Editing IBM 5100 Troubleshooting Guide (section)

== Diagnostic ROS ==

A '''keyboard sequence at power-on''' enters Diagnostic ROS — the operator can read and write RAM, video memory, PALM registers, interrupt vectors and the clock counter in hex, effectively assembly-language access without an operating system.<ref>https://en.wikipedia.org/wiki/IBM_5100</ref>

The Diagnostic ROS is documented in MIM SY31-0405. Once in Diagnostic ROS, the operator can:

* Read PALM registers and verify the processor is alive.
* Read / write RAM cells to check the RWS subsystem.
* Read Language ROS to verify the interpreter is intact.
* Trace PALM microcode execution to isolate a failing instruction class.