[TUHS] Did System V Really Prevent 5BSD?

[Paul Winalski via TUHS]

On Sun, Dec 28, 2025 at 9:53 PM Jon Forrest via TUHS <tuhs at tuhs.org> wrote:

> In the early 1980s David Kashtan at SRI wrote Eunics, which was a way to
> run Unix (I don't recall which version) on top of VMS. It was an
> emulation layer, not a virtual machine. (I'm not aware of any VMs that
> ran on top of VMS).
>

It is not possible to implement a virtual machine hypervisor on the
original VAX architecture.  The reason is the PROBER and PROBEW
instructions.  These allow one to test a memory address for read
accessibility (PROBER) or write accessibility (PROBEW).  The problem is
that they are non-privileged instructions.  To implement a virtual machine
the hypervisor must be able to trap both of these instructions and to "lie"
to the virtual machine about the accessibility of the address.

-Paul W.