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

//

// Filename:    wbudecompress.v

//

// Project:     XuLA2 board

//

// Purpose:     Compression via this interface is simply a lookup table.

//              When writing, if requested, rather than writing a new 36-bit

//      word, we may be asked to repeat a word that's been written recently.

//      That's the goal of this routine: if given a word's (relative) address

//      in the write stream, we use that address, else we expect a full 32-bit

//      word to come in to be written.

//

//

// Creator:     Dan Gisselquist, Ph.D.

//              Gisselquist Technology, LLC

//

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

//

// Copyright (C) 2015, 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.

//

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

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

//

//

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

//

//

module  wbudecompress(i_clk, i_stb, i_word, o_stb, o_word);

        input                   i_clk, i_stb;

        input           [35:0]   i_word;

        output  reg             o_stb;

        output  reg     [35:0]   o_word;

        // Clock zero

        //      { o_stb, r_stb } = 0

        wire    cmd_write_not_compressed = (i_word[35:33] == 3'h3);

        // Clock one: { o_stb, r_stb } = 4'h1 when done

        reg     [7:0]    wr_addr;

        initial wr_addr = 8'h0;

        always @(posedge i_clk)

                if ((i_stb)&&(cmd_write_not_compressed))

                        wr_addr <= wr_addr + 8'h1;

        reg     [31:0]   compression_tbl [0:255];

        always @(posedge i_clk)

                if (i_stb)

                compression_tbl[wr_addr] <= { i_word[32:31], i_word[29:0] };

        reg     [35:0]   r_word;

        always @(posedge i_clk)

                if (i_stb)

                        r_word <= i_word;

        // Clock two, calculate the table address ... 1 is the smallest address

        //      { o_stb, r_stb } = 4'h2 when done

        reg     [7:0]    cmd_addr;

        always @(posedge i_clk)

                cmd_addr = wr_addr - { r_word[32:31], r_word[29:24] };

        // Let's also calculate the address, in case this is a compressed

        // address word

        reg     [24:0]   r_addr;

        always @(posedge i_clk)

                case(r_word[32:30])

                3'b000: r_addr <= { 19'h0, r_word[29:24] };

                3'b010: r_addr <= { 13'h0, r_word[29:18] };

                3'b100: r_addr <= {  7'h0, r_word[29:12] };

                3'b110: r_addr <= {  1'h0, r_word[29: 6] };

                3'b001: r_addr <= { {(19){ r_word[29]}}, r_word[29:24] };

                3'b011: r_addr <= { {(13){ r_word[29]}}, r_word[29:18] };

                3'b101: r_addr <= { {( 7){ r_word[29]}}, r_word[29:12] };

                3'b111: r_addr <= { {( 1){ r_word[29]}}, r_word[29: 6] };

                endcase

        wire    [31:0]   w_addr;

        assign  w_addr = { {(7){r_addr[24]}}, r_addr };

        reg     [9:0]    rd_len;

        always @(posedge i_clk)

                if (~r_word[34])

                        rd_len <= 10'h01 + { 6'h00, r_word[33:31] };

                else

                        rd_len <= 10'h08 + { 1'b0, r_word[33:31], r_word[29:24] };

        // Clock three, read the table value

        //      { o_stb, r_stb } = 4'h4 when done

        // Maintaining ...

        //      r_word (clock 1)

        //      r_addr, rd_len (clock 2)

        reg     [31:0]   cword;

        always @(posedge i_clk)

                cword <= compression_tbl[cmd_addr];

        // Pipeline the strobe signal to create an output strobe, 3 clocks later

        reg     [2:0]    r_stb;

        initial r_stb = 0;

        always @(posedge i_clk)

                r_stb <= { r_stb[1:0], i_stb };

        // Clock four, now that the table value is valid, let's set our output

        // word.

        //      { o_stb, r_stb } = 4'h8 when done

        always @(posedge i_clk)

                o_stb <= r_stb[2];

        // Maintaining ...

        //      r_word          (clock 1)

        //      r_addr, rd_len  (clock 2)

        //      cword           (clock 3)

        //              Any/all of these can be pipelined for faster operation

        // However, speed is really limited by the speed of the I/O port.  At

        // it's fastest, it's 1 bit per clock, 48 clocks per codeword therefore,

        // thus ... things will hold still for much longer than just 5 clocks.

        always @(posedge i_clk)

                if (r_word[35:30] == 6'b101110)

                        o_word <= r_word;

                else casez(r_word[35:30])

                // Set address from something compressed ... unsigned

                6'b001??0: o_word <= { 4'h0, w_addr[31:0] };

                // Set a new address as a signed offset from the last (set) one

                //      (The last address is kept further down the chain,

                //      we just mark here that the address is to be set

                //      relative to it, and by how much.)

                6'b001??1: o_word <= { 3'h1, w_addr[31:30], 1'b1, w_addr[29:0]};

                // Write a value to the bus, with the value given from our

                // codeword table

                6'b010???: o_word <=

                        { 3'h3, cword[31:30], r_word[30], cword[29:0] };

                // Read, highly compressed length (1 word)

                6'b10????: o_word <= { 5'b11000, r_word[30], 20'h00, rd_len };

                // Read, two word (3+9 bits) length

                6'b11????: o_word <= { 5'b11000, r_word[30], 20'h00, rd_len };

                default: o_word <= r_word;

                endcase

endmodule