Writing TLM2.0-compliant timed SystemC simulation models for SoCLib

Authors : Alain Greiner, François Pêcheux, Emmanuel Viaud, Nicolas Pouillon, Aline Vieira de Mello

A) Introduction

This document is still under development. It describes the modeling rules for writing TLM-T SystemC simulation models for SoCLib that are compliant with the new TLM2.0 OSCI standard. These rules enforce the PDES (Parallel Discrete Event Simulation) principles. Each PDES process involved in the simulation has its own local time, and PDES processes synchronize through messages piggybacked with time information. Models complying to these rules can be used with the "standard" OSCI simulation engine (SystemC 2.x) and the TLM2.0 protocol, but can also be used also with others simulation engines, especially distributed, parallelized simulation engines.

The interested user should also look at the general SoCLib rules.

B) Single VCI initiator and single VCI target

Figure 1 presents a minimal TLM-T system containing one single initiator, my_initiator , and one single target, my_target . The my_initiator module behavior is modeled by the SC_THREAD execLoop(), that contains an infinite loop. The call-back function my_nb_transport_bw() is executed when a VCI response packet is received by the initiator module.

Unlike the initiator, the target module has a purely reactive behaviour and is therefore modeled as a simple call-back function. In other words, there is no need to use a SC_THREAD for these simple target components: the target behaviour is entirely described by the call-back function my_nb_transport_fw(), that is executed when a VCI command packet is received by the target module.

The VCI communication channel is a point-to-point bi-directionnal channel, encapsulating two separated uni-directionnal channels: one to transmit the VCI command packet, one to transmit the VCI response packet.

C) VCI initiator Modeling

In the proposed example, the initiator module is modeled by the my_initiator class. This class inherits from the standard SystemC sc_core::sc_module class, that acts as the root class for all TLM-T modules.

The initiator local time is contained in a member variable named m_localTime, of type sc_core::sc_time. The local time can be accessed with the following accessors: addLocalTime(), setLocalTime() and getLocalTime().

  sc_core::sc_time m_localTime;                     // the initiator local time
  ...
  void addLocalTime(sc_core::sc_time t);            // add a value to the local time
  void setLocalTime(sc_core::sc_time& t);           // set the local time
  sc_core::sc_time getLocalTime(void);              // get the local time

The initiator activity corresponds to the boolean member m_activity that indicates if the initiator is currently active (i.e. true, wants to participate in the arbitration in the interconnect) or inactive (i.e. false, does not want to participate in the arbitration in the interconnect). The corresponding access functions are setActivity() and getActivity().

  bool m_activity;
  ...
  void setActivity(bool t);                         // set the activity status (true if the component is active)
  bool getActivity(void);                           // get the activity state

The execLoop() method, describing the initiator behaviour must be declared as a member function of the my_initiator class.

Finally, the class my_initiator must contain a member variable p_vci_init, of type tlmt_simple_initiator_socket. This member variable represents the VCI initiator port. It has 3 template parameters, two of which are used to help connecting the response callback function (my_initiator in the example, first template parameter) to the port and defining the port type (soclib_vci_types in the following example, third template parameter). soclib_vci_types is indeed a C++ structure containing two typedef: the first typedef defines the payload type as VCI, and the other defines the TLM phase type. The phase type can either be TLMT_CMD (i.e. the transaction indicates the emission of a command by an initiator and its reception by a target), TLMT_RSP (i.e. the transaction indicates the emission of a response by a target and its reception by an initiator), or TLMT_INFO (i.e. a TLM-T transaction emitted by one side of a link (vci, irq or fifo) to get information such as time and activity on the other side of the link).

C.1) Sending a VCI command packet

To send a VCI command packet, the execLoop() method must use the nb_transport_fw() method, that is a member function of the p_vci_init port. The prototype of this method is the following:

  tlm::tlm_sync_enum nb_transport_fw               /// sync status
  ( soclib_vci_types::tlm_payload_type &payload,      ///< VCI payload pointer
    soclib_vci_types::tlm_phase_type   &phase,       ///< transaction phase
    sc_core::sc_time                   &time);        ///< time

The first parameter of this member function is the VCI packet, the second represents the phase (TLMT_CMD in this case), and the third parameter contains the initiator local time.

