## Known differences between w11a and KB11-C (11/70)

### 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.

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.

Notes:
- on a 11/70 an access 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 conficuration
  the address range 17000000:17757777 will be normal memory.
- 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.
