[jan-jaap]

Well POD IS POWER ON DIAGNOSTICS...the CPU hasn't started yet.  Am I wrong?  The L1 doesn't use the Main MIPS CPU, so change in POD are real as POD is before boot, yes?  So is there really a problem here?  Aren't we already before CPU boot?

Yeah this is a great question; I do wonder what this is all running on!  What does the L1 run on?  There must be a microcontroller in there somewhere.  PROM code is definitely MIPS, so is running on the CPU.  L1, POD, etc.. is POD part of the L1 code? Trying to disassemble L1 and I can't even figure out if it's mips, arm, (powerpc, sh4..), nothing seems to fit!  My guess would have been ARM (SGI has used ARM microprocessors elsewhere, e.g. the IR gbricks), but doesn't look like it.

The L1 is a small embedded CPU. Don't know about the architecture.

POD is indeed Power-On-Diagnostics. It's the code executed by the main MIPS CPU to test functionality of the system. It's a subset of the diagnostics that takes little enough time that you can run it every time the system is powered on. The most basic  routines even run without RAM because they run entirely from ROM and only access registers.