To prepare a VCI packet for sending, the execLoop function must declare two objects locally, payload and phase.

  soclib_vci_types::tlm_payload_type payload;
  soclib_vci_types::tlm_phase_type phase;

A payload of type soclib_vci_types::tlm_payload_type corresponds to a tlmt_vci_transaction. It contains three groups of information:

• TLM2.0 related fields
• TLM-T related fields
• VCI related fields

The contents of a tlmt_vci_transaction is defined below:

class tlmt_vci_transaction
{
	...
private:
  // TLM2.0 related fields and common structure

  sc_dt::uint64        m_address; 		// address
  unsigned char*       m_data; 			// buf
  unsigned int         m_length; 		// nword
  tlmt_response_status m_response_status; 	// rerror
  bool                 m_dmi; 			// nothing
  unsigned char*       m_byte_enable; 		// be
  unsigned int         m_byte_enable_length; 
  unsigned int         m_streaming_width; 	// 

  // TLM-T related fields
  
  bool*                m_activity_ptr;
  sc_core::sc_time*    m_local_time_ptr;

  // VCI related fields
  
  tlmt_command         m_command; 		// cmd
  unsigned int         m_src_id; 		// srcid
  unsigned int         m_trd_id; 		// trdid
  unsigned int         m_pkt_id; 		// pktid

The TLM2.0 compliant accessors allow to set the TLM2.0 related fields, such as the transaction address, the byte enable array pointer and its associated size in bytes, and the data array pointer and its associated size in bytes. The byte enable array allows to build versatile packets thanks to a powerful but slow data masking scheme. Further experiments are currently done to evaluate the performance degradation incurred by the byte formatting. It is therefore possible that the types of the m_data and m_byte_enable of the tlmt_vci_transaction will be changed to uint32* in a near future.

Dedicated VCI accessors are used to define the VCI transaction type, that can either be set_read() (for read command), set_write() (for write command),set_locked_read() (for atomic locked read), and set_store_cond() (for atomic store conditional). The set_src_id(), set_trd_id() and set_pkt_id() functions respectively set the VCI source, thread and packet identifiers. The following example describes a VCI write command:

    payload.set_address(0x10000000);//ram 0
    payload.set_byte_enable_ptr(byte_enable);
    payload.set_byte_enable_length(nbytes);
    payload.set_data_ptr(data);
    payload.set_data_length(nbytes); // 5 words of 32 bits

    payload.set_write();
    payload.set_src_id(m_id);
    payload.set_trd_id(0);
    payload.set_pkt_id(pktid);

    phase= soclib::tlmt::TLMT_CMD;
    sendTime = getLocalTime();

    p_vci_init->nb_transport_fw(payload, phase, sendTime);

The nb_transport_fw() function is non-blocking. To implement a blocking transaction (such as a cache line read, where the processor is stalled during the VCI transaction), the model designer must use the SystemC sc_core::wait(x) primitive (x being of type sc_core::sc_event): the execLoop() thread is then suspended, and will be reactivated when the response packet is actually received.

C.2) Receiving a VCI response packet

To receive a VCI response packet, a call-back function must be defined as a member function of the class my_initiator. This call-back function (named vci_rsp_received() in the example), must be declared in the my_initiator class and is executed each time a VCI response packet is received on the p_vci_init port. The function name is not constrained, but the arguments must respect the following prototype:

  tlm::tlm_sync_enum vci_rsp_received                // for resp messages 
    ( soclib_vci_types::tlm_payload_type &payload,     // payload
      soclib_vci_types::tlm_phase_type   &phase,      // transaction phase
      sc_core::sc_time                   &time);       // resp time

The return value (type tlm::tlm_sync_enum) must be sytematically set to tlm::TLM_COMPLETED in this implementation The function parameters are identical to those described in the forward transport function

In the general case, the actions executed by the call-back function depend on the response transaction type (m_command field), as well as the pktid and trdid fields.

For sake of simplicity, in the proposed example the call-back function does not make any distinction between VCI transaction types :

C.3) Initiator Constructor

The constructor of the class my_initiator must initialize all the member variables, including the p_vci_init port. The my_nb_transport_bw() call-back function being executed in the context of the thread sending the response packet, a link between the p_vci_init port and the call-back function must be established. The TLMT_INFO transaction allows the target to get information on the initiator that actually sends VCI packets (the initiator local time, the initiator activity, etc).

The my_initiator constructor for the p_vci_init object must be called with the following arguments:

  p_vci_init.register_nb_transport_bw(this, &my_initiator::my_nb_transport_bw);

where my_nb_transport_bw is the name of the callback function

C.4) Lookahead parameter

The SystemC simulation engine behaves as a cooperative, non-preemptive multi-tasks system. Any thread in the system must stop execution after at some point, in order to allow the other threads to execute. With the proposed approach, a TLM-T initiator will never stop if it does not execute blocking communication (such as a processor that has all code and data in the L1 caches).

