This message often occurs at boot time, sometimes along with a Watchdog Reset error. It comes from the OpenBoot PROM monitor, which was passed a processor trap from the hardware. This error indicates that some program tried to access a register window that was not accessible from the processor.
On some system architectures the problem could be that different capacity memory chips are mixed together. Someone might have placed 1-Mbyte SIMMs in the same bank with 4-Mbyte SIMMs. If this is so, rearrange the memory chips. Make sure to put higher-capacity SIMMs in the first bank(s), and lower-capacity SIMMs in the remaining bank(s); never mix different capacity SIMMs in the same bank.
The problem could also be that cache memory on the motherboard has gone bad and needs replacement. If main memory is installed correctly, try swapping the motherboard.
The best way to isolate the problem is to look at the %pc register to see where it got its arguments, and why the arguments were bad. If you can reproduce the condition causing this message, your system vendor might be able to help diagnose the problem.