Refactored into core and top.

2 files changed, 927 insertions, 775 deletions

diff --git a/src/rtl/modexp.v b/src/rtl/modexp.v
index 88060ea..e5158ed 100644
--- a/src/rtl/modexp.v
+++ b/src/rtl/modexp.v
@@ -104,34 +104,9 @@ module modexp(
   localparam DEFAULT_MODLENGTH     = 8'h80; // 2048 bits.
   localparam DEFAULT_EXPLENGTH     = 8'h80;
 
-  localparam MONTPROD_SELECT_ONE_NR = 3'h0;
-  localparam MONTPROD_SELECT_X_NR   = 3'h1;
-  localparam MONTPROD_SELECT_Z_P    = 3'h2;
-  localparam MONTPROD_SELECT_P_P    = 3'h3;
-  localparam MONTPROD_SELECT_Z_ONE  = 3'h4;
-
-  localparam MONTPROD_DEST_Z        = 2'b00;
-  localparam MONTPROD_DEST_P        = 2'b01;
-  localparam MONTPROD_DEST_NOWHERE  = 2'b10;
-
-  localparam CTRL_IDLE           = 4'h0;
-  localparam CTRL_RESIDUE        = 4'h1;
-  localparam CTRL_CALCULATE_Z0   = 4'h2;
-  localparam CTRL_CALCULATE_P0   = 4'h3;
-  localparam CTRL_ITERATE        = 4'h4;
-  localparam CTRL_ITERATE_Z_P    = 4'h5;
-  localparam CTRL_ITERATE_P_P    = 4'h6;
-  localparam CTRL_ITERATE_END    = 4'h7;
-  localparam CTRL_CALCULATE_ZN   = 4'h8;
-  localparam CTRL_DONE           = 4'h9;
-
-  localparam EXPONATION_MODE_SECRET_SECURE = 1'b0;
-  localparam EXPONATION_MODE_PUBLIC_FAST   = 1'b1; //for rsa, c=M^65537 etc, there is no need to slow down to hide the exponent
-
-
-  localparam CORE_NAME0          = 32'h6d6f6465; // "mode"
-  localparam CORE_NAME1          = 32'h78702020; // "xp  "
-  localparam CORE_VERSION        = 32'h302e3531; // "0.51"
+  localparam CORE_NAME0            = 32'h6d6f6465; // "mode"
+  localparam CORE_NAME1            = 32'h78702020; // "xp  "
+  localparam CORE_VERSION          = 32'h302e3532; // "0.52"
 
 
   //----------------------------------------------------------------
@@ -145,146 +120,36 @@ module modexp(
   reg [07 : 0] modulus_length_new;
   reg          modulus_length_we;
 
-  reg [07 : 0] length_m1_reg;
-  reg [07 : 0] length_m1_new;
-
   reg          start_reg;
   reg          start_new;
-  reg          start_we;
-  reg          clear_start;
-
-  reg          ready_reg;
-  reg          ready_new;
-  reg          ready_we;
-
-  reg [2 : 0]  montprod_select_reg;
-  reg [2 : 0]  montprod_select_new;
-  reg          montprod_select_we;
-  reg [1 : 0]  montprod_dest_reg;
-  reg [1 : 0]  montprod_dest_new;
-  reg          montprod_dest_we;
-
-  reg [3 : 0]  modexp_ctrl_reg;
-  reg [3 : 0]  modexp_ctrl_new;
-  reg          modexp_ctrl_we;
-
-  reg [31 : 0] one_reg;
-  reg [31 : 0] one_new;
-  reg [31 : 0] b_one_reg;
-  reg [31 : 0] b_one_new;
-
-  reg [12 : 0] loop_counter_reg;
-  reg [12 : 0] loop_counter_new;
-  reg          loop_counter_we;
-
-  reg [07 : 0] E_word_index;
-  reg [04 : 0] E_bit_index;
-  reg          last_iteration;
-
-  reg          ei_reg;
-  reg          ei_new;
-  reg          ei_we;
-
-  reg          exponation_mode_reg;
-  reg          exponation_mode_new;
-  reg          exponation_mode_we;
-
-  reg [31 : 0] cycle_ctr_low_reg;
-  reg [31 : 0] cycle_ctr_low_new;
-  reg          cycle_ctr_low_we;
-  reg [31 : 0] cycle_ctr_high_reg;
-  reg [31 : 0] cycle_ctr_high_new;
-  reg          cycle_ctr_high_we;
-  reg          cycle_ctr_state_reg;
-  reg          cycle_ctr_state_new;
-  reg          cycle_ctr_state_we;
-  reg          cycle_ctr_start;
-  reg          cycle_ctr_stop;
 
 
   //----------------------------------------------------------------
   // Wires.
   //----------------------------------------------------------------
-  reg [07 : 0]  modulus_mem_int_rd_addr;
-  wire [31 : 0] modulus_mem_int_rd_data;
-  wire [31 : 0] modulus_mem_api_rd_data;
-
-  reg [07 : 0]  message_mem_int_rd_addr;
-  wire [31 : 0] message_mem_int_rd_data;
-  wire [31 : 0] message_mem_api_rd_data;
-
-  reg [07 : 0]  exponent_mem_int_rd_addr;
-  wire [31 : 0] exponent_mem_int_rd_data;
-  wire [31 : 0] exponent_mem_api_rd_data;
-
-  wire [31 : 0] result_mem_api_rd_data;
-  reg  [07 : 0] result_mem_int_rd_addr;
-  wire [31 : 0] result_mem_int_rd_data;
-  reg  [07 : 0] result_mem_int_wr_addr;
-  reg  [31 : 0] result_mem_int_wr_data;
-  reg           result_mem_int_we;
-
-  reg  [07 : 0] p_mem_rd0_addr;
-  wire [31 : 0] p_mem_rd0_data;
-  reg  [07 : 0] p_mem_rd1_addr;
-  wire [31 : 0] p_mem_rd1_data;
-  reg  [07 : 0] p_mem_wr_addr;
-  reg  [31 : 0] p_mem_wr_data;
-  reg           p_mem_we;
-
-  reg [31 : 0]  tmp_read_data;
-
-  reg           montprod_calc;
-  wire          montprod_ready;
-  reg [07 : 0]  montprod_length;
-
-  wire [07 : 0] montprod_opa_addr;
-  reg [31 : 0]  montprod_opa_data;
-
-  wire [07 : 0] montprod_opb_addr;
-  reg [31 : 0]  montprod_opb_data;
-
-  wire [07 : 0] montprod_opm_addr;
-  reg [31 : 0]  montprod_opm_data;
-
-  wire [07 : 0] montprod_result_addr;
-  wire [31 : 0] montprod_result_data;
-  wire          montprod_result_we;
-
-  reg           residue_calculate;
-  wire          residue_ready;
-  reg [14 : 0]  residue_nn;
-  reg  [07 : 0] residue_length;
-  wire [07 : 0] residue_opa_rd_addr;
-  wire [31 : 0] residue_opa_rd_data;
-  wire [07 : 0] residue_opa_wr_addr;
-  wire [31 : 0] residue_opa_wr_data;
-  wire          residue_opa_wr_we;
-  wire [07 : 0] residue_opm_addr;
-  reg [31 : 0]  residue_opm_data;
-
-  reg  [07 : 0] residue_mem_montprod_read_addr;
-  wire [31 : 0] residue_mem_montprod_read_data;
-
-  reg           residue_valid_reg;
-  reg           residue_valid_new;
-  reg           invalidate_residue;
-  reg           residue_valid_int_validated;
+  reg           exponent_mem_api_rst;
+  reg           exponent_mem_api_cs;
+  reg           exponent_mem_api_wr;
+  wire [31 : 0] exponent_mem_api_read_data;
 
   reg           modulus_mem_api_rst;
   reg           modulus_mem_api_cs;
   reg           modulus_mem_api_wr;
-
-  reg           exponent_mem_api_rst;
-  reg           exponent_mem_api_cs;
-  reg           exponent_mem_api_wr;
+  wire [31 : 0] modulus_mem_api_read_data;
 
   reg           message_mem_api_rst;
   reg           message_mem_api_cs;
   reg           message_mem_api_wr;
+  wire [31 : 0] message_mem_api_read_data;
 
   reg           result_mem_api_rst;
   reg           result_mem_api_cs;
+  wire [31 : 0] result_mem_api_read_data;
+
+  wire          ready;
+  wire [63 : 0] cycles;
+
+  reg [31 : 0]  tmp_read_data;
 