To solve this issue, it is necessary to define -for each initiator module- a lookahead parameter. This parameter defines the maximum number of cycles that can be executed by the thread before it is automatically stopped. The lookahead parameter allows the system designer to bound the de-synchronization time interval between threads.

A small value for this parameter results in a better timing accuracy for the simulation, but implies a larger number of context switches, and a slower simulation speed.

C.4) VCI initiator example

////////////////////////// my_initiator.h ////////////////////////////////
#ifndef __MY_INITIATOR_H__
#define __MY_INITIATOR_H__

#include "tlm.h"                                // TLM headers
#include "tlmt_transactions.h"			// VCI headers
#include "tlmt_simple_initiator_socket.h"       // VCI socket
#include "mapping_table.h"

class my_initiator                              // my_initiator 
:         public sc_core::sc_module             // inherit from SC module base class
{
private:
  //Variables
  typedef soclib::tlmt::VciParams<uint32_t,uint32_t,4> vci_param;
  sc_core::sc_event m_rspEvent;
  sc_core::sc_time m_localTime;
  bool m_activity;
  uint32_t m_initid;
  uint32_t m_counter;
  uint32_t m_lookahead;
 
  /////////////////////////////////////////////////////////////////////////////////////
  // Fuctions
  /////////////////////////////////////////////////////////////////////////////////////
  void execLoop(void);                              // initiator thread
  void addLocalTime(sc_core::sc_time t);            // add a value to the local time
  void setLocalTime(sc_core::sc_time& t);           // set the local time
  sc_core::sc_time getLocalTime(void);              // get the local time
  void setActivity(bool t);                         // set the activity status (true if the component is active)
  bool getActivity(void);                           // get the activity state

  /////////////////////////////////////////////////////////////////////////////////////
  // Virtual Fuctions  tlm::tlm_bw_transport_if (VCI INITIATOR SOCKET)
  /////////////////////////////////////////////////////////////////////////////////////

  /// Receive rsp from target
  tlm::tlm_sync_enum my_nb_transport_bw                // for resp messages 
    ( soclib_vci_types::vci_payload_type &payload,     // payload
      soclib_vci_types::tlmt_phase_type   &phase,      // transaction phase
      sc_core::sc_time                   &time);       // resp time

protected:
  SC_HAS_PROCESS(my_initiator);
  
public:
  tlmt_simple_initiator_socket<my_initiator,32,soclib_vci_types> p_vci_init;   // VCI initiator port 

  //constructor
  my_initiator(                                              // constructor
	       sc_core::sc_module_name name,                 // module name
	       const soclib::common::IntTab &index,          // VCI initiator index 
	       const soclib::common::MappingTable &mt,       // mapping table
               uint32_t lookahead                            // lookahead
	       );
  
}; 
 #endif /* __MY_INITIATOR_H__ */

////////////////////////// my_initiator.cpp ////////////////////////////////

#include "my_initiator.h"                       // Our header

#ifndef MY_INITIATOR_DEBUG
#define MY_INITIATOR_DEBUG 1
#endif

#define tmpl(x) x my_initiator

///Constructor
tmpl (/**/)::my_initiator
	    ( sc_core::sc_module_name name,           // module name
	      const soclib::common::IntTab &index,    // index of mapping table
	      const soclib::common::MappingTable &mt, // mapping table
	      uint32_t lookahead                      // lookahead
	      )
	    : sc_module(name)                         // init module name
	   , p_vci_init("p_vci_init")                 // vci initiator socket name
{
  //register callback function
  p_vci_init.register_nb_transport_bw(this, &my_initiator::my_nb_transport_bw);

  // initiator identification
  m_initid = mt.indexForId(index);

  //lookahead control
  m_counter = 0;
  m_lookahead = lookahead;

  //initialize the local time
  m_localTime= 0 * UNIT_TIME;

  // initialize the activity variable
  setActivity(true);

