wfjm.w11/doc/w11a_diff_70_red_stack_abort.md

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

'fatal stack errors' lose PSW, a 0 is pushed onto the stack

The 11/70, together with the 11/45, has the most elaborate stack protection system of all PDP-11 models. A stack push via kernel stack is aborted when the stack pointer is in the 'red zone' 16 words below the stack limit.

This is considered a 'fatal stack error', other cases are aborts due to odd address, non-existing memory or MMU aborts. In case of a 'fatal stack error' an emergency stack is set up, SP is set to 4, and PSW and PC are stored.

The w11a loses the PSW, a 0 is pushed.

'fatal stack errors' are never recovered, all OS treat them as fatal errors. This difference is therefore considered an acceptable implementation difference.