Renesas R8C/Tiny Series Software Manual page 285

Q
What is the difference between the user stack pointer (USP) and the interrupt stack pointer (ISP)?
What are their roles?
A
USP is used when using the OS. When several tasks are run, the OS secures stack areas to save
the contents of registers for 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 efficient use
of stack areas.
Q&A-4
CPU