cvw/src/privileged/csru.sv

///////////////////////////////////////////
// csru.sv
//
// Written: David_Harris@hmc.edu 9 January 2021
// Modified: 
//
// Purpose: User-Mode Control and Status Registers for Floating Point
// 
// Documentation: RISC-V System on Chip Design Chapter 5
//
// A component of the CORE-V-WALLY configurable RISC-V project.
// 
// Copyright (C) 2021-23 Harvey Mudd College & Oklahoma State University
//
// SPDX-License-Identifier: Apache-2.0 WITH SHL-2.1
//
// Licensed under the Solderpad Hardware License v 2.1 (the “License”); you may not use this file 
// except in compliance with the License, or, at your option, the Apache License version 2.0. You 
// may obtain a copy of the License at
//
// https://solderpad.org/licenses/SHL-2.1/
//
// Unless required by applicable law or agreed to in writing, any work distributed under the 
// License is distributed on an “AS IS” BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, 
// either express or implied. See the License for the specific language governing permissions 
// and limitations under the License.
////////////////////////////////////////////////////////////////////////////////////////////////

`include "wally-config.vh"

module csru #(parameter 
  FFLAGS = 12'h001,
  FRM = 12'h002,
  FCSR = 12'h003) (
  input  logic             clk, reset, 
  input  logic             InstrValidNotFlushedM,
  input  logic             CSRUWriteM,
  input  logic [11:0]      CSRAdrM,
  input  logic [`XLEN-1:0] CSRWriteValM,
  input  logic [1:0]       STATUS_FS,
  output logic [`XLEN-1:0] CSRUReadValM,  
  input  logic [4:0]       SetFflagsM,
  output logic [2:0]       FRM_REGW,
  output logic             WriteFRMM, WriteFFLAGSM,
  output logic             IllegalCSRUAccessM
);

  logic [4:0]              FFLAGS_REGW;
  logic [2:0]              NextFRMM;
  logic [4:0]              NextFFLAGSM;
  logic                    SetOrWriteFFLAGSM;
  
  // Write enables
  assign WriteFRMM =    CSRUWriteM & (STATUS_FS != 2'b00) & (CSRAdrM == FRM | CSRAdrM == FCSR);
  assign WriteFFLAGSM = CSRUWriteM & (STATUS_FS != 2'b00) & (CSRAdrM == FFLAGS | CSRAdrM == FCSR);

  // Write Values
  assign NextFRMM = (CSRAdrM == FCSR) ? CSRWriteValM[7:5] : CSRWriteValM[2:0];
  assign NextFFLAGSM = WriteFFLAGSM ? CSRWriteValM[4:0] : FFLAGS_REGW | SetFflagsM;
  assign SetOrWriteFFLAGSM = WriteFFLAGSM | (|SetFflagsM & InstrValidNotFlushedM);

  // CSRs
  flopenr #(3) FRMreg(clk, reset, WriteFRMM, NextFRMM, FRM_REGW);
  flopenr   #(5) FFLAGSreg(clk, reset, SetOrWriteFFLAGSM, NextFFLAGSM, FFLAGS_REGW); 

  // CSR Reads
  always_comb begin
    if (STATUS_FS == 2'b00) begin // fpu disabled, trap
      IllegalCSRUAccessM = 1;
      CSRUReadValM = 0;
    end else begin
      IllegalCSRUAccessM = 0;
      case (CSRAdrM) 
        FFLAGS:    CSRUReadValM = {{(`XLEN-5){1'b0}}, FFLAGS_REGW};
        FRM:       CSRUReadValM = {{(`XLEN-3){1'b0}}, FRM_REGW};
        FCSR:      CSRUReadValM = {{(`XLEN-8){1'b0}}, FRM_REGW, FFLAGS_REGW};
        default: begin
                   CSRUReadValM = 0; 
                   IllegalCSRUAccessM = 1;
        end         
      endcase
    end
  end
endmodule