MMU

[Jake Janovetz]

> Charles-Antoine Gauthier wrote:
> 
> > RTEMS is meant for hard real-time systems. Such systems do not do demand
> > paging because of the phenomenally high cost of fetching pages from disk
> > (relative to CPU speeds). Virtual memory may still be used. In some BSPs
> > with caches, the MMU is usually set up "to stay out of the way" as Joel
> > puts it, but it must be programmed to control the cache. Either some
> > form of block address translation is used, or page descriptors must be
> > set up.
> >
> > One could also imagine setting up virtual addressing to build a flat
> > address space multiprocessor where each board uses the same fixed
> > physical addressing, as is common with many VME cards (IMHO usually due
> > to brain-damaged CPU designs).
> 
> There are, of course, other cases in which an MMU might prove useful. My
> current project is a prototype of a certain "secure" networking device.
> 
> While we have >some< faith that the hardware we will use will work
> correctly, we have much less faith that the software will do so, if only
> due to its (required) complexity. An MMU might be useful in limiting damage
> that might be caused by faulty software.
> 
> One design rule we must obey is not to mix cipher- and clear-text in the
> same memory area. (Rule #1: Though shalt not leak clear-text.) If we had an
> MMU (and we don't), we might be able to disable accesses by threads
> designed to work only with clear-text to areas of memory allocated to
> contain only cipher-text, and vice versa. The best we can do now is to
> perform deign and code reviews and walk-throughs which, in the end, comes
> down to little more than waving our hands.

Isn't Java an appealing alternative in such cases?  The VM performs
the protection by nature of the Java execution model.  If the VM can
be written to not depend on an MMU, then the Java programs don't 
depend on an MMU, but are restricted to their own memory space by
the VM.

Then, as long as the VM is written well, it doesn't matter how well the
Java programs are written -- they just _can't_ intrude.


    Cheers,
    Jake

-- 
   janovetz at uiuc.edu    | How can it be that mathematics, being after all a 
 University of Illinois | product of human thought independent of experience,
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