 
   //----------------------------------------------------------------
@@ -296,119 +161,40 @@ module modexp(
   //----------------------------------------------------------------
   // core instantiations.
   //----------------------------------------------------------------
-  montprod montprod_inst(
-                         .clk(clk),
-                         .reset_n(reset_n),
-
-                         .calculate(montprod_calc),
-                         .ready(montprod_ready),
-
-                         .length(montprod_length),
-
-                         .opa_addr(montprod_opa_addr),
-                         .opa_data(montprod_opa_data),
-
-                         .opb_addr(montprod_opb_addr),
-                         .opb_data(montprod_opb_data),
-
-                         .opm_addr(montprod_opm_addr),
-                         .opm_data(montprod_opm_data),
-
-                         .result_addr(montprod_result_addr),
-                         .result_data(montprod_result_data),
-                         .result_we(montprod_result_we)
-                        );
-
-
-  residue residue_inst(
-                     .clk(clk),
-                     .reset_n(reset_n),
-                     .calculate(residue_calculate),
-                     .ready(residue_ready),
-                     .nn(residue_nn),
-                     .length(residue_length),
-                     .opa_rd_addr(residue_opa_rd_addr),
-                     .opa_rd_data(residue_opa_rd_data),
-                     .opa_wr_addr(residue_opa_wr_addr),
-                     .opa_wr_data(residue_opa_wr_data),
-                     .opa_wr_we(residue_opa_wr_we),
-                     .opm_addr(residue_opm_addr),
-                     .opm_data(residue_opm_data)
-                    );
-
-  blockmem2r1w residue_mem(
-                           .clk(clk),
-                           .read_addr0(residue_opa_rd_addr),
-                           .read_data0(residue_opa_rd_data),
-                           .read_addr1(residue_mem_montprod_read_addr),
-                           .read_data1(residue_mem_montprod_read_data),
-                           .wr(residue_opa_wr_we),
-                           .write_addr(residue_opa_wr_addr),
-                           .write_data(residue_opa_wr_data)
-                          );
-
-  blockmem2r1wptr modulus_mem(
-                              .clk(clk),
-                              .reset_n(reset_n),
-                              .read_addr0(modulus_mem_int_rd_addr),
-                              .read_data0(modulus_mem_int_rd_data),
-                              .read_data1(modulus_mem_api_rd_data),
-                              .rst(modulus_mem_api_rst),
-                              .cs(modulus_mem_api_cs),
-                              .wr(modulus_mem_api_wr),
-                              .write_data(write_data)
-                             );
-
-
-  blockmem2r1wptr message_mem(
-                              .clk(clk),
-                              .reset_n(reset_n),
-                              .read_addr0(message_mem_int_rd_addr),
-                              .read_data0(message_mem_int_rd_data),
-                              .read_data1(message_mem_api_rd_data),
-                              .rst(message_mem_api_rst),
-                              .cs(message_mem_api_cs),
-                              .wr(message_mem_api_wr),
-                              .write_data(write_data)
-                             );
-
-
-  blockmem2r1wptr exponent_mem(
-                               .clk(clk),
-                               .reset_n(reset_n),
-                               .read_addr0(exponent_mem_int_rd_addr),
-                               .read_data0(exponent_mem_int_rd_data),
-                               .read_data1(exponent_mem_api_rd_data),
-                               .rst(exponent_mem_api_rst),
-                               .cs(exponent_mem_api_cs),
-                               .wr(exponent_mem_api_wr),
-                               .write_data(write_data)
-                              );
-
-
-  blockmem2rptr1w result_mem(
-                             .clk(clk),
-                             .reset_n(reset_n),
-                             .read_addr0(result_mem_int_rd_addr[7 : 0]),
-                             .read_data0(result_mem_int_rd_data),
-                             .read_data1(result_mem_api_rd_data),
-                             .rst(result_mem_api_rst),
-                             .cs(result_mem_api_cs),
-                             .wr(result_mem_int_we),
-                             .write_addr(result_mem_int_wr_addr),
-                             .write_data(result_mem_int_wr_data)
-                            );
-
-  blockmem2r1w p_mem(
-                     .clk(clk),
-                     .read_addr0(p_mem_rd0_addr),
-                     .read_data0(p_mem_rd0_data),
-                     .read_addr1(p_mem_rd1_addr),
-                     .read_data1(p_mem_rd1_data),
-                     .wr(p_mem_we),
-                     .write_addr(p_mem_wr_addr),
-                     .write_data(p_mem_wr_data)
-                    );
+  modexp_core core_inst(
+                        .clk(clk),
+                        .reset_n(reset_n),
+
+                        .start(start_reg),
+                        .ready(ready),
+
+                        .exponent_length(exponent_length_reg),
+                        .modulus_length(modulus_length_reg),
+
+                        .cycles(cycles),
+
+                        .exponent_mem_api_cs(exponent_mem_api_cs),
+                        .exponent_mem_api_wr(exponent_mem_api_wr),
+                        .exponent_mem_api_rst(exponent_mem_api_rst),
+                        .exponent_mem_api_write_data(write_data),
+                        .exponent_mem_api_read_data(exponent_mem_api_read_data),
+
+                        .modulus_mem_api_cs(modulus_mem_api_cs),
+                        .modulus_mem_api_wr(modulus_mem_api_wr),
+                        .modulus_mem_api_rst(modulus_mem_api_rst),
+                        .modulus_mem_api_write_data(write_data),
+                        .modulus_mem_api_read_data(modulus_mem_api_read_data),
+
+                        .message_mem_api_cs(message_mem_api_cs),
+                        .message_mem_api_wr(message_mem_api_wr),
+                        .message_mem_api_rst(message_mem_api_rst),
+                        .message_mem_api_write_data(write_data),
+                        .message_mem_api_read_data(message_mem_api_read_data),
+
+                        .result_mem_api_cs(result_mem_api_cs),
+                        .result_mem_api_rst(result_mem_api_rst),
+                        .result_mem_api_read_data(result_mem_api_read_data)
+                       );
 
 
   //----------------------------------------------------------------
@@ -422,73 +208,23 @@ module modexp(
     begin
       if (!reset_n)
         begin
+          start_reg           <= 1'b0;
           exponent_length_reg <= DEFAULT_EXPLENGTH;
           modulus_length_reg  <= DEFAULT_MODLENGTH;
-          start_reg           <= 1'b0;
-          ready_reg           <= 1'b1;
-          montprod_select_reg <= MONTPROD_SELECT_ONE_NR;
-          montprod_dest_reg   <= MONTPROD_DEST_NOWHERE;
-          modexp_ctrl_reg     <= CTRL_IDLE;
-          one_reg             <= 32'h0;
-          b_one_reg           <= 32'h0;
-          length_m1_reg       <= 8'h0;
-          loop_counter_reg    <= 13'b0;
-          ei_reg              <= 1'b0;
-          residue_valid_reg   <= 1'b0;
-          exponation_mode_reg <= EXPONATION_MODE_SECRET_SECURE;
-          cycle_ctr_low_reg   <= 32'h00000000;
-          cycle_ctr_high_reg  <= 32'h00000000;
-          cycle_ctr_state_reg <= 1'b0;
         end
       else
         begin
-          one_reg             <= one_new;
-          b_one_reg           <= b_one_new;
-          residue_valid_reg   <= residue_valid_new;
+          start_reg <= start_new;
 
           if (exponent_length_we)
             begin
-              exponent_length_reg <= exponent_length_new;
+              exponent_length_reg <= write_data[7 : 0];
             end
 
           if (modulus_length_we)
             begin
-              modulus_length_reg <= modulus_length_new;
-              length_m1_reg       <= length_m1_new;
+              modulus_length_reg <= write_data[7 : 0];
             end
-
-          if (start_we)
-            start_reg <= start_new;
-
-          if (ready_we)
-            ready_reg <= ready_new;
-
-          if (montprod_select_we)
-            montprod_select_reg <= montprod_select_new;
-
-          if (montprod_dest_we)
-            montprod_dest_reg <= montprod_dest_new;
-
-          if (loop_counter_we)
-            loop_counter_reg <= loop_counter_new;
-
-          if (ei_we)
-            ei_reg <= ei_new;
-
-          if (exponation_mode_we)
-            exponation_mode_reg <= exponation_mode_new;
-
-          if (cycle_ctr_low_we)
-            cycle_ctr_low_reg <= cycle_ctr_low_new;
-
-          if (cycle_ctr_high_we)
-            cycle_ctr_high_reg <= cycle_ctr_high_new;
-
-          if (cycle_ctr_state_we)
-            cycle_ctr_state_reg <= cycle_ctr_state_new;
-
-          if (modexp_ctrl_we)
-            modexp_ctrl_reg <= modexp_ctrl_new;
         end
     end // reg_update
 
