void trap (tf_t *tf)
{
    // TODO
    uctx_pool[get_curid()] = *tf;
    set_pdir_base(0);

    if(tf->trapno >= T_DIVIDE && tf->trapno <= T_SECEV)
    {
      exception_handler();
    }

    else if(tf->trapno >= T_IRQ0 + IRQ_TIMER && tf->trapno <= T_IRQ0 + IRQ_IDE2)
    {
      interrupt_handler();
    }
    else if(tf->trapno == T_SYSCALL)
    {
      syscall_dispatch();
    }
      // Trap handled: call proc_start_user() to initiate return from trap.
    proc_start_user ();
}