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

// Debug Unit SPR cache: definition

// Copyright (C) 2008  Embecosm Limited <info@embecosm.com>

// Contributor Jeremy Bennett <jeremy.bennett@embecosm.com>

// This file is part of the GDB interface to the cycle accurate model of the

// OpenRISC 1000 based system-on-chip, ORPSoC, built using Verilator.

// This program is free software: you can redistribute it and/or modify it

// under the terms of the GNU Lesser General Public License as published by

// the Free Software Foundation, either version 3 of the License, or (at your

// option) any later version.

// This program is distributed in the hope that it will be useful, but WITHOUT

// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

// FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public

// License for more details.

// You should have received a copy of the GNU Lesser General Public License

// along with this program.  If not, see <http://www.gnu.org/licenses/>.

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

// $Id: SprCache.h 331 2009-03-12 17:01:48Z jeremy $

#ifndef SPR_CACHE__H

#define SPR_CACHE__H

#include <stdint.h>

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

//! Module for cacheing SPR accesses by the debug unit

//! SPR reads and writes through the Debug Unit via JTAG are time

//! consuming - of the order of 1000 CPU clock cycles. However when the

//! processor is stalled the values cannot change, other than through the

//! debug unit, so it makes sense to cache values.

//! @note It is not strictly true that SPRs do not change. If the NPC is

//!       written, it flushes the pipeline, and subsequent reads will return

//!       zero until the processor is unstalled and the pipeline has

//!       refilled. However for our purposes, it is convenient to return the

//!       value written into the NPC in such circumstances.

//!

//! The cache is represented as a closed hash table, which is generally

//! allowed to be no more than 70% full (however NPC is always

//! cacheable). The hash function is a simple modulo function, stepping

//! forward to the first free slot. This works because there is no function to

//! delete an entry - just to clear the whole table, so holes cannot appear.

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

class SprCache

{

public:

  // Constructor and destructor

  SprCache (int  _tableSize = 257);

  ~SprCache ();

  // Functions

  void  clear ();

  void  write (uint16_t  sprNum,

               uint32_t  value,

               bool      force);

  bool  read (uint16_t  sprNum,

              uint32_t &value);

private:

  //! The size of the hash table

  int  tableSize;

  //! Maximum amount of cache left to use, before cacheing is rejected.

  int  maxToUse;

  // The cache, keyed by sprNum. Done as two parallel vectors,

  // allowing unambiguous clearing by use of memset for efficiency.

  bool     *sprIsValid;

  uint16_t *sprKeyNum;

  uint32_t *sprValue;

};      // SprCache ()

#endif  // SPR_CACHE__H