ZAP is effectively reset for the micro-architecture, forcing the ucode address to
known/initial value. It has multiple sources throughout the processor, including tendrils
into some of trap handling hardware. (Caveat: my experience is based off extensive work
with the '11/45, but the micro-architecture as I understand it for the '11/70 is
For the '45, there was a very handy "KB11-A,D Maintenance Manual", which
explained the logic of such internal signals and the board by board internal operation of
the CPU to a very useful level of detail; I'm sure similar is available for the
KB11-B,C? It's worth a read through if you haven't already, though its quite a
bit to take in.
I would imagine the next step would be to throw the RAC board out on extenders, verify
that ZAP is asserted, and if so pursue the driving source.
Do you know if you have a KB11-B or C?