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15 lines
682 B
Markdown
15 lines
682 B
Markdown
## Known differences between w11a and a SimH 11/70
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### SimH: implicit stack pops not recorded in MMR1
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The MMU abort behavior for instructions with implicit stack pops
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(`RTS`, `MTPI`, `MTPD`) differs on SimH from w11 and a real 11/70.
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SimH updates the stack pointer _after_ the stack value has been
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read. If this read is aborted by the MMU, the state is `SP` unchanged
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and `MMR1` zero. w11 and a real 11/70 update `SP` and record that in
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`MMR1` before the stack value is accessed and an MMU abort detected.
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In both cases the register change state and the `MMR1` state
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are consistent, so MMU vector 250 handlers will work correctly.
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This difference is only detected in test codes.
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