[TUHS] PG3 or Gen3.0?

[Dan Cross via TUHS]

On Fri, May 8, 2026 at 2:17 PM segaloco via TUHS <tuhs at tuhs.org> wrote:
> Granted this is now getting quite far afield from the original subject.  To bring it back around, the modern pedagogical V6 port xv6 runs on RISC-V now, so that means there is also hope for bootstrapping other historic UNIX systems (like PG3) on RISC-V.

Yeah, as Warner pointed out, this isn't quite accurate.

xv6 isn't a port in any meaningful way, and in particular it shares no
code with 6th Ed. Rather, it is an independent reimplementation along
similar design lines. Even under that very loose constraint, however,
much is different.

Some things were added to be there from the start: xv6 protects kernel
data structures with fine-grained locks, and has supported SMP
machines from inception.  Additionally, virtual memory segments are
constructed on top of a page-based virtual address space.

Some were removed: xv6 doesn't really support a notion of "users" or
terminal devices, file permissions really exist (partly as a
consequence of not having users), and a number of system calls were
deliberately not added.

These differences are not gratuitous, but rather pragmatic: it's a
pedagogical system, so the selection is primarily based on educational
relevance. Some things (a few system calls, etc) are left as exercises
for students.

        - Dan C.