@@ -500,11 +236,9 @@ module modexp(
   //----------------------------------------------------------------
   always @*
     begin : api
-      modulus_length_we   = 1'b0;
-      exponent_length_we  = 1'b0;
-      start_new           = 1'b0;
-      start_we            = 1'b0;
-      invalidate_residue  = 1'b0;
+      modulus_length_we    = 1'b0;
+      exponent_length_we   = 1'b0;
+      start_new            = 1'b0;
 
       modulus_mem_api_rst  = 1'b0;
       modulus_mem_api_cs   = 1'b0;
@@ -521,24 +255,7 @@ module modexp(
       result_mem_api_rst   = 1'b0;
       result_mem_api_cs    = 1'b0;
 
-
-      //TODO: Add API code to enable fast exponation for working with public exponents.
-      exponation_mode_we  = 1'b0;
-
-      exponation_mode_new = EXPONATION_MODE_SECRET_SECURE;
-      modulus_length_new  = write_data[7 : 0];
-      exponent_length_new = write_data[7 : 0];
-      length_m1_new       = write_data[7 : 0] - 8'h1;
-
-      tmp_read_data       = 32'h00000000;
-
-      // We need to be able to clear start bit after we have
-      // started a modexp operation.
-      if (clear_start)
-        begin
-          start_new = 1'b0;
-          start_we  = 1'b1;
-        end
+      tmp_read_data        = 32'h00000000;
 
       if (cs)
         begin
@@ -551,7 +268,6 @@ module modexp(
                       ADDR_CTRL:
                         begin
                           start_new = write_data[0];
-                          start_we  = 1'b1;
                         end
 
                       ADDR_MODULUS_LENGTH:
@@ -573,7 +289,6 @@ module modexp(
                         begin
                           modulus_mem_api_cs = 1'b1;
                           modulus_mem_api_wr = 1'b1;
-                          invalidate_residue = 1'b1;
                         end
 
                       ADDR_EXPONENT_PTR_RST:
@@ -624,13 +339,13 @@ module modexp(
                         tmp_read_data = {31'h00000000, start_reg};
 
                       ADDR_STATUS:
-                        tmp_read_data = {31'h00000000, ready_reg};
+                        tmp_read_data = {31'h00000000, ready};
 
                       ADDR_CYCLES_HIGH:
-                        tmp_read_data = cycle_ctr_high_reg;
+                        tmp_read_data = cycles[63 : 32];
 
                       ADDR_CYCLES_LOW:
-                        tmp_read_data = cycle_ctr_low_reg;
+                        tmp_read_data = cycles[31 : 0];
 
                       ADDR_MODULUS_LENGTH:
                         tmp_read_data = {24'h000000, modulus_length_reg};
@@ -641,25 +356,25 @@ module modexp(
                       ADDR_MODULUS_DATA:
                         begin
                           modulus_mem_api_cs = 1'b1;
-                          tmp_read_data      = modulus_mem_api_rd_data;
+                          tmp_read_data      = modulus_mem_api_read_data;
                         end
 
                       ADDR_EXPONENT_DATA:
                         begin
                           exponent_mem_api_cs = 1'b1;
-                          tmp_read_data       = exponent_mem_api_rd_data;
+                          tmp_read_data       = exponent_mem_api_read_data;
                         end
 
                       ADDR_MESSAGE_DATA:
                         begin
                           message_mem_api_cs = 1'b1;
-                          tmp_read_data      = message_mem_api_rd_data;
+                          tmp_read_data      = message_mem_api_read_data;
                         end
 
                       ADDR_RESULT_DATA:
                         begin
                           result_mem_api_cs = 1'b1;
-                          tmp_read_data     = result_mem_api_rd_data;
+                          tmp_read_data     = result_mem_api_read_data;
                         end
 
                       default:
@@ -677,432 +392,6 @@ module modexp(
         end // if (cs)
     end // api
 
