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92 lines
5.3 KiB
Systemverilog
92 lines
5.3 KiB
Systemverilog
///////////////////////////////////////////
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// shiftcorrection.sv
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//
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// Written: me@KatherineParry.com
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// Modified: 7/5/2022
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//
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// Purpose: shift correction
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//
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// Documentation: RISC-V System on Chip Design Chapter 13
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//
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// A component of the CORE-V-WALLY configurable RISC-V project.
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//
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// Copyright (C) 2021-23 Harvey Mudd College & Oklahoma State University
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//
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// SPDX-License-Identifier: Apache-2.0 WITH SHL-2.1
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//
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// Licensed under the Solderpad Hardware License v 2.1 (the “License”); you may not use this file
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// except in compliance with the License, or, at your option, the Apache License version 2.0. You
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// may obtain a copy of the License at
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//
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// https://solderpad.org/licenses/SHL-2.1/
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//
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// Unless required by applicable law or agreed to in writing, any work distributed under the
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// License is distributed on an “AS IS” BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND,
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// either express or implied. See the License for the specific language governing permissions
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// and limitations under the License.
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////////////////////////////////////////////////////////////////////////////////////////////////
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module shiftcorrection import cvw::*; #(parameter cvw_t P) (
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input logic [P.NORMSHIFTSZ-1:0] Shifted, // the shifted sum before LZA correction
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// divsqrt
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input logic DivOp, // is it a divsqrt opperation
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input logic DivResSubnorm, // is the divsqrt result subnormal
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input logic [P.NE+1:0] DivQe, // the divsqrt result's exponent
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input logic DivSubnormShiftPos, // is the subnorm divider shift amount positive (ie not underflowed)
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//fma
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input logic FmaOp, // is it an fma opperation
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input logic [P.NE+1:0] NormSumExp, // exponent of the normalized sum not taking into account Subnormal or zero results
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input logic FmaPreResultSubnorm, // is the result subnormal - calculated before LZA corection
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input logic FmaSZero,
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// output
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output logic [P.NE+1:0] FmaMe, // exponent of the normalized sum
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output logic [P.CORRSHIFTSZ-1:0] Mf, // the shifted sum before LZA correction
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output logic [P.NE+1:0] Qe // corrected exponent for divider
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);
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logic [3*P.NF+3:0] CorrSumShifted; // the shifted sum after LZA correction
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logic [P.CORRSHIFTSZ-1:0] CorrQm0, CorrQm1; // portions of Shifted to select for CorrQmShifted
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logic [P.CORRSHIFTSZ-1:0] CorrQmShifted; // the shifted divsqrt result after one bit shift
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logic ResSubnorm; // is the result Subnormal
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logic LZAPlus1; // add one or two to the sum's exponent due to LZA correction
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logic LeftShiftQm; // should the divsqrt result be shifted one to the left
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// LZA correction
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assign LZAPlus1 = Shifted[P.NORMSHIFTSZ-1];
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// correct the shifting error caused by the LZA
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// - the only possible mantissa for a plus two is all zeroes
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// - a one has to propigate all the way through a sum. so we can leave the bottom statement alone
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mux2 #(P.NORMSHIFTSZ-2) lzacorrmux(Shifted[P.NORMSHIFTSZ-3:0], Shifted[P.NORMSHIFTSZ-2:1], LZAPlus1, CorrSumShifted);
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// correct the shifting of the divsqrt caused by producing a result in (2, .5] range
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// condition: if the msb is 1 or the exponent was one, but the shifted quotent was < 1 (Subnorm)
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assign LeftShiftQm = (LZAPlus1|(DivQe==1&~LZAPlus1));
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assign CorrQm0 = Shifted[P.NORMSHIFTSZ-3:P.NORMSHIFTSZ-P.CORRSHIFTSZ-2];
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assign CorrQm1 = Shifted[P.NORMSHIFTSZ-2:P.NORMSHIFTSZ-P.CORRSHIFTSZ-1];
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mux2 #(P.CORRSHIFTSZ) divcorrmux(CorrQm0, CorrQm1, LeftShiftQm, CorrQmShifted);
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// if the result of the divider was calculated to be subnormal, then the result was correctly normalized, so select the top shifted bits
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always_comb
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if(FmaOp) Mf = {CorrSumShifted, {P.CORRSHIFTSZ-(3*P.NF+4){1'b0}}};
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else if (DivOp&~DivResSubnorm) Mf = CorrQmShifted;
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else Mf = Shifted[P.NORMSHIFTSZ-1:P.NORMSHIFTSZ-P.CORRSHIFTSZ];
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// Determine sum's exponent
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// main exponent issues:
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// - LZA was one too large
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// - LZA was two too large
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// - if the result was calulated to be subnorm but it's norm and the LZA was off by 1
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// - if the result was calulated to be subnorm but it's norm and the LZA was off by 2
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// if plus1 If plus2 kill if the result Zero or actually subnormal
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// | | |
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assign FmaMe = (NormSumExp+{{P.NE+1{1'b0}}, LZAPlus1} +{{P.NE+1{1'b0}}, FmaPreResultSubnorm}) & {P.NE+2{~(FmaSZero|ResSubnorm)}};
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// recalculate if the result is subnormal after LZA correction
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assign ResSubnorm = FmaPreResultSubnorm&~Shifted[P.NORMSHIFTSZ-2]&~Shifted[P.NORMSHIFTSZ-1];
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// the quotent is in the range [.5,2) if there is no early termination
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// if the quotent < 1 and not Subnormal then subtract 1 to account for the normalization shift
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assign Qe = (DivResSubnorm & DivSubnormShiftPos) ? '0 : DivQe - {(P.NE+1)'(0), ~LZAPlus1};
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endmodule
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