2.12 “i” and “d” Spaces

In the PDP11/45 and 11/70 systems, there are additional sets of segmentation registers. Addresses created using the pc register (r7) are said to belong to “i” space, and are translated by a different set of segmentation registers from those used for the remaining addresses which are said to belong to “d” space.

The advantage of this arrangement is that both “i” and “d” spaces may occupy up to 32K words, thus allowing the maximum space which can be allocated to a program to be increased to twice the space available on the PDP11/40.