Lots of things become possible once one is willing to decap ICs to get at the internals. I'd expect security consious parts (ie: all of the theoretical "loader

Lots of things become possible once one is willing to decap ICs to get at the internals. I'd expect security consious parts (ie: all of the theoretical "loader A") would need to run in SRAM (ie: ram that is in the same IC and thus harder to get at than external DRAM chips) or some other mechanism. At that point, it becomes a question of physical hardening within the ICs. Some manufacturers have done things like put metal layers over fuses (to prevent them from being changed), I'd imagine the same could be done (at some cost) for a larger area of the chip. I'd imagine HSMs (hardware security modules) and TPMs (though these aren't as good) probably implement some of that. There also exist some chips targeted towards security purposes (not aware of any processors off hand) that could be used .