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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