-
-  //----------------------------------------------------------------
-  // cycle_ctr
-  //
-  // Implementation of the cycle counter
-  //----------------------------------------------------------------
-  always @*
-    begin : cycle_ctr
-      cycle_ctr_low_new   = 32'h00000000;
-      cycle_ctr_low_we    = 1'b0;
-      cycle_ctr_high_new  = 32'h00000000;
-      cycle_ctr_high_we   = 1'b0;
-      cycle_ctr_state_new = 1'b0;
-      cycle_ctr_state_we  = 1'b0;
-
-      if (cycle_ctr_start)
-        begin
-          cycle_ctr_low_new   = 32'h00000000;
-          cycle_ctr_low_we    = 1'b1;
-          cycle_ctr_high_new  = 32'h00000000;
-          cycle_ctr_high_we   = 1'b1;
-          cycle_ctr_state_new = 1'b1;
-          cycle_ctr_state_we  = 1'b1;
-        end
-
-      if (cycle_ctr_stop)
-        begin
-          cycle_ctr_state_new = 1'b0;
-          cycle_ctr_state_we  = 1'b1;
-        end
-
-      if (cycle_ctr_state_reg)
-        begin
-          cycle_ctr_low_new = cycle_ctr_low_reg + 1'b1;
-          cycle_ctr_low_we  = 1'b1;
-
-          if (cycle_ctr_low_new == 32'h00000000)
-            begin
-              cycle_ctr_high_new  = cycle_ctr_high_reg + 1'b1;
-              cycle_ctr_high_we   = 1'b1;
-            end
-        end
-    end // cycle_ctr
-
-
-  //----------------------------------------------------------------
-  // one
-  //
-  // generates the big integer one ( 00... 01 )
-  //----------------------------------------------------------------
-  always @*
-    begin : one_process
-      one_new   = 32'h00000000;
-      b_one_new = 32'h00000000;
-
-      if (montprod_opa_addr == length_m1_reg)
-        one_new = 32'h00000001;
-
-      if (montprod_opb_addr == length_m1_reg)
-        b_one_new = 32'h00000001;
-    end
-
-
-  //----------------------------------------------------------------
-  // Read mux for modulus. Needed since it is being
-  // addressed by two sources.
-  //----------------------------------------------------------------
-  always @*
-    begin : modulus_mem_reader_process
-      if (modexp_ctrl_reg == CTRL_RESIDUE)
-        modulus_mem_int_rd_addr = residue_opm_addr;
-      else
-        modulus_mem_int_rd_addr = montprod_opm_addr;
-    end
-
-
-  //----------------------------------------------------------------
-  // Feeds residue calculator.
-  //----------------------------------------------------------------
-  always @*
-    begin : residue_process
-      //N*2, N=length*32, *32 = shl5, *64 = shl6
-      residue_nn = { 1'b0, modulus_length_reg, 6'h0 };
-      residue_length = modulus_length_reg;
-      residue_opm_data = modulus_mem_int_rd_data;
-    end
-
-
-  //----------------------------------------------------------------
-  // Sets the register that decides if residue is valid or not.
-  //----------------------------------------------------------------
-  always @*
-    begin : residue_valid_process
-      if (invalidate_residue)
-        residue_valid_new = 1'b0;
-      else if ( residue_valid_int_validated == 1'b1)
-        residue_valid_new = 1'b1;
-     else
-        residue_valid_new = residue_valid_reg;
-    end
-
-
-  //----------------------------------------------------------------
-  // montprod_op_select
-  //
-  // Select operands used during montprod calculations depending
-  // on what operation we want to do.
-  //----------------------------------------------------------------
-  always @*
-    begin : montprod_op_select
-
-      montprod_length          = modulus_length_reg;
-
-      result_mem_int_rd_addr   = montprod_opa_addr;
-      message_mem_int_rd_addr  = montprod_opa_addr;
-      p_mem_rd0_addr           = montprod_opa_addr;
-
-      residue_mem_montprod_read_addr = montprod_opb_addr;
-      p_mem_rd1_addr                 = montprod_opb_addr;
-
-      montprod_opm_data = modulus_mem_int_rd_data;
-      case (montprod_select_reg)
-        MONTPROD_SELECT_ONE_NR:
-          begin
-            montprod_opa_data  = one_reg;
-            montprod_opb_data  = residue_mem_montprod_read_data;
-          end
-
-        MONTPROD_SELECT_X_NR:
-          begin
-            montprod_opa_data  = message_mem_int_rd_data;
-            montprod_opb_data  = residue_mem_montprod_read_data;
-          end
-
-        MONTPROD_SELECT_Z_P:
-          begin
-            montprod_opa_data  = result_mem_int_rd_data;
-            montprod_opb_data  = p_mem_rd1_data;
-          end
-
-        MONTPROD_SELECT_P_P:
-          begin
-            montprod_opa_data  = p_mem_rd0_data;
-            montprod_opb_data  = p_mem_rd1_data;
-          end
-
-        MONTPROD_SELECT_Z_ONE:
-          begin
-            montprod_opa_data  = result_mem_int_rd_data;
-            montprod_opb_data  = b_one_reg;
-          end
-
-        default:
-          begin
-            montprod_opa_data  = 32'h00000000;
-            montprod_opb_data  = 32'h00000000;
-          end
-      endcase // case (montprod_selcect_reg)
-    end
-
-
-  //----------------------------------------------------------------
-  // memory write mux
-  //
-  // Direct memory write signals to correct memory.
-  //----------------------------------------------------------------
-  always @*
-    begin : memory_write_process
-      result_mem_int_wr_addr = montprod_result_addr;
-      result_mem_int_wr_data = montprod_result_data;
-      result_mem_int_we      = 1'b0;
-
-      p_mem_wr_addr = montprod_result_addr;
-      p_mem_wr_data = montprod_result_data;
-      p_mem_we      = 1'b0;
-
-      case (montprod_dest_reg)
-        MONTPROD_DEST_Z:
-          result_mem_int_we = montprod_result_we;
-        MONTPROD_DEST_P:
-          p_mem_we = montprod_result_we;
-        default:
-          begin
-          end
-      endcase
-
-      // inhibit Z=Z*P when ei = 0
-      if (modexp_ctrl_reg == CTRL_ITERATE_Z_P)
-        result_mem_int_we = result_mem_int_we & ei_reg;
-    end
-
-
-  //----------------------------------------------------------------
-  // loop_counter
-  //
-  // Calculate the loop counter and related variables.
-  //----------------------------------------------------------------
-  always @*
-    begin : loop_counters_process
-      loop_counter_new = 13'b0;
-      loop_counter_we  = 1'b0;
-
-      if (loop_counter_reg == { length_m1_reg, 5'b11111 })
-        last_iteration = 1'b1;
-      else
-        last_iteration = 1'b0;
-
-      case (modexp_ctrl_reg)
-        CTRL_CALCULATE_P0:
-          begin
-            loop_counter_new = 13'b0;
-            loop_counter_we  = 1'b1;
-          end
-
-        CTRL_ITERATE_END:
-          begin
-            loop_counter_new = loop_counter_reg + 1'b1;
-            loop_counter_we  = 1'b1;