  // register thread process
  SC_THREAD(execLoop);
}

/////////////////////////////////////////////////////////////////////////////////////
// Fuctions
/////////////////////////////////////////////////////////////////////////////////////
tmpl (sc_core::sc_time)::getLocalTime()
{
  return m_localTime;
}

tmpl (bool)::getActivity()
{
  return m_activity;
}

tmpl (void)::setLocalTime(sc_core::sc_time &t)
{
  m_localTime=t;
}

tmpl (void)::addLocalTime(sc_core::sc_time t)
{
  m_localTime= m_localTime + t;
}

tmpl (void)::setActivity(bool t)
{
  m_activity =t;
}

tmpl (void)::execLoop(void)   // initiator thread
{
  soclib_vci_types::vci_payload_type payload;
  soclib_vci_types::tlmt_phase_type phase;
  sc_core::sc_time sendTime;
  unsigned char data[32];
  unsigned char byte_enable[32];
  int pktid = 0;
  int nbytes = 4; // 1 word of 32 bits

  uint32_t int_data = 12345678;
  std::ostringstream name;
  name << "" << int_data;
  std::cout << "NAME = " << std::dec << name << std::endl;

  for(int i=0; i<nbytes; i++)
    byte_enable[i] = TLMT_BYTE_ENABLED;

  for(int i=0; i<32; i++)
    data[i] = 0x0;

  data[0]='a';
  data[1]='b';
  data[2]='c';
  data[3]='d';
  data[4]='\0';

  std ::cout<< "DATA = " << data << std::endl;

  while ( 1 ){
    addTime(10 * UNIT_TIME);

    payload.set_address(0x10000000);//ram 0
    payload.set_byte_enable_ptr(byte_enable);
    payload.set_byte_enable_length(nbytes);
    payload.set_data_ptr(data);
    payload.set_data_length(nbytes); // 5 words of 32 bits

    payload.set_write();
    payload.set_src_id(m_id);
    payload.set_trd_id(0);
    payload.set_pkt_id(pktid);

    phase= soclib::tlmt::TLMT_CMD;
    sendTime = getLocalTime();

#if MY_INITIATOR_DEBUG
    std::cout << "[INITIATOR " << m_id << "] send cmd packet id = " << payload.get_pkt_id() << " time = " << getLocalTime().value() << std::endl;
#endif

    p_vci_init->nb_transport_fw(payload, phase, sendTime);
    wait(m_rspEvent);

#if MY_INITIATOR_DEBUG
    std::cout << "[INITIATOR " << m_id << "] receive rsp packet id = " << payload.get_pkt_id() << " time = " << getLocalTime().value() << std::endl;
#endif

    pktid++;

    addTime(10 * UNIT_TIME);

    payload.set_address(0x10000000);//ram 0
    payload.set_byte_enable_ptr(byte_enable);
    payload.set_byte_enable_length(nbytes);
    payload.set_data_ptr(data);
    payload.set_data_length(nbytes); // 5 words of 32 bits

    payload.set_read();
    payload.set_src_id(m_id);
    payload.set_trd_id(0);
    payload.set_pkt_id(pktid);

    phase= soclib::tlmt::TLMT_CMD;
    sendTime = getLocalTime();

#if MY_INITIATOR_DEBUG
    std::cout << "[INITIATOR " << m_id << "] send cmd packet id = " << payload.get_pkt_id() << " time = " << getLocalTime().value() << std::endl;
#endif

    p_vci_init->nb_transport_fw(payload, phase, sendTime);
    wait(m_rspEvent);

#if MY_INITIATOR_DEBUG
    std::cout << "[INITIATOR " << m_id << "] receive rsp packet id = " << payload.get_pkt_id() << " time = " << getLocalTime().value() << std::endl;

#endif

    pktid++;

    // lookahead management
    m_counter++ ;
    if (m_counter >= m_lookahead) {
      m_counter = 0 ;
      wait(sc_core::SC_ZERO_TIME) ;
    }

  } // end while true
  setActivity(false);
} // end initiator_thread


/////////////////////////////////////////////////////////////////////////////////////
// Virtual Fuctions  tlm::tlm_bw_transport_if (VCI SOCKET)
/////////////////////////////////////////////////////////////////////////////////////

/// receive the response packet from target socket
tmpl (tlm::tlm_sync_enum)::my_nb_transport_bw // inbound nb_transport_bw
( soclib_vci_types::vci_payload_type &payload,       // VCI payload
  soclib_vci_types::tlmt_phase_type   &phase,        // tlm phase
  sc_core::sc_time                   &rspTime        // the response timestamp
 )
{
  switch(phase){
  case soclib::tlmt::TLMT_RSP :
    setLocalTime(rspTime);
    m_rspEvent.notify(0 * UNIT_TIME);
    break;
  case soclib::tlmt::TLMT_INFO :
    payload.set_local_time_ptr(&m_localTime);
    payload.set_activity_ptr(&m_activity);
    break;
  }
  return tlm::TLM_COMPLETED;
} // end backward nb transport

D) VCI target modeling

In the proposed example, the target handles all VCI commands in the same way. To simplify the model, there is no real error management.

