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34 lines
1.7 KiB
Markdown
34 lines
1.7 KiB
Markdown
## Known differences between SimH, 11/70, and w11a
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### SimH: stack limit check and vector push aborts
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In a vector flow, the 11/70 performs an individual stack limit check for the
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PS push and the PC push. Therefore, a vector flow can be aborted on the first
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or the second push, the red zone is never written.
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In case of a stack limit abort during a vector flow, the PS is restored to
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the value at the entry of the vector flow, an emergency stack is set up,
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and a vector 4 flow is started that will save the PS and PC values present
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at the beginning of the aborted vector flow.
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SimH implements a substantially different behavior for all PDP-11 models.
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A stack limit violation is checked at the end of the vector flow, after the
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writes have been performed and one or two values have been potentially written
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into the red zone. An emergency stack is set up and a vector 4 flow is started
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that will save the PS and PC values taken read in beginning of the aborted
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vector flow.
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**Note**: The SimH behavior for vector push aborts caused by an MMU abort is
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different. These aborts are detected before the actual write, and the vector
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flow handling of the abort saves the PS and PC values present at the beginning
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of the initial vector flow. This occurs both for a vector 240 flow when a push
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to a non-kernel stack failed, and a vector 4 flow when a push to kernel stack
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failed and is converted to a fatal stack error. In these cases, SimH
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implements the 11/70 behavior.
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The w11 correctly implements the 11/70 behavior in all cases.
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This is verified in a [tcode](../tools/tcode/README.md), the tests are
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skipped when executed on SimH
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(see [cpu_details](../tools/tcode/cpu_details.mac) tests A3.4 and A3.5).
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Tested with SimH V3.12-3.
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