> On Feb 20, 2025, at 5:04 PM, Mike Katz via cctalk <cctalk@classiccmp.org>
> wrote:
>
> 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.
That reminds me that the PDP-11 has the ability to send interrupts to modes
other than kernel mode, because the new processor status (which includes the
mode) is part of the interrupt vector. I doubt that has ever been done,
though. One complication is that an interrupt into user mode that interrupts
kernel mode code would be a dead end, because the RTI instruction can't restore
the state then: in user mode it can't restore a PSW value that says kernel
mode. So a user mode vector would work only if the kernel is not interruptible
at all.
paul
