When the Debugger Lies With Stale Cache Values

What happened

This raised the question of which cache was doing this. In a recent blog post by [Daniel Mangum] he goes over a scenario observed while debugging the Cortex-M33-based nRF54LM20, reading and writing values while running through a few scenarios.

After initially it seemed to go seemingly without any issues, suddenly the GDB debugger would happily return values that suggested that a previous operation had not succeeded. Or, as the case turned out to be, stale cached values were being returned. What follows is a very technical and low-level breakdown of how this MCU functions inside, especially its cryptographic features and Key Management Unit, which is used for storing sensitive information.