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Hi,

the issue with not having (1), i.e. page tables via MMU, is threefold. 
First, one obviously loses memory protection. With well-written and 
reviewed code, this should not be impossible to overcome (to sufficient 
degree, depending on your application).

Second, all application code you run must be (statically linked, which 
I'll ignore as impractical for most purposes, or) relocatable - because 
the different instances cannot obviously see the same start address 
when memory is not mappable. Regular unix binaries by default aren't 
relocatable, but .so's are. Theoretically a single address space 
solution could be devised with proper compilation flags, and 
potentially modifications to the program loader. You can simulate this 
by writing a process which loads executable code dynamically from 
shared object files - into a single shared address space.

The third, and a much more complex issue, is the in-kernel memory 
management, such as allocating (and managing) the buffer cache memory. 
All such code relying on page mapping capabilities will need to be 
reimplemented for an MMU-less environment.

Cheers,
Jussi

On Oct 19, 2004, at 20:50, Brian Rose wrote:

> My understanding is that the MMU serves two functions.
> (1) To provide the system processes with their own view of memory, 
> such that each process feels that it has access to the entire address 
> space.
>
> (2) To provide an apparant memory greater than the installed physical 
> RAM.
>
> For embedded devices, (2) could be solved by not making a swap 
> partition and essentially setting it to 0 (like a boot floppy image), 
> correct? As for (1), is there any way to work around this? My guess is 
> that emulating an MMU in software would eat up so much of the 
> processor's time, that performance would be killed (if it is possible 
> at all).
>
> If (1) is discarded is it possible to still have multiple threads in 
> the single process address space?
- --
Juha-Matti Liukkonen - Systems Architect, Cubical Solutions Ltd
tel: +358-40-5280142
e-mail: jml@cubical.fi

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