Memory Protection without MMU

Question

I would like to know how memory can be protected without MMU support. I have tried to google it, but have not seen any worthwile papers or research on it. And those which deal with it only deals it for bugs, such as uninitialized pointers and not memory corruption due to a soft error, that is, due to a hardware transient fault corrupting an instruction that writes to a memory location.

The reason I want to know this is because I am working on a proprietary manycore platform without any Memory Protection. Now my question is, can software be used to protect memory, especially for wild writes due to soft erros (as opposed to mistakes by a programmer). Any help on this would be really appreciated.

Answer 1

If you're looking for Runtime memory protection the sane only option is hardware support. Hardware is the only way to intervene in a bad memory access before it can cause damage. Any software solution would be vulnerable to the very memory errors it is trying to protect against.

With software you could possibly implement a verification/detection scheme. You could periodically check portions of memory that the currently running program should not have access and see if they have changed (probably by CRCing these areas). But of course if the rogue program damages the area where the checksums are held, or where the checking program's code is held, then all bets are off.

Even this software checking solution would be more of a debugging utility than a permanent runtime protection. It is likely that a device with no MMU is a small embedded device which won't have the spare cycles to be constantly checking the device's memory.

Usually devices without MMUs are designed to run a single program with no kernel or anything else, and thus there is nothing to protect. If you need to run multiple programs and feel you need protection, you probably need a more advanced piece of hardware that supports the kind of features you're looking for.

Answer 2

If you want software implemented memory protection, then you will need support from your compiler and its associated libraries. I expect that there is one compiler only on this platform and so you should contact the vendor. I wouldn't hold out much hope for a positive response. Even if they had such tools, I would expect the performance of software memory protection to be unacceptable.