Re: Embedded linux: With or without MMU


Does the lack of the MMU cause many limitations of which systems calls that is possible to use in user software? ucLinux does as far as I know not include the full Linux API?


AFAIK the API calls are quite different with µCLinux, but you never do direct Linux API calls. You need to link your application against one of the µC-aware libraries (instead of gLibC) and thus you will notice no difference in "normal" applications. OK there is no "fork" and you need to use vfork instead that works a bit different, but you supposedly will not use fork a lot anyway in your own code in an embedded device. If you want to do multitasking with your supposedly will use the pthread library.

If you have a safety-critical application an MMU provides a better fall-back behavior of the device in case part of the user software fails.

So another point here might be boot-time. If my application should crash the kernel will keep living and should be able to restart the application without booting the board?

You will need a hardware watchdog, of course, to detect crashing. Supposedly µCLinux will boot faster than full Linux.


Is this "mistake" in design general? How about MIPS and PPC ?

I don't know anything about MIPS. I do suppose that the PPC MMU/Cache system works like that of the X86, as this is originally meant for Desktops as well.


Great info. Why is task switches a problem? Is it because the "cached table entries" must be refreshed when changeing memory space?


If there are no task switches there in fact is no difference in the (single) running application whether it runs in full or µCLinux.

(Besides the potential difference in hardware execution speed) With any task switch the OS has to do some work to reprogram the MMU (and with ARM the cache gets invalidated).

-Michael
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