The advantage of multiple stacks is that the system stack runs ring 0
protected tasks (main operating system tasks). The user stack runs the
user(s) tasks.
On systems with some kind of memory protection this keeps any non system
task from accessing any other non-system tasks memory and possibly keeps
any individual system task from accessing any other system (or
non-system) tasks memory. On these systems, usually an MMU is involved
and an interrupt or trap instruction will cause the MMU to switch its
memory map to the system stack. Usually all interrupts go through the
OS even if the eventually interrupts handler code is in user memory.
On systems without any kind of protected memory it keeps the user level
tasks from accessing system task memory. On these systems, once the OS
enters "protected mode" only an interrupt (or trapped instruction) can
cause a switch to the system stack.
These statements are very high level and do not discuss the complexity
of operating systems that use system memory and user memory.
