The OSTickISR'question?

hi friends!

why I can't find the the OSTickISR in the UCOS_II'source codes in NIOS2?

Actually,the OS can work correctly ,but I don't find the OSTickISR() in the os_cpu_a.s file.

and I can not find the initialize and tick interrupts in the UCOS_II' software examples?

where does the OS'Time tick come from?

please help me,thanks!