Booting an IBM MP 3000 S/390 System

[Chuck Guzis]

Another reason for an attached service processor is to handle twiddling 
of things that the main machine shouldn't have access to.  Reconfiguring 
memory, adjusting operating margins, monitoring water temperature, 
getting hold of detailed status information, etc.  In at least one case, 
I'm aware of a machine where the MCU could reconfigure memory so that 
the CPU could continue running while the MCU ran diagnostics on the 
invisible (to the CPU) memory.  Other than somewhat degraded memory 
size, the CPU was none the wiser.

Some CDC MCUs employed a drum for its own program as well as for storing 
microcode--and had its own (separate) display console.

--Chuck