-          end
-
-        default:
-          begin
-          end
-      endcase
-    end
-
-
-  //----------------------------------------------------------------
-  // exponent
-  //
-  // Reads the exponent.
-  //----------------------------------------------------------------
-  always @*
-    begin : exponent_process
-      // Accessing new instead of reg - pick up update at
-      // CTRL_ITERATE_NEW to remove a pipeline stall.
-      E_word_index  = length_m1_reg - loop_counter_new[ 12 : 5 ];
-
-      E_bit_index   = loop_counter_reg[ 04 : 0 ];
-
-      exponent_mem_int_rd_addr = E_word_index;
-
-      ei_new = exponent_mem_int_rd_data[ E_bit_index ];
-
-      if (modexp_ctrl_reg == CTRL_ITERATE)
-        ei_we = 1'b1;
-      else
-        ei_we = 1'b0;
-    end
-
-
-  //----------------------------------------------------------------
-  // modexp_ctrl
-  //
-  // Control FSM logic needed to perform the modexp operation.
-  //----------------------------------------------------------------
-  always @*
-    begin
-      ready_new           = 1'b0;
-      ready_we            = 1'b0;
-      montprod_select_new = MONTPROD_SELECT_ONE_NR;
-      montprod_select_we  = 0;
-      montprod_dest_new   = MONTPROD_DEST_NOWHERE;
-      montprod_dest_we    = 0;
-      montprod_calc       = 0;
-      modexp_ctrl_new     = CTRL_IDLE;
-      modexp_ctrl_we      = 1'b0;
-      clear_start         = 1'b0;
-      cycle_ctr_start     = 1'b0;
-      cycle_ctr_stop      = 1'b0;
-
-      residue_calculate = 1'b0;
-
-      residue_valid_int_validated = 1'b0;
-
-      case (modexp_ctrl_reg)
-        CTRL_IDLE:
-          begin
-            if (start_reg)
-              begin
-                ready_new       = 1'b0;
-                ready_we        = 1'b1;
-                clear_start     = 1'b1;
-                cycle_ctr_start = 1'b1;
-
-                if (residue_valid_reg)
-                  begin
-                    //residue has alrady been calculated, start with MONTPROD( 1, Nr, MODULUS )
-                    montprod_select_new = MONTPROD_SELECT_ONE_NR;
-                    montprod_select_we  = 1;
-                    montprod_dest_new   = MONTPROD_DEST_Z;
-                    montprod_dest_we    = 1;
-                    montprod_calc       = 1;
-                    modexp_ctrl_new     = CTRL_CALCULATE_Z0;
-                    modexp_ctrl_we      = 1;
-                  end
-                else
-                  begin
-                    //modulus has been written and residue (Nr) must be calculated
-                    modexp_ctrl_new = CTRL_RESIDUE;
-                    modexp_ctrl_we  = 1;
-                    residue_calculate = 1'b1;
-                  end
-              end
-          end
-
-        CTRL_RESIDUE:
-          begin
-            if (residue_ready)
-              begin
-                montprod_select_new = MONTPROD_SELECT_ONE_NR;
-                montprod_select_we  = 1;
-                montprod_dest_new   = MONTPROD_DEST_Z;
-                montprod_dest_we    = 1;
-                montprod_calc       = 1;
-                modexp_ctrl_new     = CTRL_CALCULATE_Z0;
-                modexp_ctrl_we      = 1;
-                residue_valid_int_validated = 1'b1; //update registers telling residue is valid
-              end
-          end
-
-        CTRL_CALCULATE_Z0:
-          begin
-            if (montprod_ready)
-              begin
-                montprod_select_new = MONTPROD_SELECT_X_NR;
-                montprod_select_we  = 1;
-                montprod_dest_new   = MONTPROD_DEST_P;
-                montprod_dest_we    = 1;
-                montprod_calc       = 1;
-                modexp_ctrl_new = CTRL_CALCULATE_P0;
-                modexp_ctrl_we  = 1;
-              end
-          end
-
-        CTRL_CALCULATE_P0:
-          begin
-            if (montprod_ready == 1'b1)
-              begin
-                modexp_ctrl_new = CTRL_ITERATE;
-                modexp_ctrl_we  = 1;
-              end
-          end
-
-        CTRL_ITERATE:
-          begin
-            montprod_select_new = MONTPROD_SELECT_Z_P;
-            montprod_select_we  = 1;
-            montprod_dest_new   = MONTPROD_DEST_Z;
-            montprod_dest_we    = 1;
-            montprod_calc       = 1;
-            modexp_ctrl_new     = CTRL_ITERATE_Z_P;
-            modexp_ctrl_we      = 1;
-
-            if (ei_new == 1'b0 && exponation_mode_reg == EXPONATION_MODE_PUBLIC_FAST)
-              begin
-                //Skip the fake montgomery calculation, exponation_mode_reg optimizing for speed not blinding.
-                montprod_select_new = MONTPROD_SELECT_P_P;
-                montprod_dest_new   = MONTPROD_DEST_P;
-                modexp_ctrl_new     = CTRL_ITERATE_P_P;
-              end
-          end
-
-        CTRL_ITERATE_Z_P:
-            if (montprod_ready)
-              begin
-                montprod_select_new = MONTPROD_SELECT_P_P;
-                montprod_select_we  = 1;
-                montprod_dest_new   = MONTPROD_DEST_P;
-                montprod_dest_we    = 1;
-                montprod_calc       = 1;
-                modexp_ctrl_new     = CTRL_ITERATE_P_P;
-                modexp_ctrl_we      = 1;
-              end
-
-        CTRL_ITERATE_P_P:
-            if (montprod_ready == 1'b1)
-              begin
-                modexp_ctrl_new = CTRL_ITERATE_END;
-                modexp_ctrl_we  = 1;
-              end
-
-        CTRL_ITERATE_END:
-          begin
-            if (!last_iteration)
-              begin
-                modexp_ctrl_new = CTRL_ITERATE;
-                modexp_ctrl_we  = 1;
-              end
-            else
-              begin
-                montprod_select_new = MONTPROD_SELECT_Z_ONE;
-                montprod_select_we  = 1;
-                montprod_dest_new   = MONTPROD_DEST_Z;
-                montprod_dest_we    = 1;
-                montprod_calc       = 1;
-                modexp_ctrl_new     = CTRL_CALCULATE_ZN;
-                modexp_ctrl_we      = 1;
-              end
-          end
-
-        CTRL_CALCULATE_ZN:
-          begin
-            if (montprod_ready)
-              begin
-                modexp_ctrl_new = CTRL_DONE;
-                modexp_ctrl_we  = 1;
-              end
-          end
-
-        CTRL_DONE:
-          begin
-            cycle_ctr_stop  = 1'b1;
-            ready_new       = 1'b1;
-            ready_we        = 1'b1;
-            modexp_ctrl_new = CTRL_IDLE;
-            modexp_ctrl_we  = 1;
-          end
-
-        default:
-          begin
-          end
-
-      endcase // case (modexp_ctrl_reg)
-    end
-
 endmodule // modexp
 
