache_stall);

// Return read data either from the wishbone bus or the cache

assign o_fetch_instruction = sel_cache                  ? cache_read_data :

                             uncached_instruction_read  ? i_wb_read_data  :

                                                          32'hffeeddcc    ;

// Stall the instruction decode and execute stages of the core

// when the fetch stage needs more than 1 cycle to return the requested

// read data

assign o_fetch_stall    = !i_system_rdy || wait_wb || cache_stall;

assign o_wb_address     = i_iaddress;

assign o_wb_req         = icache_wb_req || uncached_instruction_read;

assign o_wb_qword       = icache_wb_req;

assign wait_wb          = (o_wb_req || wb_req_r) && !i_wb_ready;

always @(posedge i_clk)

    wb_req_r <= o_wb_req && !i_wb_ready;

assign core_stall = o_fetch_stall || i_mem_stall || i_conflict;

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

// L1 Instruction Cache

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

a25_icache u_cache (

    .i_clk                      ( i_clk                 ),

    .i_core_stall               ( core_stall            ),

    .o_stall                    ( cache_stall           ),

    .i_select                   ( sel_cache             ),

    .i_address                  ( i_iaddress            ),

    .i_address_nxt              ( i_iaddress_nxt        ),

    .i_cache_enable             ( i_cache_enable        ),

    .i_cache_flush              ( i_cache_flush         ),

    .o_read_data                ( cache_read_data       ),

    .o_wb_req                   ( icache_wb_req         ),

    .i_wb_read_data             ( i_wb_read_data        ),

    .i_wb_ready                 ( i_wb_ready            )

);

endmodule