The class my_target inherits from the class sc_core::sc_module. The class my_target contains a member variable p_vci_target of type tlmt_simple_target_socket. This object has 3 template parameters, that are identical to those used for declaring initiator ports (see above).

D.1) Receiving a VCI command packet

To receive a VCI command packet, a call-back function must be defined as a member function of the class my_target. This call-back function (named my_nb_transport_fw() in the example), will be executed each time a VCI command packet is received on the p_vci_target port. The function name is not constrained, but the arguments must respect the following prototype:

  tlm::tlm_sync_enum my_nb_transport_fw               /// sync status
  ( soclib_vci_types::vci_payload_type &payload,      ///< VCI payload pointer
    soclib_vci_types::tlmt_phase_type   &phase,       ///< transaction phase
    sc_core::sc_time                   &time);        ///< time

D.2) Sending a VCI response packet

To send a VCI response packet the my_nb_transport_fw() function must use the nb_transport_bw() method, that is a member function of the class tlmt_simple_target_socket, and has the following prototype:

	payload.set_response_status(soclib::tlmt::TLMT_OK_RESPONSE);
  
	phase = soclib::tlmt::TLMT_VCI_RSP;
	time = time + (nwords * UNIT_TIME);
  
	p_vci_target->nb_transport_bw(payload, phase, time);

For a reactive target, the response packet time is computed as the command packet time plus the target intrinsic latency.

D.3) Target Constructor

The constructor of the class my_target must initialize all the member variables, including the p_vci_target port. The my_nb_transport_fw() call-back function being executed in the context of the thread sending the command packet, a link between the p_vci_target port and the call-back function must be established. The my_target constructor must be called with the following arguments:

  p_vci_target.register_nb_transport_fw(this, &my_target::my_nb_transport_fw);

where my_nb_transport_fw is the name of the forward callback function.

D.4) VCI target example

////////////////////////// my_target.h ////////////////////////////////

#ifndef __MY_TARGET_H__ 
#define __MY_TARGET_H__

#include "tlm.h"                        // TLM headers
#include "tlmt_transactions.h"		// VCI headers
#include "tlmt_simple_target_socket.h"  // VCI SOCKET
#include "mapping_table.h"
#include "soclib_endian.h"

class my_target 
  : public sc_core::sc_module
{
 private:
  typedef soclib::tlmt::VciParams<uint32_t,uint32_t,4> vci_param;

  uint32_t m_targetid;
  soclib::common::MappingTable m_mt;


  /////////////////////////////////////////////////////////////////////////////////////
  // Virtual Fuctions  tlm::tlm_fw_transport_if (VCI SOCKET)
  /////////////////////////////////////////////////////////////////////////////////////
  
  // receive command from initiator
  tlm::tlm_sync_enum my_nb_transport_fw               /// sync status
  ( soclib_vci_types::vci_payload_type &payload,      ///< VCI payload pointer
    soclib_vci_types::tlmt_phase_type   &phase,       ///< transaction phase
    sc_core::sc_time                   &time);        ///< time

 protected:
  SC_HAS_PROCESS(my_target);
 public:
  tlmt_simple_target_socket<my_target,32,soclib_vci_types> p_vci_target;        ///<  VCI target socket

  my_target(sc_core::sc_module_name            name,
	 const soclib::common::IntTab       &index,
	 const soclib::common::MappingTable &mt);
  
  ~my_target();
};

#endif

////////////////////////// my_target.cpp ////////////////////////////////

#include "my_target.h"

#ifndef MY_TARGET_DEBUG
#define MY_TARGET_DEBUG 1
#endif

#define tmpl(x) x my_target

//////////////////////////////////////////////////////////////////////////////////////////
// CONSTRUCTOR
//////////////////////////////////////////////////////////////////////////////////////////
tmpl(/**/)::my_target
	   ( sc_core::sc_module_name name,
	     const soclib::common::IntTab &index,
	     const soclib::common::MappingTable &mt)
	   : sc_module(name),
	   m_mt(mt),
	   p_vci_target("p_vci_target")
{
  //register callback fuction
  p_vci_target.register_nb_transport_fw(this, &my_target::my_nb_transport_fw);
  
  m_id = m_mt.indexForId(index);
}

tmpl(/**/)::~my_target(){}

/////////////////////////////////////////////////////////////////////////////////////
// Virtual Fuctions  tlm::tlm_fw_transport_if VCI SOCKET
/////////////////////////////////////////////////////////////////////////////////////