 //======================================================================
diff --git a/src/rtl/modexp_core.v b/src/rtl/modexp_core.v
new file mode 100644
index 0000000..b8c4403
--- /dev/null
+++ b/src/rtl/modexp_core.v
@@ -0,0 +1,863 @@
+//======================================================================
+//
+// modexp_core.v
+// -------------
+// Modular exponentiation core for implementing public key algorithms
+// such as RSA, DH, ElGamal etc.
+//
+// The core calculates the following function:
+//
+//   C = M ** e mod N
+//
+//   M is a message with a length of n bits
+//   e is the exponent with a length of m bits
+//   N is the modulus  with a length of n bits
+//
+//   n can be 32 and up to and including 8192 bits in steps
+//   of 32 bits.
+//   m can be one and up to and including 8192 bits in steps
+//   of 32 bits.
+//
+// The core has access ports for the exponent, modulus, message and
+// result memories.
+//
+//
+// Author: Joachim Strombergson, Peter Magnusson
+// Copyright (c) 2015, Assured AB
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or
+// without modification, are permitted provided that the following
+// conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright
+//    notice, this list of conditions and the following disclaimer.
+//
+// 2. 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 OWNER 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 modexp_core(
+                   input wire           clk,
+                   input wire           reset_n,
+
+                   input wire           start,
+                   output wire          ready,
+
+                   input wire [07 : 0]  exponent_length,
+                   input wire [07 : 0]  modulus_length,
+
+                   output wire [63 : 0] cycles,
+
+                   input wire           exponent_mem_api_cs,
+                   input wire           exponent_mem_api_wr,
+                   input wire           exponent_mem_api_rst,
+                   input wire [31 : 0]  exponent_mem_api_write_data,
+                   output wire [31 : 0] exponent_mem_api_read_data,
+
+                   input wire           modulus_mem_api_cs,
+                   input wire           modulus_mem_api_wr,
+                   input wire           modulus_mem_api_rst,
+                   input wire [31 : 0]  modulus_mem_api_write_data,
+                   output wire [31 : 0] modulus_mem_api_read_data,
+
+                   input wire           message_mem_api_cs,
+                   input wire           message_mem_api_wr,
+                   input wire           message_mem_api_rst,
+                   input wire [31 : 0]  message_mem_api_write_data,
+                   output wire [31 : 0] message_mem_api_read_data,
+
+                   input wire           result_mem_api_cs,
+                   input wire           result_mem_api_rst,
+                   output wire [31 : 0] result_mem_api_read_data
+                  );
+
+
+  //----------------------------------------------------------------
+  // Internal constant and parameter definitions.
+  //----------------------------------------------------------------
+  localparam MONTPROD_SELECT_ONE_NR = 3'h0;
+  localparam MONTPROD_SELECT_X_NR   = 3'h1;
+  localparam MONTPROD_SELECT_Z_P    = 3'h2;
+  localparam MONTPROD_SELECT_P_P    = 3'h3;
+  localparam MONTPROD_SELECT_Z_ONE  = 3'h4;
+
+  localparam MONTPROD_DEST_Z        = 2'b00;
+  localparam MONTPROD_DEST_P        = 2'b01;
+  localparam MONTPROD_DEST_NOWHERE  = 2'b10;
+
+  localparam CTRL_IDLE              = 4'h0;
+  localparam CTRL_RESIDUE           = 4'h1;
+  localparam CTRL_CALCULATE_Z0      = 4'h2;
+  localparam CTRL_CALCULATE_P0      = 4'h3;
+  localparam CTRL_ITERATE           = 4'h4;
+  localparam CTRL_ITERATE_Z_P       = 4'h5;
+  localparam CTRL_ITERATE_P_P       = 4'h6;
+  localparam CTRL_ITERATE_END       = 4'h7;
+  localparam CTRL_CALCULATE_ZN      = 4'h8;
+  localparam CTRL_DONE              = 4'h9;
+
+  //for rsa, c=M^65537 etc, there is no need to slow down to hide the exponent
+  localparam EXPONATION_MODE_SECRET_SECURE = 1'b0;
+  localparam EXPONATION_MODE_PUBLIC_FAST   = 1'b1;
+
+
+  //----------------------------------------------------------------
+  // Registers including update variables and write enable.
+  //----------------------------------------------------------------
+  reg          ready_reg;
+  reg          ready_new;
+  reg          ready_we;
+
+  reg [2 : 0]  montprod_select_reg;
+  reg [2 : 0]  montprod_select_new;
+  reg          montprod_select_we;
+  reg [1 : 0]  montprod_dest_reg;
+  reg [1 : 0]  montprod_dest_new;
+  reg          montprod_dest_we;
+
+  reg [3 : 0]  modexp_ctrl_reg;
+  reg [3 : 0]  modexp_ctrl_new;
+  reg          modexp_ctrl_we;
+
+  reg [31 : 0] one_reg;
+  reg [31 : 0] one_new;
+  reg [31 : 0] b_one_reg;
+  reg [31 : 0] b_one_new;
+
+  reg [12 : 0] loop_counter_reg;
+  reg [12 : 0] loop_counter_new;
+  reg          loop_counter_we;
+
+  reg [07 : 0] E_word_index;
+  reg [04 : 0] E_bit_index;
+  reg          last_iteration;
+
+  reg          ei_reg;
+  reg          ei_new;
+  reg          ei_we;
+
+  reg          exponation_mode_reg;
+  reg          exponation_mode_new;
+  reg          exponation_mode_we;
+
+  reg [31 : 0] cycle_ctr_low_reg;
+  reg [31 : 0] cycle_ctr_low_new;
+  reg          cycle_ctr_low_we;
+  reg [31 : 0] cycle_ctr_high_reg;
+  reg [31 : 0] cycle_ctr_high_new;
+  reg          cycle_ctr_high_we;
+  reg          cycle_ctr_state_reg;
+  reg          cycle_ctr_state_new;
+  reg          cycle_ctr_state_we;
+  reg          cycle_ctr_start;
+  reg          cycle_ctr_stop;
+
+
+  //----------------------------------------------------------------
+  // Wires.
+  //----------------------------------------------------------------
+  reg [07 : 0]  modulus_mem_int_rd_addr;
+  wire [31 : 0] modulus_mem_int_rd_data;
+
+  reg [07 : 0]  message_mem_int_rd_addr;
+  wire [31 : 0] message_mem_int_rd_data;
+
+  reg [07 : 0]  exponent_mem_int_rd_addr;
+  wire [31 : 0] exponent_mem_int_rd_data;
+
+  reg  [07 : 0] result_mem_int_rd_addr;
+  wire [31 : 0] result_mem_int_rd_data;
+  reg  [07 : 0] result_mem_int_wr_addr;
+  reg  [31 : 0] result_mem_int_wr_data;
+  reg           result_mem_int_we;
+
+  reg  [07 : 0] p_mem_rd0_addr;
+  wire [31 : 0] p_mem_rd0_data;
+  reg  [07 : 0] p_mem_rd1_addr;
+  wire [31 : 0] p_mem_rd1_data;
+  reg  [07 : 0] p_mem_wr_addr;
+  reg  [31 : 0] p_mem_wr_data;
+  reg           p_mem_we;
+
+  reg [31 : 0]  tmp_read_data;
+
+  reg           montprod_calc;
+  wire          montprod_ready;
+  reg [07 : 0]  montprod_length;
+
+  wire [07 : 0] montprod_opa_addr;
+  reg [31 : 0]  montprod_opa_data;
+
+  wire [07 : 0] montprod_opb_addr;
+  reg [31 : 0]  montprod_opb_data;
+
+  wire [07 : 0] montprod_opm_addr;
+  reg [31 : 0]  montprod_opm_data;
+
+  wire [07 : 0] montprod_result_addr;
+  wire [31 : 0] montprod_result_data;
+  wire          montprod_result_we;
+
+  reg           residue_calculate;
+  wire          residue_ready;
+  reg [14 : 0]  residue_nn;
+  reg  [07 : 0] residue_length;
+  wire [07 : 0] residue_opa_rd_addr;
+  wire [31 : 0] residue_opa_rd_data;
+  wire [07 : 0] residue_opa_wr_addr;
+  wire [31 : 0] residue_opa_wr_data;
+  wire          residue_opa_wr_we;
+  wire [07 : 0] residue_opm_addr;
+  reg [31 : 0]  residue_opm_data;
+
+  reg  [07 : 0] residue_mem_montprod_read_addr;
+  wire [31 : 0] residue_mem_montprod_read_data;
+
+  reg           residue_valid_reg;
+  reg           residue_valid_new;
+  reg           residue_valid_int_validated;
+
+  wire [7 : 0]  length_m1;
+
+
+  //----------------------------------------------------------------
+  // Concurrent connectivity for ports etc.
+  //----------------------------------------------------------------
+  assign ready  = ready_reg;
+  assign cycles = {cycle_ctr_high_reg, cycle_ctr_low_reg};
+
+  assign length_m1 = modulus_length - 8'h1;
+
+
+  //----------------------------------------------------------------
+  // core instantiations.
+  //----------------------------------------------------------------
+  montprod montprod_inst(
+                         .clk(clk),
+                         .reset_n(reset_n),
+
+                         .calculate(montprod_calc),
+                         .ready(montprod_ready),
+
+                         .length(montprod_length),
+
+                         .opa_addr(montprod_opa_addr),
+                         .opa_data(montprod_opa_data),
+
+                         .opb_addr(montprod_opb_addr),
+                         .opb_data(montprod_opb_data),
+
+                         .opm_addr(montprod_opm_addr),
+                         .opm_data(montprod_opm_data),
+
+                         .result_addr(montprod_result_addr),
+                         .result_data(montprod_result_data),
+                         .result_we(montprod_result_we)
+                        );
+
+
+  residue residue_inst(
+                     .clk(clk),
+                     .reset_n(reset_n),
+                     .calculate(residue_calculate),
+                     .ready(residue_ready),
+                     .nn(residue_nn),
+                     .length(residue_length),
+                     .opa_rd_addr(residue_opa_rd_addr),
+                     .opa_rd_data(residue_opa_rd_data),
+                     .opa_wr_addr(residue_opa_wr_addr),
+                     .opa_wr_data(residue_opa_wr_data),
+                     .opa_wr_we(residue_opa_wr_we),
+                     .opm_addr(residue_opm_addr),
