the UCOS_II'question ?

why I can't find the the OSTickISR() in the UCOS_II'source code in NIOS_II?

Actually,the OS can work correctly ,but I don't find the OSTickISR() in the os_cpu_a.s file. The clock tick certainly come from the cpu' timer.But why does the OS not have the OSTickISR() ? How does it work? And who does call the OSTimeTick() function?

I can not find the answer,please help me!