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57 lines
2.9 KiB
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57 lines
2.9 KiB
Markdown
# Summary of known differences and limitations for w11a CPU and systems
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This file descibes differences and limitations of the w11 CPU and systems.
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The issues of the w11 CPU and systems are listed in a separate document
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[README_known_issues.md](README_known_issues.md).
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### Table of content
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- [Known differences between w11a and KB11-C (11/70)](#user-content-diff)
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- [Known limitations](#user-content-lim)
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### <a id="diff">Known differences between w11a and KB11-C (11/70)</a>
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- the `SPL` instruction in the 11/70 always fetched the next instruction
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regardless of pending device or even console interrupts. This is known
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as the infamous _spl bug_, see
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- https://minnie.tuhs.org/pipermail/tuhs/2006-September/002692.html
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- https://minnie.tuhs.org/pipermail/tuhs/2006-October/002693.html
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- https://minnie.tuhs.org/pipermail/tuhs/2006-October/002694.html
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- https://minnie.tuhs.org/pipermail/tuhs/2006-October/002695.html
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- https://minnie.tuhs.org/pipermail/tuhs/2006-October/002701.html
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- https://minnie.tuhs.org/pipermail/tuhs/2006-October/002695.html
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- https://minnie.tuhs.org/pipermail/tuhs/2006-October/002702.html
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In the w11a the `SPL` has 11/70 semantics in kernel mode, thus next no
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traps or interrupts, but in supervisor and user mode `SPL` really acts as
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`NOOP`, so traps and interrupts are taken as for all other instructions.
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**--> The w11a isn't bug compatible with the 11/70.**
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- A 'red stack violation' looses PSW, a 0 is pushed in stack.
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- The 'instruction complete flag' in `SSR0` is not implemented, it is
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permanently '0', `SSR2` will not record vector addresses in case of a
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vector fetch fault. Recovery of vector fetch faults is therefore not
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possible, but only 11/45 and 11/70 supported this, no OS used that, and
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it's even unclear whether it can be practically used.
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- the 11/70 maps the 18 bit UNIBUS address space into the upper part of
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the 22bit extended mode address space. With UNIBUS mapping enabled, this
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allowed to access via 17000000:17757777 the memory exactly as a UNIBUS
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device would see it. The w11a doesn't implement this remapping, an access
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in the range 17000000:17757777 causes a NXM fault.
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All four points relate to very 11/70 specific behaviour, no operating system
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depends on them, therefore they are considered acceptable implementation
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differences.
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### <a id="lim">Known limitations</a>
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- some programs use timing loops based on the execution speed of the
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original processors. This can lead to spurious timeouts, especially
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in old test programs.
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**--> a 'CPU throttle mechanism' will be added in a future version to
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circumvent this for some old test codes.**
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- the emulated I/O can lead to apparently slow device reaction times,
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especially when the server runs as normal user process. This can lead
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to timeout, again mostly in test programs.
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**--> a 'watch dog' mechanism will be added in a future version which
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suspends the CPU when the server doesn't respond fast enough.**
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