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// ============================================================================

//        __

//   \\__/ o\    (C) 2006-2020  Robert Finch, Waterloo

//    \  __ /    All rights reserved.

//     \/_//     robfinch@finitron.ca

//       ||

//

//      DFPRound.sv

//    - decimal floating point rounding unit

//    - parameterized width

//    - IEEE 754 representation

//

//

// BSD 3-Clause License

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// 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.

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//    this list of conditions and the following disclaimer in the documentation

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//    this software without specific prior written permission.

//

// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"

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// ============================================================================

import fp::*;

`ifdef MIN_LATENCY

`define PIPE_ADV  *

`else

`define PIPE_ADV  (posedge clk)

`endif

module DFPRound(clk, ce, rm, i, o);

input clk;

input ce;

input [2:0] rm;                 // rounding mode

input [119:0] i;                // intermediate format input

output [127:0] o;               // rounded output

parameter ROUND_CEILING = 3'd0;

parameter ROUND_FLOOR = 3'd1;

parameter ROUND_HALF_UP = 3'd2;

parameter ROUND_HALF_EVEN = 3'd3;

parameter ROUND_DOWN = 3'd4;

//------------------------------------------------------------

// variables

wire [3:0] so;

wire [15:0] xo;

reg  [95:0] mo;

reg [15:0] xo1;

reg [95:0] mo1;

wire xInf = i[115:100]==16'h9999;

wire so0 = i[118];

assign o = {12'hDF0,so,xo,mo};

wire [3:0] l = i[7:4];

wire [3:0] r = i[3:0];

reg rnd;

//------------------------------------------------------------

// Clock #1

// - determine round amount (add 1 or 0)

//------------------------------------------------------------

always @`PIPE_ADV

if (ce) xo1 <= i[115:100];

always @`PIPE_ADV

if (ce) mo1 <= i[99:4];

// Compute the round bit

// Infinities and NaNs are not rounded!

always @`PIPE_ADV

if (ce)

        if (|so[1:0])

                rnd = 1'b0;

        else

                case (rm)

                ROUND_CEILING:  rnd <= (r == 4'd0 || so[2]==1'b0) ? 1'b0 : 1'b1;

                ROUND_FLOOR:            rnd <= (r == 4'd0 || so[2]==1'b1) ? 1'b0 : 1'b1;

                ROUND_HALF_UP:  rnd <= r >= 4'h5;

                ROUND_HALF_EVEN:        rnd <= r==4'h5 ? l[0] : r > 4'h5 ? 1'b1 : 1'b0;

                ROUND_DOWN:                     rnd <= 1'b0;

                default:                                rnd <= 1'b0;

                endcase

//------------------------------------------------------------

// Clock #2

// round the number, check for carry

// note: inf. exponent checked above (if the exponent was infinite already, then no rounding occurs as rnd = 0)

// note: exponent increments if there is a carry (can only increment to infinity)

//------------------------------------------------------------

wire [111:0] rounded1;

wire co1;

BCDAddN #(.N(29)) ubcdan1

(

        .ci(1'b0),

        .a({xo1,mo1}),

        .b({111'd0,rnd}),

        .o(rounded1),

        .co(co1)

);

reg [111:0] rounded2;

reg carry2;

reg rnd2;

reg dn2;

wire [15:0] xo2;

always @`PIPE_ADV

        if (ce) rounded2 <= rounded1;

always @`PIPE_ADV

        if (ce) carry2 <= co1;

always @`PIPE_ADV

        if (ce) rnd2 <= rnd;

always @`PIPE_ADV

        if (ce) dn2 <= !(|xo1);

assign xo2 = rounded2[111:96];

//------------------------------------------------------------

// Clock #3

// - shift mantissa if required.

//------------------------------------------------------------

`ifdef MIN_LATENCY

assign so = i[119:116];

assign xo = xo2;

`else

delay3 #(4) u21 (.clk(clk), .ce(ce), .i(i[119:116]), .o(so));

delay1 #(16) u22 (.clk(clk), .ce(ce), .i(xo2), .o(xo));

`endif

always @`PIPE_ADV

if (ce)

        casez({rnd2,xo2==16'h9999,carry2,dn2})

        4'b0??0:        mo <= mo1[95:0];                                                        // not rounding, not denormalized

        4'b0??1:        mo <= mo1[95:0];                                                        // not rounding, denormalized

        4'b1000:        mo <= rounded2[95: 0];                          // exponent didn't change, number was normalized

        4'b1001:        mo <= rounded2[95: 0];                          // exponent didn't change, but number was denormalized

        4'b1010:        mo <= {4'h1,rounded2[95: 4]};   // exponent incremented (new MSD generated), number was normalized

        4'b1011:        mo <= rounded2[95:0];                                   // exponent incremented (new MSB generated), number was denormalized, number became normalized

        4'b11??:        mo <= 96'd0;                                                                    // number became infinite, no need to check carry etc., rnd would be zero if input was NaN or infinite

        endcase

endmodule