//nb_transport_fw implementation calls from initiators 
tmpl(tlm::tlm_sync_enum)::my_nb_transport_fw                            // non-blocking transport call through Bus
			 ( soclib_vci_types::vci_payload_type &payload, // VCI payload pointer
			   soclib_vci_types::tlmt_phase_type   &phase,  // transaction phase
			   sc_core::sc_time                   &time)    // time
{
  int nwords = payload.get_data_length() / vci_param::nbytes;
  switch(payload.get_command()){
  case soclib::tlmt::VCI_READ_COMMAND:
  case soclib::tlmt::VCI_WRITE_COMMAND:
  case soclib::tlmt::VCI_LOCKED_READ_COMMAND:
  case soclib::tlmt::VCI_STORE_COND_COMMAND:
      {
#if MY_TARGET_DEBUG
	std::cout << "[RAM " << m_id << "] Receive from source " << payload.get_src_id() <<" a packet "<< payload.get_pkt_id() << " Time = "  << time.value() << std::endl;
#endif

	payload.set_response_status(soclib::tlmt::TLMT_OK_RESPONSE);
  
	phase = soclib::tlmt::TLMT_VCI_RSP;
	time = time + (nwords * UNIT_TIME);
  
#if MY_TARGET_DEBUG
	std::cout << "[RAM " << m_id << "] Send to source "<< payload.get_src_id() << " a anwser packet " << payload.get_pkt_id() << " Time = "  << time.value() << std::endl;
#endif
  
	p_vci_target->nb_transport_bw(payload, phase, time);
	return tlm::TLM_COMPLETED;
      }
      break;
    default:
      break;
  }
  
  //send error message
  payload.set_response_status(soclib::tlmt::TLMT_ERROR_RESPONSE);
  
  phase = soclib::tlmt::TLMT_VCI_RSP;
  time = time + nwords * UNIT_TIME;
  
#if MY_TARGET_DEBUG
  std::cout << "[RAM " << m_id << "] Address " << payload.get_address() << " does not match any segment " << std::endl;
  std::cout << "[RAM " << m_id << "] Send to source "<< payload.get_src_id() << " a error packet with time = "  << time.value() << std::endl;
#endif
  p_vci_target->nb_transport_bw(payload, phase, time);
  return tlm::TLM_COMPLETED;
}

E) VCI Interconnect

The VCI interconnect used for the TLM-T simulation is a generic simulation model, named VciVgmn. The two main parameters are the number of initiators, and the number of targets. In TLM-T simulation, we don't want to reproduce the cycle-accurate behavior of a particular interconnect. We only want to simulate the contention in the network, when several VCI intitiators try to reach the same VCI target. Therefore, the network is actually modeled as a complete cross-bar : In a physical network such as the multi-stage network described in Figure 2.a, conflicts can appear at any intermediate switch. In the VciVgmn network described in Figure 2.b, conflicts can only happen at the output ports. It is possible to specify a specific latency for each input/output couple. As in most physical interconnects, the general arbitration policy is round-robin.

E.1) Generic network modeling

There is actually two fully independent networks for VCI command packets and VCI response packets. There is a routing function for each input port, and an arbitration function for each output port, but the two networks are not symmetrical :

• For the command network, the arbitration policy is distributed: there is one arbitration thread for each output port (i.e. one arbitration thread for each VCI target). Each arbitration thread is modeled by a SC_THREAD, and contains a local clock.
• For the response network, there are no conflicts, and there is no need for arbitration. Therefore, there is no thread (and no local time) and the response network is implemented by simple function calls.

This is illustrated in Figure 3 for a network with 2 initiators and three targets :

E.2) VCI initiators and targets synchronizations

As described in sections B & C, each VCI initiator TLM-T module contains a thread and a local clock. But, in order to increase the TLM-T simulation speed, the VCI targets are generally described as reactive call-back functions. Therefore, there is no thread, and no local clock in the TLM-T module describing a VCI target. For a VCI target, the local clock is actually the clock associated to the corresponding arbitration thread contained in the VciVgmn module.