Re: Embedded linux: With or without MMU

Xenu The Enturbulator wrote:
Michael Schnell wrote:

(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).

Are you sure about this ? If it were true, the performance would completely suck. Remember, ARM was originally designed for desktop use as well, AFAIK .. wasn't it used in those Acorn Archimedes machines ?

The first ARM was for the Acorn Archimedes machines, but it did not have an MMU, and I'm not sure that it even had a cache (it ran at 8 MHz IIRC, and in those days memory was not much slower than cpus).

There are two ways to handle cache and MMU - you can cache by physical address (which causes slower access to the cached data, as addresses need to be translated before accessing the cache), or you can cache by virtual address (which is faster for the cpu to access as the logical addresses are used directly, but it requires a cache flush when changing the MMU maps).

I don't know which method the ARM uses. I've a vague feeling that on larger processors, L1 caches use virtual addresses while L2 (and L3) use physical addresses, but that could be wrong.
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