[TUHS] history of virtual address space

[John R Levine via TUHS]

On Tue, 20 Jan 2026, Paul Winalski wrote:
> The Memory Management chapter is describing the memory layout of a VAX/VMS
> process.  If you look at the hardware architecture side of things, you will
> see that there is absolutely no requirement or constraint at the hardware
> level to have all of the virtual addresses with the high bit on be reserved
> to the OS and those with the high bit off to user space.  Each page table
> entry has its own protection bits that describe the read and write
> permissions for each execution mode (user/supervisor/executive/kernel).

Did you miss the bit about the S page tables being in real memory and the 
P0 and P1 in S memory, with P1 growing down from 2GB?  Really, the 
hardware dictates the layout.  You might want to find a copy of the 
handbook and refresh your memory.

> One could have, if one wanted, done the opposite and put user address space
> in the address range with the high bit on and the kernel address space in
> the low part of the address space.  It would be fully conformant with the
> VAX page table layout and supported by all then-extant (11/780/750/730) and
> future VAXen.

Nope.  Please reead the handbook.

Regards,
John Levine, johnl at taugh.com, Taughannock Networks, Trumansburg NY
Please consider the environment before reading this e-mail. https://jl.ly