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/*      MODULE: openfire_fetch

        DESCRIPTION: The fetch module interfaces with the instruction memory and

fetches the next instruction.

TO DO:

- Add prefetch buffer

- Add LMB interface

AUTHOR:

Stephen Douglas Craven

Configurable Computing Lab

Virginia Tech

scraven@vt.edu

REVISION HISTORY:

Revision 0.2, 8/10/2005 SDC

Initial release

Revision 0.3, 12/17/2005 SDC

Fixed PC size bug

Revision 0.4  27/03/2007 Antonio J Anton

Instruction port wait states

COPYRIGHT:

Copyright (c) 2005 Stephen Douglas Craven

Permission is hereby granted, free of charge, to any person obtaining a copy of

this software and associated documentation files (the "Software"), to deal in

the Software without restriction, including without limitation the rights to

use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies

of the Software, and to permit persons to whom the Software is furnished to do

so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all

copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR

IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,

FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE

AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER

LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,

OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE

SOFTWARE. */

`include "openfire_define.v"

module openfire_fetch(

        stall, clock, reset,                                                                    // top level

        branch_taken, pc_branch, idata,                                         // inputs

        instruction, imem_addr, pc_decode, imem_re              // outputs

);

// From top level -- all active high unless otherwise noted

input           stall;

input           reset;

input           clock;

// From EXECUTE module

input                   branch_taken;   // strobe for latching in new pc

// PCs are A_SPACE + 1 because lower 2 bits are always zero

//      A_SPACE is addr space in words ... 2^A_SPACE * 4 = # Bytes

input   [`A_SPACE+1:0]   pc_branch;      // PC of branch

// From Instr Mem

input   [31:0]           idata;

output  [31:0]           imem_addr;

output                                  imem_re;

output  [`A_SPACE+1:0]   pc_decode;

output  [31:0]           instruction;

reg     [`A_SPACE+1:0]   pc_fetch;       // PCs only need to contain addressable instr mem

reg     [`A_SPACE+1:0]   pc_decode;      // delayed PC for DECODE

reg     [31:0]                   instruction;

reg                                             imem_re;

assign imem_addr[31:`A_SPACE+2] = 0;     // pad unused bits with zeros, 

assign imem_addr[`A_SPACE+1:0]  = pc_fetch;

always@(posedge clock)

begin

        if (reset)

        begin

                pc_fetch         <= `RESET_PC_ADDRESS;

                pc_decode        <= `RESET_PC_ADDRESS;

                imem_re          <= 1;

                instruction  <= `NoOp;          // Execute NoOp on reset

        end

        else

        begin                                                                   // update PC to branch or increment pc (if stall --> pc on hold)

                if(!stall)

                begin

                  pc_fetch        <= branch_taken ? pc_branch : pc_fetch + 4;

                  pc_decode   <= pc_fetch;

                  instruction <= idata;

`ifdef DEBUG_FETCH

                  $display("FETCH : pc_fetch=%x, pc_decode=%x, instruction=%x", pc_fetch, pc_decode, instruction);

`endif

                end

                imem_re <= !stall;

        end

end

endmodule