PDP-11 param.h - address space

[54394gt at hocda.UUCP]

You do not get 56K bytes in the PDP-11 kernel data space.  The bottom segment
is physical address 0, where UNIX boots itself and where traps go; they must
work with or without memory management operating.  In addition, another data
segmentation register points to the user process structure for the currrently
running user process.  At least the kernel borrows trap/interrupt stacks from
the user, so there is no stack segment.  That is 48K counting the bottom seg.
I don't know whether your version of UNIX allows the 'kernel' to use
supervisor as well as kernel addressing modes; ours does; that gives more
address space (I haven't looked closely enough at the source to tell if it
uses both instruction and data space in supervisor mode).