Re: For the AdaOS folks

["Warren W. Gay VE3WWG" <ve3wwg@NoSPAM.cogeco.ca>]

Warren W. Gay VE3WWG wrote:
> Marven Lee wrote:
> 
>> Warren W. Gay VE3WWG wrote:
>>
>>> Dmitry A. Kazakov wrote:
>>>
>>>> Warren W. Gay VE3WWG wrote:
>>>>
>>>>> The problem then is that the drivers add to the code in "real-mode",
>>>>> which is one thing that the mk tries to avoid (for additional safety
>>>>> and modularity).
>>>>
>>>>
>>>> I am still not sure what you mean here. If you mean modular kernel
>>>> design and more fine-grained system of modes than all-or-nothing,
>>>> then yes, sure. There is no reason, why a driver should not be
>>>> treated as an application.
>>>
>>>
>>> That's right. Just remember that the purpose of the mk is to keep the
>>> "priviledged-mode" code to a bare minimum (hence the "micro").
>>
>>
>> Some microkernels and single address space operating systems
>> implement cross-domain call mechanisms that enables a thread
>> to cross from one protection domain into another in a similar
>> way as a segmented system performs far calls or interrupts
>> and traps can transfer from user-mode to kernel-mode.
>>
>> In Mach it's called "migrating threads".
> 
> 
> Yes, I've seen references to this in the Mach
> literature.
> 
>> The Spring Nucleus (and Solaris) has cross-domain calls through "Doors".
>> In the Pebble Operating System it's called "Portal Traversal"
>> Mungi has "Protection Domain eXtensions".
>> I think the Grasshopper kernel has something similar.
>> Maybe LPC in Windows is a form of cross-domain call, I'm not sure.
>>
>> You can end up with a microkernel that only handles cross-domain
>> calls.  Everything else, including what you normally think
>> a microkernel should at least include,
> 
> 
> I'm glad you said "normally" here. ;-)
> 
>> such as memory management,
>> scheduling and IPC can be implemented outside of the microkernel.
>>
>> Marv 
> 
> 
> Exokernel?
> 
>   http://www.pdos.lcs.mit.edu/exo.html
> 
> I've never been too keen on the single address space systems. But
> I did like the way the ring mechanism worked in PrimOS (not a mk),
> which was probably borrowed from Multics. I liked the way the
> links were "snapped" and the way that you could call into different
> levels of protection.
> 
> Cross domain threads is an interesting idea though. I'll check it
> out.

For those interested, a Mach paper on migrating threads is
available at:

http://www.usenix.org/publications/library/proceedings/sf94/full_papers/ford.pdf

In the Mach case, it is only a modification of the thread
model (semantics are changed somewhat). A major benefit was
the speedup it offers RPC calls. However the overall microkernel
structure does not change (you still have priv-mode code in mk,
and user-mode code is outside).

-- 
Warren W. Gay VE3WWG
http://home.cogeco.ca/~ve3wwg