User timer tick (INT 1Ch)

        This interrupt is called (in the IBM-BIOS) at the end of each
        time-update operation by the real-time clock routines (INT 08h).
        It normally points to an IRET.

        Warning: When this interrupt is called, the stack may contain
                 more than the pushed flags and CS:IP of the inter-
                 rupted program. Therefore this interrupt, can't be
                 used for a task-switcher in a multitasking system.