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

//

// Filename:    wbucompress.v

//

// Project:     XuLA2 board

//

// Purpose:     When reading many words that are identical, it makes no sense

//              to spend the time transmitting the same thing over and over

//      again, especially on a slow channel.  Hence this routine uses a table

//      lookup to see if the word to be transmitted was one from the recent

//      past.  If so, the word is replaced with an address of the recently

//      transmitted word.  Mind you, the table lookup takes one clock per table

//      entry, so even if a word is in the table it might not be found in time.

//      If the word is not in the table, or if it isn't found due to a lack of

//      time, the word is placed into the table while incrementing every other

//      table address.

//

//      Oh, and on a new address--the table is reset and starts over.  This way,

//      any time the host software changes, the host software will always start

//      by issuing a new address--hence the table is reset for every new piece

//      of software that may wish to communicate.

//

//

// 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

//

//

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

//

//

// All input words are valid codewords.  If we can, we make them

// better here.

module  wbucompress(i_clk, i_stb, i_codword, o_stb, o_cword, i_busy);

        parameter       DW=32, CW=36, TBITS=10;

        input                           i_clk, i_stb;

        input           [(CW-1):0]       i_codword;

        output  wire                    o_stb;

        output  wire    [(CW-1):0]       o_cword;

        input                           i_busy;

        //

        //

        // First stage is to compress the address.

        // This stage requires one clock.

        //

        //      ISTB,ICODWORD

        //      ISTB2,IWRD2     ASTB,AWORD

        //      ISTB3,IWRD3     ASTB2,AWRD2     I_BUSY(1)

        //      ISTB3,IWRD3     ASTB2,AWRD2     I_BUSY(1)

        //      ISTB3,IWRD3     ASTB2,AWRD2     I_BUSY(1)

        //      ISTB3,IWRD3     ASTB2,AWRD2     

        //      ISTB4,IWRD4     ASTB3,AWRD3     I_BUSY(2)

        //      ISTB4,IWRD4     ASTB3,AWRD3     I_BUSY(2)

        //      ISTB4,IWRD4     ASTB3,AWRD3     I_BUSY(2)

        reg             a_stb;

        reg     [35:0]   a_addrword;

        wire    [31:0]   w_addr;

        assign  w_addr = i_codword[31:0];

        always @(posedge i_clk)

                if ((i_stb)&&(~a_stb))

                begin

                        if (i_codword[35:32] != 4'h2)

                        begin

                                a_addrword <= i_codword;

                        end else if (w_addr[31:6] == 26'h00)

                                a_addrword <= { 6'hc, w_addr[ 5:0], 24'h00 };

                        else if (w_addr[31:12] == 20'h00)

                                a_addrword <= { 6'hd, w_addr[11:0], 18'h00 };

                        else if (w_addr[31:18] == 14'h00)

                                a_addrword <= { 6'he, w_addr[17:0], 12'h00 };

                        else if (w_addr[31:24] == 8'h00)

                                a_addrword <= { 6'hf, w_addr[23:0],  6'h00 };

                        else begin

                                a_addrword <= i_codword;

                        end

                end

        initial a_stb = 1'b0;

        always @(posedge i_clk)

                if ((i_stb)&&(~a_stb))

                        a_stb <= i_stb;

                else if (~i_busy)

                        a_stb <= 1'b0;

        //

        //

        // The next stage attempts to replace data codewords with previous

        // codewords that may have been sent.  The memory is only allowed

        // to be as old as the last new address command.  In this fashion,

        // any program that wishes to talk to the device can start with a

        // known compression table by simply setting the address and then

        // reading from the device.

        //

        // We start over any time a new value shows up, and 

        // the follow-on isn't busy and can take it.  Likewise,

        // we reset the writer on the compression any time a

        // i_clr value comes through (i.e., ~i_cyc or new

        // address)

        wire    w_accepted;

        assign  w_accepted = (a_stb)&&(~i_busy);

        reg             r_stb;

        always @(posedge i_clk)

                r_stb <= a_stb;

        wire    [35:0]   r_word;

        assign  r_word = a_addrword;

        //

        // First step of the compression is keeping track of a compression

        // table.  And the first part of that is keeping track of what address

        // to write into the compression table, and whether or not the entire

        // table is full or not.  This logic follows:

        //

        reg     [(TBITS-1):0]    tbl_addr;

        reg                     r_compressed, tbl_filled;

        // First part, write the compression table

        always @(posedge i_clk)

                // If we send a new address, then reset the table to empty

                if (w_accepted)

                begin

                        // Reset on new address (0010xx) and on new compressed

                        // addresses (0011ll).

