The 11/70 includes location 177776 in the red stack zone. This is not
documented in the Processor Handbooks, only mentioned in the Technical
Manual. It was added to protect the PSW in case a further stack push
is done after an emergency stack was set up, the vector flow of a fatal
stack errors had concluded, and the handler does a stack push when SP
is still 0.
SimH doesn't support this behavior.
w11 implements the PSW protection. This is verified in a
tcode, the test is skipped when executed on SimH
(see cpu_details.mac test A3.2).
Tested with SimH V3.12-3.