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wfjm.w11/doc/w11a_diff_simh_mmr1_rts_mtp.md

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Known differences between w11a and a SimH 11/70

SimH: implicit stack pops not recorded in MMR1

The MMU abort behavior for instructions with implicit stack pops (RTS, MTPI, MTPD) differs on SimH from w11 and a real 11/70. SimH updates the stack pointer after the stack value has been read. If this read is aborted by the MMU, the state is SP unchanged and MMR1 zero. w11 and a real 11/70 update SP and record that in MMR1 before the stack value is accessed and an MMU abort detected. In both cases the register change state and the MMR1 state are consistent, so MMU vector 250 handlers will work correctly.

This difference is only detected in test codes.