need help to understand this example ?????

[mohi@608]

this is a simple example for a counter :

// TLM counter
// compiler: Visual C++ 2008 Express Edition
//
#include <stdio.h>
#include <systemc.h>
#include "tlm.h"
#include "tlm_utils/simple_initiator_socket.h"
#include "tlm_utils/simple_target_socket.h"
using namespace tlm;
using namespace tlm_utils;
//
// C++ counter
//
unsigned int counter(unsigned int cnt)
{
   return (cnt+1);
}
//
// SystemC counter module
//
class counter_module : public sc_module
{
   SC_HAS_PROCESS(counter_module);
   virtual void increment_request_transport(tlm_generic_payload& tx, sc_time& dt);
   public:
      simple_target_socket<counter_module> increment_socket;
   private:
      unsigned int cnt;
   public:
      counter_module(sc_module_name nm) : sc_module(nm),
         increment_socket("increment_socket")
      {
         increment_socket.register_b_transport(this, &counter_module::increment_request_transport);
	     cnt=0;
      }
   const char* hdl_name() const { return "counter_module"; }
};
//
// Transport Handler
//
void counter_module::increment_request_transport(tlm_generic_payload& tx, sc_time& dt)
{
   if(tx.get_command() == TLM_READ_COMMAND)
   {
      cnt = counter(cnt);
      tx.set_data_length(4);
	  unsigned char rv[5];
	  rv[0] = cnt & 0xFF;
	  rv[1] = (cnt>>8) & 0xFF;
	  rv[2] = (cnt>>16) & 0xFF;
	  rv[3] = (cnt>>24) & 0xFF;
//cout << " cnt = " << (int)rv[0] << " " << (int)rv[1] << " " << (int)rv[2] << " " << (int)rv[3] << endl;
      tx.set_data_ptr(rv);
      tx.set_response_status(TLM_OK_RESPONSE);
   }
   else if(tx.get_command() == TLM_WRITE_COMMAND) // Reset actually
   {
      cnt = 0;
	  unsigned char rv[5];
	  rv[0] = cnt & 0xFF;
	  rv[1] = (cnt>>8) & 0xFF;
	  rv[2] = (cnt>>16) & 0xFF;
	  rv[3] = (cnt>>24) & 0xFF;
      tx.set_data_length(4);
      tx.set_data_ptr(rv);//conversion from int to char iccurs
      tx.set_response_status(TLM_OK_RESPONSE);
   }
   else
      tx.set_response_status(TLM_ADDRESS_ERROR_RESPONSE);
}
//
// Stimulus generator
//
class stimulus: public sc_module
{
   SC_HAS_PROCESS(stimulus);
   public:
      simple_initiator_socket<counter_module> read_increment;
      tlm_generic_payload tx;
      void run();

      void reset_counter();
      unsigned int read_counter();

   stimulus(sc_module_name nm) :
      sc_module(nm),
      read_increment("read_increment")
   {
      SC_THREAD(run);
   }
   const char* hdl_name() const { return "stimulus"; }
};
//
// Stimulus main process
//
void stimulus::run()
{
   reset_counter();
   wait(1, SC_NS);
   for(int i=1; i<22; i++)
   {
      cout << "Counter = " << read_counter() << " at cycle " << i << endl;
      wait(1, SC_NS);
   }
   reset_counter();
   wait(1, SC_NS);
   for(int i=23; i<100; i++)
   {
      cout << "Counter = " << read_counter() << " at cycle " << i << endl;
      wait(1, SC_NS);
   }
   sc_stop();
}
//
// Reset counter request
//
void stimulus::reset_counter()
{
   tx.set_address(0);
   unsigned char value[5] = {0,0,0,0,0};
   tx.set_data_ptr(value);
   tx.set_data_length(1);
   tx.set_command(TLM_WRITE_COMMAND);
   sc_time to(SC_ZERO_TIME);
   read_increment->b_transport(tx, to);
   wait(1, SC_NS);
}
//
// Read counter (increment occurs before reading)
//
unsigned int stimulus::read_counter()
{
   unsigned int data;
   tx.set_address(0);
   tx.set_data_length(1);
   sc_time to(SC_ZERO_TIME);
   tx.set_command(TLM_READ_COMMAND);
   read_increment->b_transport(tx, to);
   data = *reinterpret_cast<unsigned int*>(tx.get_data_ptr());
   wait(20, SC_NS);
   return data;
}
//
// Top-level module
//
int sc_main (int argc, char *argv[])
{
   counter_module counter_module_dut("counter_module_dut");
   stimulus stimulus_dut ("stimulus_dut");
   stimulus_dut.read_increment(counter_module_dut.increment_socket);

   sc_start();
   return 0;
}