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/*
 * Copyright (c) 2014, Aleksander Osman
 * All rights reserved.
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 * 
 * * Redistributions of source code must retain the above copyright notice, this
 *   list of conditions and the following disclaimer.
 * 
 * * Redistributions in binary form must reproduce the above copyright notice,
 *   this list of conditions and the following disclaimer in the documentation
 *   and/or other materials provided with the distribution.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
 * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
module decode_regs(
    input               clk,
    input               rst_n,
 
    input               dec_reset,
 
    input       [3:0]   fetch_valid,
    input       [63:0]  fetch,
 
    input       [3:0]   prefix_count,
    input       [3:0]   consume_count,
 
    output      [3:0]   dec_acceptable,
 
    output reg  [95:0]  decoder,
    output reg  [3:0]   decoder_count
);
 
//------------------------------------------------------------------------------
 
wire [3:0] after_consume_count;
 
wire [4:0] total_count;
 
wire [3:0] acceptable_1;
wire [3:0] acceptable_2;
 
wire [3:0] accepted;
 
wire [95:0] after_consume;
 
wire [95:0] decoder_next;
 
//------------------------------------------------------------------------------
 
//------------------------------------------------------------------------------
 
assign after_consume_count = decoder_count - consume_count;
 
assign total_count         = prefix_count + decoder_count;
 
//------------------------------------------------------------------------------
 
// acceptable by decoder
assign acceptable_1     = 4'd12 - decoder_count + consume_count;
 
// acceptable by total instruction length
assign acceptable_2     = (total_count < 5'd15)? 4'd15 - total_count[3:0] : 4'd0;
 
assign dec_acceptable   = (dec_reset)?                      4'd0 :
                          (acceptable_1 < acceptable_2)?    acceptable_1 : acceptable_2;
 
assign accepted         = (dec_acceptable > fetch_valid)? fetch_valid : dec_acceptable;
 
//------------------------------------------------------------------------------
 
assign after_consume = 
    (consume_count == 4'd0)?            decoder :
    (consume_count == 4'd1)?  { 8'd0,   decoder[95:8] } :
    (consume_count == 4'd2)?  { 16'd0,  decoder[95:16] } :
    (consume_count == 4'd3)?  { 24'd0,  decoder[95:24] } :
    (consume_count == 4'd4)?  { 32'd0,  decoder[95:32] } :
    (consume_count == 4'd5)?  { 40'd0,  decoder[95:40] } :
    (consume_count == 4'd6)?  { 48'd0,  decoder[95:48] } :
    (consume_count == 4'd7)?  { 56'd0,  decoder[95:56] } :
    (consume_count == 4'd8)?  { 64'd0,  decoder[95:64] } :
    (consume_count == 4'd9)?  { 72'd0,  decoder[95:72] } :
    (consume_count == 4'd10)? { 80'd0,  decoder[95:80] } :
                              { 88'd0,  decoder[95:88] };
 
assign decoder_next =
    (after_consume_count == 4'd0)?   { 32'd0,fetch } :
    (after_consume_count == 4'd1)?   { 24'd0,fetch,       after_consume[7:0] } :
    (after_consume_count == 4'd2)?   { 16'd0,fetch,       after_consume[15:0] } :
    (after_consume_count == 4'd3)?   { 8'd0, fetch,       after_consume[23:0] } :
    (after_consume_count == 4'd4)?   {       fetch,       after_consume[31:0] } :
    (after_consume_count == 4'd5)?   {       fetch[55:0], after_consume[39:0] } :
    (after_consume_count == 4'd6)?   {       fetch[47:0], after_consume[47:0] } :
    (after_consume_count == 4'd7)?   {       fetch[39:0], after_consume[55:0] } :
    (after_consume_count == 4'd8)?   {       fetch[31:0], after_consume[63:0] } :
    (after_consume_count == 4'd9)?   {       fetch[23:0], after_consume[71:0] } :
    (after_consume_count == 4'd10)?  {       fetch[15:0], after_consume[79:0] } :
    (after_consume_count == 4'd11)?  {       fetch[7:0],  after_consume[87:0] } :
                                                          after_consume;
 
//------------------------------------------------------------------------------
 
always @(posedge clk or negedge rst_n) begin
    if(rst_n == 1'b0)   decoder <= 96'd0;
    else                decoder <= decoder_next;
end
 
always @(posedge clk or negedge rst_n) begin
    if(rst_n == 1'b0)   decoder_count <= 4'd0;
    else if(dec_reset)  decoder_count <= 4'd0;
    else                decoder_count <= after_consume_count + accepted;
end
 
//------------------------------------------------------------------------------
 
//------------------------------------------------------------------------------
 
endmodule