Why my variable become to zero??

hi all

This is a piece of my coce,it complier by large model

In main.c

uchar volatile xdata *data ptr_reg;
unsigned char pdata g_size;

void dma_task_execute (void) small
{
  size =  g_size;
  do_something();
}

void main(void)
{
 while (1)
{
EA = 0;
dma_task_execute();
EA = 1;
}

}

In Int.c

extern unsigned char volatile xdata *data ptr_reg;

extern unsigned char pdata g_size;


void int_handler(void) interrupt 2

{
  EA = 0;
  g_size = ptr_reg[X_addr];

  EA = 1;

}

I found the data store into g_size in int_handler function is correct,but why g_size will become to zero when run dma_task_execute() sometimes ?? The vlaue of g_size is never will be zero,but why it will become to zero in dma_task_execute() sometimes?? Sorry my poor english any comment is welcome Best regards