Direct Memory Access with JTAG in Trust Zone

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I'd like to know what prevents me from accessing memory in a DMA fashion, in the Trust Zone, when using JTAG?

Thanks in advance.

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jmg On BEST ANSWER

TrustZone is a kind of secure world inside an ARM processor. Untrusted software should not be able to even observe things that run in TrustZone.

Protection is done by hardware. ARM provides for debug the CoreSight architecture. You can download after registration the CoreSight Architecture Manual from the ARM website.

This article on arm infocenter introduces the SPIDEN and SPNIDEN signals that enable secure invasive and secure non-invasive debug.

How these signals are enabled depends on the implementation of the ASIC.