path: root/mod_subtractor.v

//------------------------------------------------------------------------------
//
// mod_subtractor.v
// -----------------------------------------------------------------------------
// Modular subtractor.
//
// Authors: Pavel Shatov
//
// Copyright (c) 2016, NORDUnet A/S
//
// 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.
//
// - Neither the name of the NORDUnet nor the names of its contributors may be
//   used to endorse or promote products derived from this software without
//   specific prior written permission.
//
// 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 mod_subtractor
  (
   clk, rst_n,
   ena, rdy,
   ab_addr, n_addr, d_addr, d_wren,
   a_din, b_din, n_din, d_dout
   );


    //
    // Settings
    //
    `include "ed25519_settings.vh"


   //
   // Parameters
   //
   parameter	OPERAND_NUM_WORDS	= 8;
   parameter	WORD_COUNTER_WIDTH	= 3;


   //
   // Handy Numbers
   //
   localparam	[WORD_COUNTER_WIDTH-1:0]	WORD_INDEX_ZERO	= 0;
   localparam	[WORD_COUNTER_WIDTH-1:0]	WORD_INDEX_LAST	= OPERAND_NUM_WORDS - 1;


   //
   // Handy Functions
   //
   function	[WORD_COUNTER_WIDTH-1:0]	WORD_INDEX_NEXT_OR_ZERO;
      input	[WORD_COUNTER_WIDTH-1:0]	WORD_INDEX_CURRENT;
      begin
	 WORD_INDEX_NEXT_OR_ZERO = (WORD_INDEX_CURRENT < WORD_INDEX_LAST) ?
				   WORD_INDEX_CURRENT + 1'b1 : WORD_INDEX_ZERO;
      end
   endfunction


   //
   // Ports
   //
   input		wire										clk;			// system clock
   input		wire										rst_n;		// active-low async reset

   input		wire										ena;			// enable input
   output	wire 											rdy;			// ready output

   output	wire [WORD_COUNTER_WIDTH-1:0] 								ab_addr;		// index of current A and B words
   output	wire [WORD_COUNTER_WIDTH-1:0] 								n_addr;		// index of current N word
   output	wire [WORD_COUNTER_WIDTH-1:0] 								d_addr;		// index of current D word
   output	wire 											d_wren;		// store current D word now

   input		wire [                  31:0] 							a_din;		// A
   input		wire [                  31:0] 							b_din;		// B
   input		wire [                  31:0] 							n_din;		// N
   output	wire [                  31:0] 								d_dout;		// D = (A - B) mod N


   //
   // Word Indices
   //
   reg [WORD_COUNTER_WIDTH-1:0] 									index_ab;
   reg [WORD_COUNTER_WIDTH-1:0] 									index_n;
   reg [WORD_COUNTER_WIDTH-1:0] 									index_d;

   /* map registers to output ports */
   assign ab_addr	= index_ab;
   assign n_addr	= index_n;
   assign d_addr	= index_d;


   //
   // Subtractor
   //
   wire [31: 0] 											sub32_d;
   wire 												sub32_b_in;
   wire 												sub32_b_out;

   `ED25519_SUB32_PRIMITIVE subtractor32
     (
      .clk		(clk),
      .a			(a_din),
      .b			(b_din),
      .d			(sub32_d),
      .b_in		(sub32_b_in),
      .b_out	(sub32_b_out)
      );


   //
   // Adder
   //
   wire [31: 0] 											add32_s;
   wire 												add32_c_in;
   wire 												add32_c_out;

   `ED25519_ADD32_PRIMITIVE adder32
     (
      .clk		(clk),
      .a			(sub32_d),
      .b			(n_din),
      .s			(add32_s),
      .c_in		(add32_c_in),
      .c_out	(add32_c_out)
      );


   //
   // FSM
   //

   localparam FSM_SHREG_WIDTH = 2*OPERAND_NUM_WORDS + 5;

   reg [FSM_SHREG_WIDTH-1:0] 										fsm_shreg;

   assign rdy = fsm_shreg[0];

   wire [OPERAND_NUM_WORDS-1:0] 									fsm_shreg_inc_index_ab	= fsm_shreg[FSM_SHREG_WIDTH - (0 * OPERAND_NUM_WORDS + 1) : FSM_SHREG_WIDTH - (1 * OPERAND_NUM_WORDS + 0)];
   wire [OPERAND_NUM_WORDS-1:0] 									fsm_shreg_inc_index_n		= fsm_shreg[FSM_SHREG_WIDTH - (0 * OPERAND_NUM_WORDS + 2) : FSM_SHREG_WIDTH - (1 * OPERAND_NUM_WORDS + 1)];
   wire [OPERAND_NUM_WORDS-1:0] 									fsm_shreg_store_dif_ab	= fsm_shreg[FSM_SHREG_WIDTH - (0 * OPERAND_NUM_WORDS + 3) : FSM_SHREG_WIDTH - (1 * OPERAND_NUM_WORDS + 2)];
   wire [OPERAND_NUM_WORDS-1:0] 									fsm_shreg_store_dif_ab_n	= fsm_shreg[FSM_SHREG_WIDTH - (0 * OPERAND_NUM_WORDS + 4) : FSM_SHREG_WIDTH - (1 * OPERAND_NUM_WORDS + 3)];
   wire [OPERAND_NUM_WORDS-1:0] 									fsm_shreg_store_data_d	= fsm_shreg[FSM_SHREG_WIDTH - (1 * OPERAND_NUM_WORDS + 4) : FSM_SHREG_WIDTH - (2 * OPERAND_NUM_WORDS + 3)];
   wire [OPERAND_NUM_WORDS-1:0] 									fsm_shreg_inc_index_d		= fsm_shreg[FSM_SHREG_WIDTH - (1 * OPERAND_NUM_WORDS + 5) : FSM_SHREG_WIDTH - (2 * OPERAND_NUM_WORDS + 4)];

