Re: Privilege Levels Below User

On Sat, 8 Jun 2024 17:37:46 +0000, MitchAlsup1 wrote:

VAX was before common era Hypervisors, do you think VAX could have
supported secure mode and hypervisor with their 4 levels ??

“Virtualization” was bandied about in the 1980s more as an idle,
theoretical concept rather than a practical one.

The question was: was the instruction set defined so that code that was
designed to run in a privileged mode be run unprivileged, so that any
attempt to do privileged things would be trapped and emulated by the real
privileged code? And there was nothing it could do to discover it wasn’t
running in privileged mode?

(Obviously performance was not the issue here, but correctness was.)

For example, the VAX had a MOVPSL instruction that allowed read-only
access to the entire processor status register. Through this,
nonprivileged user-mode code could discover it was running in user mode,
which would blow the illusion.

The Motorola 680x0 family was I think properly virtualizable in this
sense. Or maybe the 68020 and 68030 were, but the 68040 was. I think the
Motorola engineers working on the ’040 asked if any customers were
interested in preserving the self-virtualization feature, and nobody
seemed to care.