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What happens if ITCM and DTCM overlap on ARM926EJ-S?

Frédéric Hoguin over 11 years ago

Note: This was originally posted on 23rd January 2012 at http://forums.arm.com

Hello,

I noticed a bug in ITCM and DTCM initialization in some bootloader code.

The result is that the same value is written in the ITCM region register and in the DTCM region register, so both are configured with the same address and size.

It looks like in this case, ITCM gets priority over DTCM since everything is working correctly when executing code stored in this region of memory. If DTCM had priority, fetching an instruction from this region of memory would cause a prefetch abort exception.

Now, that's my understanding of what happens, but I couldn't find anything about it in the ARM926EJ-S documentation.

Can anybody confirm my theory, or explain what happens in detail?

Thanks,

Frédéric.

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Frédéric Hoguin over 11 years ago

Note: This was originally posted on 24th January 2012 at http://forums.arm.com

I finally got my answer. It's there, at the bottom of the page: http://infocenter.ar...e/I1002021.html

The instruction TCM must not be programmed to the same base address as the data TCM. If the two TCMs are of different sizes, the regions in physical memory must not overlap. If they do overlap, it is Unpredictable which memory is accessed.

So it's undefined behavior, and really it is luck that it works.
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Frédéric Hoguin over 11 years ago

Note: This was originally posted on 24th January 2012 at http://forums.arm.com

I finally got my answer. It's there, at the bottom of the page: http://infocenter.ar...e/I1002021.html

The instruction TCM must not be programmed to the same base address as the data TCM. If the two TCMs are of different sizes, the regions in physical memory must not overlap. If they do overlap, it is Unpredictable which memory is accessed.

So it's undefined behavior, and really it is luck that it works.
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