wfjm.w11/doc/simh_diff_unibus_mapping.md

Known differences between SimH, 11/70, and w11a

18-bit UNIBUS address space not mapped into 22-bit address space

The 11/70 maps the 18 bit UNIBUS address space into the upper part of the 22-bit extended mode address space. With UNIBUS mapping enabled, this allows to access via 17000000:17757777 the memory exactly as a UNIBUS device would see it. On an 11/70, an access to non-existing memory via the UNIBUS map will result in a UNIBUS timeout and set the ITO bit CPUERR.

SimH doesn't implement the UNIBUS window, in a 4 MB memory configuration the address range 17000000:17757777 will be normal memory.

Notes:

• The w11a doesn't implement this remapping, an access in the range 17000000:17757777 causes an NXM fault and will set the NXM bit in CPUERR.
• E11 implements the UNIBUS window and from V7.4 on access to main memory via the UNIBUS map. This can be disabled via set cpu nouwin. When used with set memory 3840 an E11 system will be behave like the w11.