   wire 												fsm_latch_msb_borrow	= fsm_shreg[FSM_SHREG_WIDTH - (1 * OPERAND_NUM_WORDS + 2)];

   wire 												inc_index_ab		= |fsm_shreg_inc_index_ab;
   wire 												inc_index_n		= |fsm_shreg_inc_index_n;
   wire 												store_dif_ab		= |fsm_shreg_store_dif_ab;
   wire 												store_dif_ab_n	= |fsm_shreg_store_dif_ab_n;
   wire 												store_data_d		= |fsm_shreg_store_data_d;
   wire 												inc_index_d		= |fsm_shreg_inc_index_d;

   always @(posedge clk or negedge rst_n)
     //
     if (rst_n == 1'b0)
       //
       fsm_shreg <= {{FSM_SHREG_WIDTH-1{1'b0}}, 1'b1};
   //
     else begin
	//
	if (rdy)	fsm_shreg <= {ena, {FSM_SHREG_WIDTH-2{1'b0}}, ~ena};
	//
	else		fsm_shreg <= {1'b0, fsm_shreg[FSM_SHREG_WIDTH-1:1]};
	//
     end


   //
   // Borrow & Carry Masking Logic
   //
   reg	sub32_b_mask;
   reg	add32_c_mask;


   always @(posedge clk) begin
      //
      sub32_b_mask <= (index_ab == WORD_INDEX_ZERO) ? 1'b1 : 1'b0;
      add32_c_mask <= (index_n  == WORD_INDEX_ZERO) ? 1'b1 : 1'b0;
      //
   end

   assign sub32_b_in = sub32_b_out & ~sub32_b_mask;
   assign add32_c_in = add32_c_out & ~add32_c_mask;



   //
   // Borrow & Carry Latch Logic
   //
   reg sub32_borrow_latch;

   always @(posedge clk) begin
      //
      if (fsm_latch_msb_borrow) sub32_borrow_latch <= sub32_b_out;
      //
   end


   //
   // Intermediate Results
   //
   reg	[32*OPERAND_NUM_WORDS-1:0]		d_ab;
   reg [32*OPERAND_NUM_WORDS-1:0] 		d_ab_n;

   always @(posedge clk)
     //
     if (store_data_d) begin
	//
	d_ab		<= {{32{1'bX}}, d_ab[32*OPERAND_NUM_WORDS-1:32]};
	d_ab_n	<= {{32{1'bX}}, d_ab_n[32*OPERAND_NUM_WORDS-1:32]};
	//
     end else begin
	//
	if (store_dif_ab) d_ab <= {sub32_d, d_ab[32*OPERAND_NUM_WORDS-1:32]};
	if (store_dif_ab_n) d_ab_n <= {add32_s, d_ab_n[32*OPERAND_NUM_WORDS-1:32]};
	//
     end


   //
   // Word Index Increment Logic
   //
   always @(posedge clk)
     //
     if (rdy) begin
	//
	index_ab		<= WORD_INDEX_ZERO;
	index_n		<= WORD_INDEX_ZERO;
	index_d		<= WORD_INDEX_ZERO;
	//
     end else begin
	//
	if (inc_index_ab) index_ab <= WORD_INDEX_NEXT_OR_ZERO(index_ab);
	if (inc_index_n)	index_n	<= WORD_INDEX_NEXT_OR_ZERO(index_n);
	if (inc_index_d)	index_d	<= WORD_INDEX_NEXT_OR_ZERO(index_d);
	//
     end


   //
   // Output Sum Selector
   //
   wire	mux_select_ab_n = sub32_borrow_latch;


   //
   // Output Data and Write Enable Logic
   //
   reg 	d_wren_reg;
   reg [31: 0] d_dout_reg;
   wire [31: 0] d_dout_mux = mux_select_ab_n ? d_ab_n[31:0] : d_ab[31:0];

   assign d_wren = d_wren_reg;
   assign d_dout = d_dout_reg;

   always @(posedge clk)
     //
     if (rdy) begin
	//
	d_wren_reg	<= 1'b0;
	d_dout_reg	<= {32{1'bX}};
	//
     end else begin
	//
	d_wren_reg <= store_data_d;
	d_dout_reg <= store_data_d ? d_dout_mux : {32{1'bX}};
	//
     end


endmodule


//------------------------------------------------------------------------------
// End-of-File
//------------------------------------------------------------------------------