The PSP is slightly different though as it not remotely accessible. It does sit in a similar position to the ME (can interact with the computer outside of the main x86 cores) but yes not listen on a external interface. Interestingly enough the PSP actually uses the ARM TrustZone extensions to implement some of its security.
AMD's DMTF DASH is absolutely remotely accessible, that is its main purpose. See, for example, https://developer.amd.com/tools-for-dmtf-dash/ (AMD's developer site is currently down, but there's an archived copy at http://archive.is/HHEwh) which states "DASH is a web services based standard for secure out-of-band and remote management of desktops and mobile systems. Client systems that support out-of-band management help IT administrators perform tasks independent of the power state of the machine or the state of the operating system. Examples of out-of-band management tasks include: 1) Securely starting up a system remotely, even if it is currently powered off; 2) Viewing asset inventory information for a system that is powered off; 3) Retrieving health information about system components even if the OS is unavailable." All of these things require remote access.
DMTF DASH does look to be remote out of band management of AMD computers but I cannot find a connection to the PSP itself. I suspect it may run on the PSP but nothing I have seen so far has shown that the PSP is externally accessible or implements DMTF DASH functionality.
AMD's implementation of DMTF DASH is called SimFire, and Intel's is called AMT. They both implement the same standard. I don't see where else it run if not on the PSP if it works even when the machine is (supposedly) switched off? And even if it were not to run on the PSP, does that matter? It's still an opaque binary that runs below your OS.
its actually more interesting imo if it _doesn't_ run on the PSP; because that means that there is another processor that runs at a lower level than the x86 cores that can interact with the host and listens externally.
After doing a bit of reading DMTF DASH appears to be implemented in the NIC from Broadcom or Realtek and uses the PCIE interface with the chipset from the NIC.
I would love for me to tell you that you were joking, but this sounds just crazy enough to be probably true. A cursory search confirms that the NIC seems to have a rather active role in DASH implementations:
"To support out-of-band management, a client system typically requires a Network Controller that supports out-of-band management tasks, such as the Broadcom 5761." https://www.amd.com/system/files/documents/out-of-band-clien... (out-of-band management is the "it works while the machine is supposedly off" stuff)
But it also needs to talk to the system BIOS / UEFI, so I guess something else needs to be running as well? "DASH functionality requires a communication channel between the hardware platform features, system BIOS, and the Broadcom NetXtreme™ Gigabit Ethernet Controller." (http://h10032.www1.hp.com/ctg/Manual/c01944865). I mean, the PCIe interface is useless if the other end is actually off (and not just supposedly off).
