Editing IBM PALM processor (section)

== Process Check and Diagnostic Features ==

The front-panel '''PROCESS CHECK''' indicator lights when:

* A '''storage parity error''' is detected (the 2 parity bits on the R/W bus do not match the data).
* An '''invalid X/Y device address''' pattern is presented on the I/O bus (more than one X line active, or more than one Y line, or none).
* Certain illegal opcodes are decoded.

By default the processor halts on Process Check. A jumper on the backplane can be removed to disable the halt for diagnostic purposes (so the processor continues running with parity errors logged but not fatal).

The front-panel '''"Display Registers / RAM Hex"''' switch on the 5100 shows the first 512 bytes of RWS — which is the register-bank memory-mapped window — live in hex on the 5-inch CRT. This is the field engineer's primary diagnostic tool when the system cannot reach the BASIC / APL banner.

A '''Diagnostic ROS''' mode is entered via a keyboard sequence at power-on; in this mode the operator can read and write RAM, video memory, PALM registers, interrupt vectors and the clock counter in hex — effectively assembly-language access to PALM without an operating system.