                        if (o_cword[35:33]==3'h1)

                                tbl_addr <= 0;

                        // Otherwise, on any valid return result that wasn't

                        // from our table, for whatever reason (such as didn't

                        // have the clocks to find it, etc.), increment the

                        // address to add another value into our table

                        else if (o_cword[35:33] == 3'b111)

                                tbl_addr <= tbl_addr + {{(TBITS-1){1'b0}},1'b1};

                end

        always @(posedge i_clk)

                if ((w_accepted)&&(o_cword[35:33]==3'h1)) // on new address

                        tbl_filled <= 1'b0;

                else if (tbl_addr == 10'h3ff)

                        tbl_filled <= 1'b1;

        // Now that we know where we are writing into the table, and what

        // values of the table are valid, we need to actually write into

        // the table.

        //

        // We can keep this logic really simple by writing on every clock

        // and writing junk on many of those clocks, but we'll need to remember

        // that the value of the table at tbl_addr is unreliable until tbl_addr

        // changes.

        //

        reg     [31:0]   compression_tbl [0:((1<<TBITS)-1)];

        // Write new values into the table

        always @(posedge i_clk)

                compression_tbl[tbl_addr] <= { r_word[32:31], r_word[29:0] };

        // Now that we have a working table, can we use it?

        // On any new word, we'll start looking through our codewords.

        // If we find any that matches, we're there.  We might (or might not)

        // make it through the table first.  That's irrelevant.  We just look

        // while we can.

        reg     [(TBITS-1):0]    rd_addr;

        wire    [(TBITS-1):0]    nxt_rd_addr;

        assign  nxt_rd_addr = rd_addr - { {(TBITS-1){1'b0}}, 1'b1 };

        initial rd_addr = 0;

        always @(posedge i_clk)

                if ((w_accepted)||(~a_stb))

                        // Keep in mind, if a write was just accepted, then

                        // rd_addr will need to be reset on the next clock

                        // when (~a_stb).  Hence this must be a two clock

                        // update

                        rd_addr <= tbl_addr + {(TBITS){1'b1}};

                else if ((nxt_rd_addr != tbl_addr)&&(~match)

                                &&((~nxt_rd_addr[TBITS-1])||(tbl_filled)))

                        rd_addr <= nxt_rd_addr;

        reg     [(DW-1):0]       cword;

        reg     [(TBITS-1):0]    caddr;

        always @(posedge i_clk)

        begin

                cword <= compression_tbl[rd_addr];

                caddr <= rd_addr;

        end

        reg             match;

        reg     [9:0]    matchaddr;

        always @(posedge i_clk)

                if((w_accepted)||(~a_stb)||(~r_stb))// Reset upon any write

                        match <= 1'b0;

                else if (~match)

                begin

                        // To be a match, the table must not be empty,

                        match <= (({1'b0, caddr } < {tbl_filled, tbl_addr}))

                                        // the word we are matching to must be

                                        // in the form of a read command

                                        &&(r_word[35:33] == 3'b111)

                                        // And the word in the table must be

                                        // identical to the word we are about

                                        // to send.

                                        &&(cword == { r_word[32:31], r_word[29:0] });

                        matchaddr <= tbl_addr-caddr;

                end

        // Did we find something?

        wire    [(TBITS-1):0]    adr_diff;

        wire    [9:0]            adr_dbld;

        wire    [2:0]            adr_hlfd;

        assign  adr_diff = matchaddr;

        assign  adr_hlfd = matchaddr[2:0]- 3'd2;

        assign  adr_dbld = matchaddr- 10'd10;

        initial r_compressed = 1'b0;

        reg     [(CW-1):0]       r_cword; // Record our result

        always @(posedge i_clk)

        begin

                if ((~a_stb)||(~r_stb)||(w_accepted))//Reset whenever word gets written

                        r_compressed <= 1'b0; // to our output

                else if (r_compressed)//Already compressed, wait 'til sent

                        ;

                else if ((match)&&(matchaddr < 10'd521))

                begin

                        if (matchaddr == 10'h1)

                                r_cword[35:30] <= { 5'h3, r_word[30] };

                        else if (adr_diff < 10'd10)

                                r_cword[35:30] <= { 2'b10, adr_hlfd, r_word[30] };

                        else // if (adr_diff < 10'd521)

                                r_cword[35:24] <= { 2'b01, adr_dbld[8:6],

                                                r_word[30], adr_dbld[5:0] };

                        r_compressed <= 1'b1;

                end else

                        r_cword <= r_word;

        end

        // Can we do this without a clock delay?

        assign  o_stb = a_stb;

        assign  o_cword = (r_compressed)?(r_cword):(a_addrword);

endmodule