+                     .opm_data(residue_opm_data)
+                    );
+
+  blockmem2r1w residue_mem(
+                           .clk(clk),
+                           .read_addr0(residue_opa_rd_addr),
+                           .read_data0(residue_opa_rd_data),
+                           .read_addr1(residue_mem_montprod_read_addr),
+                           .read_data1(residue_mem_montprod_read_data),
+                           .wr(residue_opa_wr_we),
+                           .write_addr(residue_opa_wr_addr),
+                           .write_data(residue_opa_wr_data)
+                          );
+
+
+  blockmem2r1w p_mem(
+                     .clk(clk),
+                     .read_addr0(p_mem_rd0_addr),
+                     .read_data0(p_mem_rd0_data),
+                     .read_addr1(p_mem_rd1_addr),
+                     .read_data1(p_mem_rd1_data),
+                     .wr(p_mem_we),
+                     .write_addr(p_mem_wr_addr),
+                     .write_data(p_mem_wr_data)
+                    );
+
+
+  blockmem2r1wptr exponent_mem(
+                               .clk(clk),
+                               .reset_n(reset_n),
+                               .read_addr0(exponent_mem_int_rd_addr),
+                               .read_data0(exponent_mem_int_rd_data),
+                               .read_data1(exponent_mem_api_read_data),
+                               .rst(exponent_mem_api_rst),
+                               .cs(exponent_mem_api_cs),
+                               .wr(exponent_mem_api_wr),
+                               .write_data(exponent_mem_api_write_data)
+                              );
+
+
+  blockmem2r1wptr modulus_mem(
+                              .clk(clk),
+                              .reset_n(reset_n),
+                              .read_addr0(modulus_mem_int_rd_addr),
+                              .read_data0(modulus_mem_int_rd_data),
+                              .read_data1(modulus_mem_api_read_data),
+                              .rst(modulus_mem_api_rst),
+                              .cs(modulus_mem_api_cs),
+                              .wr(modulus_mem_api_wr),
+                              .write_data(modulus_mem_api_write_data)
+                             );
+
+
+  blockmem2r1wptr message_mem(
+                              .clk(clk),
+                              .reset_n(reset_n),
+                              .read_addr0(message_mem_int_rd_addr),
+                              .read_data0(message_mem_int_rd_data),
+                              .read_data1(message_mem_api_read_data),
+                              .rst(message_mem_api_rst),
+                              .cs(message_mem_api_cs),
+                              .wr(message_mem_api_wr),
+                              .write_data(message_mem_api_write_data)
+                             );
+
+
+  blockmem2rptr1w result_mem(
+                             .clk(clk),
+                             .reset_n(reset_n),
+                             .read_addr0(result_mem_int_rd_addr[7 : 0]),
+                             .read_data0(result_mem_int_rd_data),
+                             .read_data1(result_mem_api_read_data),
+                             .rst(result_mem_api_rst),
+                             .cs(result_mem_api_cs),
+                             .wr(result_mem_int_we),
+                             .write_addr(result_mem_int_wr_addr),
+                             .write_data(result_mem_int_wr_data)
+                            );
+
+
+  //----------------------------------------------------------------
+  // reg_update
+  //
+  // Update functionality for all registers in the core.
+  // All registers are positive edge triggered with asynchronous
+  // active low reset. All registers have write enable.
+  //----------------------------------------------------------------
+  always @ (posedge clk or negedge reset_n)
+    begin
+      if (!reset_n)
+        begin
+          ready_reg           <= 1'b1;
+          montprod_select_reg <= MONTPROD_SELECT_ONE_NR;
+          montprod_dest_reg   <= MONTPROD_DEST_NOWHERE;
+          modexp_ctrl_reg     <= CTRL_IDLE;
+          one_reg             <= 32'h0;
+          b_one_reg           <= 32'h0;
+          loop_counter_reg    <= 13'b0;
+          ei_reg              <= 1'b0;
+          residue_valid_reg   <= 1'b0;
+          exponation_mode_reg <= EXPONATION_MODE_SECRET_SECURE;
+          cycle_ctr_low_reg   <= 32'h00000000;
+          cycle_ctr_high_reg  <= 32'h00000000;
+          cycle_ctr_state_reg <= 1'b0;
+        end
+      else
+        begin
+          one_reg             <= one_new;
+          b_one_reg           <= b_one_new;
+          residue_valid_reg   <= residue_valid_new;
+
+          if (ready_we)
+            ready_reg <= ready_new;
+
+          if (montprod_select_we)
+            montprod_select_reg <= montprod_select_new;
+
+          if (montprod_dest_we)
+            montprod_dest_reg <= montprod_dest_new;
+
+          if (loop_counter_we)
+            loop_counter_reg <= loop_counter_new;
+
+          if (ei_we)
+            ei_reg <= ei_new;
+
+          if (exponation_mode_we)
+            exponation_mode_reg <= exponation_mode_new;
+
+          if (cycle_ctr_low_we)
+            cycle_ctr_low_reg <= cycle_ctr_low_new;
+
+          if (cycle_ctr_high_we)
+            cycle_ctr_high_reg <= cycle_ctr_high_new;
+
+          if (cycle_ctr_state_we)
+            cycle_ctr_state_reg <= cycle_ctr_state_new;
+
+          if (modexp_ctrl_we)
+            modexp_ctrl_reg <= modexp_ctrl_new;
+        end
+    end // reg_update
+
+
+  //----------------------------------------------------------------
+  // cycle_ctr
+  //
+  // Implementation of the cycle counter
+  //----------------------------------------------------------------
+  always @*
+    begin : cycle_ctr
+      cycle_ctr_low_new   = 32'h00000000;
+      cycle_ctr_low_we    = 1'b0;
+      cycle_ctr_high_new  = 32'h00000000;
+      cycle_ctr_high_we   = 1'b0;
+      cycle_ctr_state_new = 1'b0;
+      cycle_ctr_state_we  = 1'b0;
+
+      if (cycle_ctr_start)
+        begin
+          cycle_ctr_low_new   = 32'h00000000;
+          cycle_ctr_low_we    = 1'b1;
+          cycle_ctr_high_new  = 32'h00000000;
+          cycle_ctr_high_we   = 1'b1;
+          cycle_ctr_state_new = 1'b1;
+          cycle_ctr_state_we  = 1'b1;
+        end
+
+      if (cycle_ctr_stop)
+        begin
+          cycle_ctr_state_new = 1'b0;
+          cycle_ctr_state_we  = 1'b1;
+        end
+
+      if (cycle_ctr_state_reg)
+        begin
+          cycle_ctr_low_new = cycle_ctr_low_reg + 1'b1;
+          cycle_ctr_low_we  = 1'b1;
+
+          if (cycle_ctr_low_new == 32'h00000000)
+            begin
+              cycle_ctr_high_new  = cycle_ctr_high_reg + 1'b1;
+              cycle_ctr_high_we   = 1'b1;
+            end
+        end
+    end // cycle_ctr
+
+
+  //----------------------------------------------------------------
+  // one
+  //
+  // generates the big integer one ( 00... 01 )
+  //----------------------------------------------------------------
+  always @*
+    begin : one_process
+      one_new   = 32'h00000000;
+      b_one_new = 32'h00000000;
+
+      if (montprod_opa_addr == length_m1)
+        one_new = 32'h00000001;
+
+      if (montprod_opb_addr == length_m1)
+        b_one_new = 32'h00000001;
+    end
+
+
+  //----------------------------------------------------------------
+  // Read mux for modulus. Needed since it is being
+  // addressed by two sources.
+  //----------------------------------------------------------------
+  always @*
+    begin : modulus_mem_reader_process
+      if (modexp_ctrl_reg == CTRL_RESIDUE)
+        modulus_mem_int_rd_addr = residue_opm_addr;
+      else
+        modulus_mem_int_rd_addr = montprod_opm_addr;
+    end
+
+
+  //----------------------------------------------------------------
+  // Feeds residue calculator.
+  //----------------------------------------------------------------
+  always @*
+    begin : residue_process
+      //N*2, N=length*32, *32 = shl5, *64 = shl6
+      residue_nn = { 1'b0, modulus_length, 6'h0 };
+      residue_length = modulus_length;
+      residue_opm_data = modulus_mem_int_rd_data;
+    end
+
+
+  //----------------------------------------------------------------
+  // Detects if modulus has been updated and we need to
+  // recalculate the residue
+  // and we need residue is valid or not.
+  //----------------------------------------------------------------
+  always @*
+    begin : residue_valid_process
+      residue_valid_new = residue_valid_reg;
+
+      if (modulus_mem_api_cs & modulus_mem_api_wr)
+        residue_valid_new = 1'b0;
+      else if ( residue_valid_int_validated == 1'b1)
+        residue_valid_new = 1'b1;
+    end
+
+
+  //----------------------------------------------------------------
+  // montprod_op_select
+  //
+  // Select operands used during montprod calculations depending
+  // on what operation we want to do.
+  //----------------------------------------------------------------
+  always @*
+    begin : montprod_op_select
+
+      montprod_length          = modulus_length;
+
+      result_mem_int_rd_addr   = montprod_opa_addr;
+      message_mem_int_rd_addr  = montprod_opa_addr;
+      p_mem_rd0_addr           = montprod_opa_addr;
+
+      residue_mem_montprod_read_addr = montprod_opb_addr;
+      p_mem_rd1_addr                 = montprod_opb_addr;
+
+      montprod_opm_data = modulus_mem_int_rd_data;
