Renesas R8C/Tiny Series Software Manual page 284

Q
What is the difference between the user stack pointer (USP) and the interrupt stack pointer (ISP)?,
What are their roles?
A
You use USP when using the OS. When several tasks run, the OS secures stack areas to save
registers of individual tasks. Also, stack areas have to be secured, task by task, to be used for
handling interrupts that occur while tasks are being executed. If you use USP and ISP in such an
instance, the stack for interrupts can be shared by these tasks; this allows you to efficiently use
stack areas.
Q&A-4
CPU