Re: Memory management

From: Dani (daninm78NOSPAM_at_yahoo.es)
Date: 03/05/05 

Date: Sat, 05 Mar 2005 14:37:41 GMT

Thanks for your comments Hans,

The real problem for me is that I would like to use the data cache in the
ARM (I hope performance to be better) but it require the MMU to be enabled
and MMU requires some RAM for working. RAM memory is very limited in the
system so it is not an easy task (we need to assign some RAM for the MMU but
there is not that ammount of free RAM in the system).

I understand caching and memory protection are similar in some points but I
would like to understand if it is necesary to reserve a part of RAM for the
MMU if I don't use virtual adressing nor memory protection. If so, I also
would like to know if this method has any disavantage I cannot see (probably
it has, because it seems to me this is not the usual way of working...)

Thanks for helping this newby guy

Daniel

"Hans-Bernhard Broeker" <broeker@physik.rwth-aachen.de> escribió en el
mensaje news:38tp65F5q8ntcU9@news.dfncis.de...
> Dani <daninm78NOSPAM@yahoo.es> wrote:
>
>> TI's OMAP requires the MMU to be enabled if you want to use the data
>> cache, but I don't understand why... is it required whatever the chip
>> is?
>
> While not all CPUs may require it, it's a sensible design decision.
> Caching is, to some extent, an MMU function --- it's a layer of
> functionality sitting between the CPU core and external memory, and
> it's supposed to be mostly transparent to the running program. If
> both caching and other MMU functions are present, they have to be
> integrated so thoroughly that it would cause major hassle for little
> gain to allow them to be turned on separately.
>
>> It seems to me that MMU manages logical memory addresses and memory
>> protection, but I do not see any relationship between these MMU task
>> and the data cache, probably due to my inexperience...
>
> Consider this: the difference between a memory protection violation
> and a cache miss is not all that large. In both cases, a simple
> memory access may cause extensive extra activity, if the exceptional
> case happens.
>
> --
> Hans-Bernhard Broeker (broeker@physik.rwth-aachen.de)
> Even if all the snow were burnt, ashes would remain.

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