wfjm.w11/doc/w11a_diff_70_ser_forced_fetch.md

Known differences between w11a and KB11-C (11/70)

No unconditional instruction fetch after stack error abort

The 11/70 suppresses the recognition of break requests at the end of a vector flow of a stack error abort. The processor manual states

6.2.2.3 Timing of Stack Error Aborts
BRQ STROBE is thus inhibited, not only during ZAP.00, but also during
SVC.90, thus guaranteeing the execution of the first instruction of the
error subroutine before any other error can be processed.

The rationale behind was apparently to ensure that the handler, which is called with SP=0 pointing to an emergency stack, can set up a new stack before any other condition interferes.

The w11 does not implement this behavior, nor does SimH. Beyond that, it would be very difficult to verify this extreme corner case.