///////////////////////////////////////////
// tlbcontrol.sv
//
// Written: David_Harris@hmc.edu 5 July 2021
// Modified: 
//
// Purpose: Control signals for TLB
// 
// A component of the Wally configurable RISC-V project.
// 
// Copyright (C) 2021 Harvey Mudd College & Oklahoma State University
//
// Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation
// files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, 
// modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software 
// is furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES 
// OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS 
// BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT 
// OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
///////////////////////////////////////////

`include "wally-config.vh"

// The TLB will have 2**ENTRY_BITS total entries
module tlbcontrol #(parameter TLB_ENTRIES = 8,
                    parameter ITLB = 0) (

  // Current value of satp CSR (from privileged unit)
  input logic  [`SVMODE_BITS-1:0] SATP_MODE,
  input logic  [`XLEN-1:0] Address,
  input logic              STATUS_MXR, STATUS_SUM, STATUS_MPRV,
  input logic  [1:0]       STATUS_MPP,
  input logic  [1:0]       PrivilegeModeW, // Current privilege level of the processeor

  // 00 - TLB is not being accessed
  // 1x - TLB is accessed for a read (or an instruction)
  // x1 - TLB is accessed for a write
  // 11 - TLB is accessed for both read and write
  input logic              ReadAccess, WriteAccess,
  input logic              DisableTranslation,
  input logic              TLBFlush, // Invalidate all TLB entries
  input logic [7:0]        PTEAccessBits,
  input logic              CAMHit,
  output logic             TLBMiss,
  output logic             TLBHit,
  output logic             TLBPageFault,
  output logic             SV39Mode,
  output logic             Translate
);

  // Sections of the page table entry
  logic [11:0]             PageOffset;
  logic [`SVMODE_BITS-1:0] SVMode;
  logic [1:0]              EffectivePrivilegeMode;

  logic PTE_D, PTE_A, PTE_U, PTE_X, PTE_W, PTE_R; // Useful PTE Control Bits
  logic                  UpperBitsUnequalPageFault;
  logic                  DAPageFault;
  logic                  TLBAccess;

  // Grab the sv mode from SATP and determine whether translation should occur
  assign EffectivePrivilegeMode = (ITLB == 1) ? PrivilegeModeW : (STATUS_MPRV ? STATUS_MPP : PrivilegeModeW); // DTLB uses MPP mode when MPRV is 1
  assign Translate = (SATP_MODE != `NO_TRANSLATE) & (EffectivePrivilegeMode != `M_MODE) & ~DisableTranslation; 
  generate
      if (`XLEN==64) begin
          assign SV39Mode = (SATP_MODE == `SV39);
          // generate page fault if upper bits aren't all the same
          logic UpperEqual39, UpperEqual48;
          assign UpperEqual39 = &(Address[63:38]) | ~|(Address[63:38]);
          assign UpperEqual48 = &(Address[63:47]) | ~|(Address[63:47]); 
          assign UpperBitsUnequalPageFault = SV39Mode ? ~UpperEqual39 : ~UpperEqual48;
      end else begin
          assign SV39Mode = 0;
          assign UpperBitsUnequalPageFault = 0;
      end           
  endgenerate

  // Determine whether TLB is being used
  assign TLBAccess = ReadAccess || WriteAccess;

  // Check whether upper bits of virtual addresss are all equal


  // unswizzle useful PTE bits
  assign {PTE_D, PTE_A} = PTEAccessBits[7:6];
  assign {PTE_U, PTE_X, PTE_W, PTE_R} = PTEAccessBits[4:1];
 
  // Check whether the access is allowed, page faulting if not.
  generate
    if (ITLB == 1) begin
      logic ImproperPrivilege;

      // User mode may only execute user mode pages, and supervisor mode may
      // only execute non-user mode pages.
      assign ImproperPrivilege = ((EffectivePrivilegeMode == `U_MODE) && ~PTE_U) ||
        ((EffectivePrivilegeMode == `S_MODE) && PTE_U);
      // fault for software handling if access bit is off
      assign DAPageFault = ~PTE_A;
      assign TLBPageFault = Translate && TLBHit && (ImproperPrivilege || ~PTE_X || DAPageFault || UpperBitsUnequalPageFault);
    end else begin
      logic ImproperPrivilege, InvalidRead, InvalidWrite;

      // User mode may only load/store from user mode pages, and supervisor mode
      // may only access user mode pages when STATUS_SUM is low.
      assign ImproperPrivilege = ((EffectivePrivilegeMode == `U_MODE) && ~PTE_U) ||
        ((EffectivePrivilegeMode == `S_MODE) && PTE_U && ~STATUS_SUM);
      // Check for read error. Reads are invalid when the page is not readable
      // (and executable pages are not readable) or when the page is neither
      // readable nor executable (and executable pages are readable).
      assign InvalidRead = ReadAccess && ~PTE_R && (~STATUS_MXR | ~PTE_X);
      // Check for write error. Writes are invalid when the page's write bit is
      // low.
      assign InvalidWrite = WriteAccess && ~PTE_W;
      // Fault for software handling if access bit is off or writing a page with dirty bit off
      assign DAPageFault = ~PTE_A | WriteAccess & ~PTE_D; 
      assign TLBPageFault = Translate && TLBHit && (ImproperPrivilege || InvalidRead || InvalidWrite || DAPageFault || UpperBitsUnequalPageFault);
    end
  endgenerate

  assign TLBHit = CAMHit & TLBAccess;
  assign TLBMiss = ~CAMHit & ~TLBFlush & Translate & TLBAccess;
endmodule