[TUHS] history of virtual address space

[Phil Budne via TUHS]

The "feature" of having user and system use non-overlapping virtual
address ranges makes reading user data (eg system call arguments) easy (*)

The VAX reserving a full half of the address space for the system
software has always struck me of typical of the mindset of
kernel/monitor developers thru the ages: Cycles expended outside of
the system state (ie; by users) are wasted(**)!!

(*) PDP-10's with paging systems all implemented special a "previous
context" flavor of the XCT (execute) instruction for moving data
between system and user spaces (systems with more physical memory than
the full/original 256K word 18-bit virtual address space were common
from the start.

(**) I first noticed it at DEC in the 80's; TOPS-20 monitor developers
hating on EMACS users wasting time by context switching on each
keystroke.