> > Hi,

> >   Can someone explain me the supervisor mode
> > in the 68000, and how it is useful for a multitasking
> > environment or OS ?

> It is to do with protection of resources.
> If you also have an MMU with the chip, then the supervisor an user
> modes give you the following:

> - different tasks can not access each other's memory (MMU)
> - user tasks can not directly access peripherals
> - user tasks can not directly control interrupts
> - user tasks always have to go through the supervisor for any system
>   services

> If you use the chip for an embedded system which runs only code you
> wrote and which users can not modify, then you don't really need all
> that and usually everything runs in supervisor mode (and no MMU). All
> the tasks are yours, so you know that they are not malicious and do not
> try to break the system.

> However, on a system where you can have multiple users (including
> hostile users) or you have to cater for programs from unknown origin
> (including hostile code), then you can use the supervisor mode + MMU to
> protect your system. A user can destroy his/her own tasks, but has no
> access to other tasks and/or shared system resources.

> It should be noted that the 68000's exception model had a flaw which
> made it nearly impossible to realise virtual memory (you could do it
> with two 68000 chips running in sync but a few clocks apart).
> Motorola fixed that in the 68010.

> Regards,

> Zoltan

> --
> Zoltán Kócsi
> Bendor Research Pty. Ltd.