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

//        __

//   \\__/ o\    (C) 2006-2022  Robert Finch, Waterloo

//    \  __ /    All rights reserved.

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

//       ||

//

//      fpRound32combo.sv

//    - floating point rounding unit

//    - IEEE 754 representation

//              - combinational logic only

//

//

// 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

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

import fp32Pkg::*;

module fpRound32combo(rm, i, o);

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

input FP32N i;          // intermediate format input

output FP32 o;          // rounded output

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

// variables

wire so;

wire [fp32Pkg::EMSB:0] xo;

reg  [fp32Pkg::FMSB:0] mo;

reg [fp32Pkg::EMSB:0] xo1;

reg [fp32Pkg::FMSB+3:0] mo1;

wire xInf = &i[fp32Pkg::MSB+2:fp32Pkg::FMSB+4];

wire so0 = i[fp32Pkg::MSB+3];

assign o.sign = so;

assign o.exp = xo;

assign o.sig = mo;

wire l = i[3];

wire g = i[2];  // guard bit: always the same bit for all operations

wire r = i[1];  // rounding bit

wire s = i[0];  // sticky bit

reg rnd;

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

// Clock #1

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

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

always_comb

        xo1 <= i[fp32Pkg::MSB+2:fp32Pkg::FMSB+4];

always_comb

        mo1 <= i[fp32Pkg::FMSB+3:0];

wire tie = g & ~(r|s);

// Compute the round bit

// Infinities and NaNs are not rounded!

always_comb

        casez ({xInf,rm})

        4'b0000:  rnd <= (g & (r|s)) | (l & tie); // round to nearest ties to even

        4'b0001:        rnd <= 1'd0;                            // round to zero (truncate)

        4'b0010:        rnd <= g & !so0;              // round towards +infinity

        4'b0011:        rnd <= g & so0;                      // round towards -infinity

        4'b0100:  rnd <= (g & (r|s)) | tie; // round to nearest ties away from zero

        4'b1???:        rnd <= 1'd0;    // no rounding if exponent indicates infinite or NaN

        default:        rnd <= 0;

        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)

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

reg [fp32Pkg::MSB:0] rounded2;

reg carry2;

reg rnd2;

reg dn2;

wire [fp32Pkg::EMSB:0] xo2;

wire [fp32Pkg::MSB:0] rounded1 = {xo1,mo1[fp32Pkg::FMSB+3:3],1'b0} + {rnd,1'b0};        // Add onto LSB, GRS=0

always_comb

        rounded2 <= rounded1;

always_comb

        carry2 <= mo1[fp32Pkg::FMSB+3] & !rounded1[fp32Pkg::FMSB+1];

always_comb

        rnd2 <= rnd;

always_comb

        dn2 <= !(|xo1);

assign xo2 = rounded2[fp32Pkg::MSB:fp32Pkg::FMSB+2];

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

// Clock #3

// - shift mantissa if required.

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

assign so = i[fp32Pkg::MSB+3];

assign xo = xo2;

always_comb

        casez({rnd2,&xo2,carry2,dn2})

        4'b0??0:        mo <= mo1[fp32Pkg::FMSB+2:2];		// not rounding, not denormalized, => hide MSB

        4'b0??1:        mo <= mo1[fp32Pkg::FMSB+3:3];           // not rounding, denormalized

        4'b1000:        mo <= rounded2[fp32Pkg::FMSB  :0];	// exponent didn't change, number was normalized, => hide MSB,

        4'b1001:        mo <= rounded2[fp32Pkg::FMSB+1:1];	// exponent didn't change, but number was denormalized, => retain MSB

        4'b1010:        mo <= rounded2[fp32Pkg::FMSB+1:1];	// exponent incremented (new MSB generated), number was normalized, => hide 'extra (FMSB+2)' MSB

        4'b1011:        mo <= rounded2[fp32Pkg::FMSB+1:1];	// exponent incremented (new MSB generated), number was denormalized, number became normalized, => hide 'extra (FMSB+2)' MSB

        4'b11??:        mo <= 1'd0;                                             // number became infinite, no need to check carry etc., rnd would be zero if input was NaN or infinite

        endcase

endmodule