[TUHS] Unix use of VAX protection modes

[Warner Losh via TUHS]

On Tue, Jan 20, 2026 at 12:48 PM Paul Winalski via TUHS <tuhs at tuhs.org>
wrote:

> The discussion of the history of virtual addressing in Unix called to my
> mind a related question:
>
> The VAX has four protection modes, from lowest to highest in privilege:
> user, supervisor, executive, kernel.  Access to certain instructions and
> hardware data structures can only be done from kernel mode.  User mode is
> of course intended for use by non-privileged code and data.  The VAX/VMS
> Digital Command Language (DCL) command interpreter ran in supervisor mode.
> Record Management Services (RMS, the primary way to access disk files) ran
> in executive mode.  The kernel, device drivers, hardware interrupt
> handlers, etc. ran in kernel mode.
>
> Obviously Unix user-mode process code and data ran in user mode.  Likewise
> the parts of the kernel that had to access privileged bits of the hardware
> (such as device drivers) must have run in kernel mode.
>
> My question is, did Unix make any use of either supervisor or executive
> mode on the VAX?
>

My 1987 OS class said it only used user and kernel mode.

Looking at the 4.3BSD kernel, it either sets both priv bits, or clears both
the bits.
I don't see where it would be using supervisor or executive mode, but this
was a super
fast grep for PSL manipulation

Warner