Related ISR in UVM

UVM
,
1

hi,
actually i’m working on interrupt service routine (ISR) using grab and ungrab method. Now, the problem is without grab and ungrab i’m able to perform the ISR operation. How can i understand the significance of grab and ungrab method. Here is the code i written for the ISR

//Sequence code 
task apb_isr::body;
begin
	`uvm_info(get_type_name(),"Message from ISR>>>>>>>>>>>>>",UVM_MEDIUM)	
	`uvm_info(get_type_name(),"Start of ISR @@@@@@@@@@@@@@@@@@@@",UVM_MEDIUM)	
		//m_sequencer.grab(this);
	req=apb_xtn::type_id::create("req");
	begin
		start_item(req);
		assert (req.randomize() with 	{ addr == `INTSTATUS_INTCLEAR;
				  		  trans_type == write;
				  		  wdata == 4'b1111;
						});
		finish_item(req);
	end
		//m_sequencer.ungrab(this);

	`uvm_info(get_type_name(),"End of ISR @@@@@@@@@@@@@@@@@@@@",UVM_MEDIUM)	
end
endtask


//Virtual sequence code
 task TX_virtual_seq::body();
    super.body();
     write=apb_write::type_id::create("write");                
    isr=apb_isr::type_id::create("isr");
  fork
  env_cfg.TX_test_flag=1;
  write.start(sqr);
  begin
  wait(env_cfg.interrupt==1)  
  isr.start(sqr);
  end
  join
endtask:body

In virtual sequence i’m using a flag,with respective flag i am checking my interrupt, as soon as i get interrupt i need to clear the interrupt using ISR.
here my question is even though i am not using grab and ungrab, i am able to clear the interrupt,Can any one explain me how can i implement and understand grab and ungrab concept.