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21 lines
994 B
Markdown
21 lines
994 B
Markdown
## Known differences between SimH, 11/70, and w11a
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### 18-bit UNIBUS address space not mapped into 22-bit address space
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The 11/70 maps the 18 bit UNIBUS address space into the upper part of
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the 22-bit extended mode address space. With UNIBUS mapping enabled, this
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allows to access via 17000000:17757777 the memory exactly as a UNIBUS
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device would see it.
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On an 11/70, an access to non-existing memory via the UNIBUS map will result
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in a UNIBUS timeout and set the `ITO` bit `CPUERR`.
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SimH doesn't implement the UNIBUS window, in a 4 MB memory configuration
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the address range 17000000:17757777 will be normal memory.
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Notes:
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- The w11a doesn't implement this remapping, an access in the range
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17000000:17757777 causes an `NXM` fault and will set the `NXM` bit in `CPUERR`.
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- E11 implements the UNIBUS window and from V7.4 on access to main memory
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via the UNIBUS map. This can be disabled via `set cpu nouwin`. When used
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with `set memory 3840` an E11 system will be behave like the w11.
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