It's amazing that so many embedded processors do this now. Back in the day, you needed to use a special-purpose processor (which had extra pinouts) to do this as part of an in-circuit emulator.

I never had access to the low-level part of embedded systems, although I did my PhD on embedded systems debugging in an embedded system company! what I saw was what was running on the PC and communicating with the JTAG external interface (basically, a `gdbserver` implementation). At that level, that was nothing more than ptrace. Under that, I can't say!