+      case (montprod_select_reg)
+        MONTPROD_SELECT_ONE_NR:
+          begin
+            montprod_opa_data  = one_reg;
+            montprod_opb_data  = residue_mem_montprod_read_data;
+          end
+
+        MONTPROD_SELECT_X_NR:
+          begin
+            montprod_opa_data  = message_mem_int_rd_data;
+            montprod_opb_data  = residue_mem_montprod_read_data;
+          end
+
+        MONTPROD_SELECT_Z_P:
+          begin
+            montprod_opa_data  = result_mem_int_rd_data;
+            montprod_opb_data  = p_mem_rd1_data;
+          end
+
+        MONTPROD_SELECT_P_P:
+          begin
+            montprod_opa_data  = p_mem_rd0_data;
+            montprod_opb_data  = p_mem_rd1_data;
+          end
+
+        MONTPROD_SELECT_Z_ONE:
+          begin
+            montprod_opa_data  = result_mem_int_rd_data;
+            montprod_opb_data  = b_one_reg;
+          end
+
+        default:
+          begin
+            montprod_opa_data  = 32'h00000000;
+            montprod_opb_data  = 32'h00000000;
+          end
+      endcase // case (montprod_selcect_reg)
+    end
+
+
+  //----------------------------------------------------------------
+  // memory write mux
+  //
+  // Direct memory write signals to correct memory.
+  //----------------------------------------------------------------
+  always @*
+    begin : memory_write_process
+      result_mem_int_wr_addr = montprod_result_addr;
+      result_mem_int_wr_data = montprod_result_data;
+      result_mem_int_we      = 1'b0;
+
+      p_mem_wr_addr = montprod_result_addr;
+      p_mem_wr_data = montprod_result_data;
+      p_mem_we      = 1'b0;
+
+      case (montprod_dest_reg)
+        MONTPROD_DEST_Z:
+          result_mem_int_we = montprod_result_we;
+        MONTPROD_DEST_P:
+          p_mem_we = montprod_result_we;
+        default:
+          begin
+          end
+      endcase
+
+      // inhibit Z=Z*P when ei = 0
+      if (modexp_ctrl_reg == CTRL_ITERATE_Z_P)
+        result_mem_int_we = result_mem_int_we & ei_reg;
+    end
+
+
+  //----------------------------------------------------------------
+  // loop_counter
+  //
+  // Calculate the loop counter and related variables.
+  //----------------------------------------------------------------
+  always @*
+    begin : loop_counters_process
+      loop_counter_new = 13'b0;
+      loop_counter_we  = 1'b0;
+
+      if (loop_counter_reg == {length_m1, 5'b11111})
+        last_iteration = 1'b1;
+      else
+        last_iteration = 1'b0;
+
+      case (modexp_ctrl_reg)
+        CTRL_CALCULATE_P0:
+          begin
+            loop_counter_new = 13'b0;
+            loop_counter_we  = 1'b1;
+          end
+
+        CTRL_ITERATE_END:
+          begin
+            loop_counter_new = loop_counter_reg + 1'b1;
+            loop_counter_we  = 1'b1;
+          end
+
+        default:
+          begin
+          end
+      endcase
+    end
+
+
+  //----------------------------------------------------------------
+  // exponent
+  //
+  // Reads the exponent.
+  //----------------------------------------------------------------
+  always @*
+    begin : exponent_process
+      // Accessing new instead of reg - pick up update at
+      // CTRL_ITERATE_NEW to remove a pipeline stall.
+      E_word_index  = length_m1 - loop_counter_new[ 12 : 5 ];
+
+      E_bit_index   = loop_counter_reg[ 04 : 0 ];
+
+      exponent_mem_int_rd_addr = E_word_index;
+
+      ei_new = exponent_mem_int_rd_data[ E_bit_index ];
+
+      if (modexp_ctrl_reg == CTRL_ITERATE)
+        ei_we = 1'b1;
+      else
+        ei_we = 1'b0;
+    end
+
+
+  //----------------------------------------------------------------
+  // modexp_ctrl
+  //
+  // Control FSM logic needed to perform the modexp operation.
+  //----------------------------------------------------------------
+  always @*
+    begin
+      ready_new           = 1'b0;
+      ready_we            = 1'b0;
+      montprod_select_new = MONTPROD_SELECT_ONE_NR;
+      montprod_select_we  = 0;
+      montprod_dest_new   = MONTPROD_DEST_NOWHERE;
+      montprod_dest_we    = 0;
+      montprod_calc       = 0;
+      modexp_ctrl_new     = CTRL_IDLE;
+      modexp_ctrl_we      = 1'b0;
+      cycle_ctr_start     = 1'b0;
+      cycle_ctr_stop      = 1'b0;
+
+      residue_calculate = 1'b0;
+
+      residue_valid_int_validated = 1'b0;
+
+      case (modexp_ctrl_reg)
+        CTRL_IDLE:
+          begin
+            if (start)
+              begin
+                ready_new       = 1'b0;
+                ready_we        = 1'b1;
+                cycle_ctr_start = 1'b1;
+
+                if (residue_valid_reg)
+                  begin
+                    //residue has alrady been calculated, start with MONTPROD( 1, Nr, MODULUS )
+                    montprod_select_new = MONTPROD_SELECT_ONE_NR;
+                    montprod_select_we  = 1;
+                    montprod_dest_new   = MONTPROD_DEST_Z;
+                    montprod_dest_we    = 1;
+                    montprod_calc       = 1;
+                    modexp_ctrl_new     = CTRL_CALCULATE_Z0;
+                    modexp_ctrl_we      = 1;
+                  end
+                else
+                  begin
+                    //modulus has been written and residue (Nr) must be calculated
+                    modexp_ctrl_new = CTRL_RESIDUE;
+                    modexp_ctrl_we  = 1;
+                    residue_calculate = 1'b1;
+                  end
+              end
+          end
+
+        CTRL_RESIDUE:
+          begin
+            if (residue_ready)
+              begin
+                montprod_select_new = MONTPROD_SELECT_ONE_NR;
+                montprod_select_we  = 1;
+                montprod_dest_new   = MONTPROD_DEST_Z;
+                montprod_dest_we    = 1;
+                montprod_calc       = 1;
+                modexp_ctrl_new     = CTRL_CALCULATE_Z0;
+                modexp_ctrl_we      = 1;
+                residue_valid_int_validated = 1'b1; //update registers telling residue is valid
+              end
+          end
+
+        CTRL_CALCULATE_Z0:
+          begin
+            if (montprod_ready)
+              begin
+                montprod_select_new = MONTPROD_SELECT_X_NR;
+                montprod_select_we  = 1;
+                montprod_dest_new   = MONTPROD_DEST_P;
+                montprod_dest_we    = 1;
+                montprod_calc       = 1;
+                modexp_ctrl_new = CTRL_CALCULATE_P0;
+                modexp_ctrl_we  = 1;
+              end
+          end
+
+        CTRL_CALCULATE_P0:
+          begin
+            if (montprod_ready == 1'b1)
+              begin
+                modexp_ctrl_new = CTRL_ITERATE;
+                modexp_ctrl_we  = 1;
+              end
+          end
+
+        CTRL_ITERATE:
+          begin
+            montprod_select_new = MONTPROD_SELECT_Z_P;
+            montprod_select_we  = 1;
+            montprod_dest_new   = MONTPROD_DEST_Z;
+            montprod_dest_we    = 1;
+            montprod_calc       = 1;
+            modexp_ctrl_new     = CTRL_ITERATE_Z_P;
+            modexp_ctrl_we      = 1;
+
+            if (ei_new == 1'b0 && exponation_mode_reg == EXPONATION_MODE_PUBLIC_FAST)
+              begin
+                //Skip the fake montgomery calculation, exponation_mode_reg optimizing for speed not blinding.
+                montprod_select_new = MONTPROD_SELECT_P_P;
+                montprod_dest_new   = MONTPROD_DEST_P;
+                modexp_ctrl_new     = CTRL_ITERATE_P_P;
+              end
+          end
+
+        CTRL_ITERATE_Z_P:
+            if (montprod_ready)
+              begin
+                montprod_select_new = MONTPROD_SELECT_P_P;
+                montprod_select_we  = 1;
+                montprod_dest_new   = MONTPROD_DEST_P;
+                montprod_dest_we    = 1;
+                montprod_calc       = 1;
+                modexp_ctrl_new     = CTRL_ITERATE_P_P;
+                modexp_ctrl_we      = 1;
+              end
+
+        CTRL_ITERATE_P_P:
+            if (montprod_ready == 1'b1)
+              begin
+                modexp_ctrl_new = CTRL_ITERATE_END;
+                modexp_ctrl_we  = 1;
+              end
+
+        CTRL_ITERATE_END:
+          begin
+            if (!last_iteration)
+              begin
+                modexp_ctrl_new = CTRL_ITERATE;
+                modexp_ctrl_we  = 1;
+              end
+            else
+              begin
+                montprod_select_new = MONTPROD_SELECT_Z_ONE;
+                montprod_select_we  = 1;
+                montprod_dest_new   = MONTPROD_DEST_Z;
+                montprod_dest_we    = 1;
+                montprod_calc       = 1;
+                modexp_ctrl_new     = CTRL_CALCULATE_ZN;
+                modexp_ctrl_we      = 1;
+              end
+          end
+
+        CTRL_CALCULATE_ZN:
+          begin
+            if (montprod_ready)
+              begin
+                modexp_ctrl_new = CTRL_DONE;
+                modexp_ctrl_we  = 1;
+              end
+          end
+
+        CTRL_DONE:
+          begin
+            cycle_ctr_stop  = 1'b1;
+            ready_new       = 1'b1;
+            ready_we        = 1'b1;
+            modexp_ctrl_new = CTRL_IDLE;
+            modexp_ctrl_we  = 1;
+          end
+
+        default:
+          begin
+          end
+
+      endcase // case (modexp_ctrl_reg)
+    end
+
+endmodule // modexp_core
+
+//======================================================================
+// EOF modexp_core.v
+//======================================================================