[TUHS] history of virtual address space

[John Levine via TUHS]

It appears that ron minnich via TUHS <rminnich at gmail.com> said:
>I was trying to remember how virtual address space evolved in Unix.
>
>When I joined the game in v6 days, processes had virtual 0->0xffff, with
>the potential for split I/D IIRC, getting you that bit of extra space. The
>kernel also had virtual 0->0xffff, also with split I/D. The kernel
> accessed user data with mov/load from previous address space instructions.
>But also on the 11, trap vectors chewed up a chunk of low physical memory,
>and the diode ROM took out  some chunk of high physical. The bootstrap you
>set from the front panel started with many 1's.
>
>Later, when we had bigger address spaces on 32- bit systems, page 0 was
>made unavailable in users address space to catch NULL pointers. Did that
>happen on the VAX at some  point, or later?

Someone said in 4.2BSD which seems right.  I fixed my share of code that
assumed that *0 == 0

>Later we got the "kernel is high virtual with bit 31 set" -- when did that
>first happen?

That's how VAX addressing worked.  User programs were in the low half
of the address space, the kernel in the high half.