[TUHS] Is there a good, even definitive, list of reimplementations of the Unix kernel? What would good cut-off criteria be?

[Luther Johnson via TUHS]

You might consider the xv6 project as a candidate for clean/pedagogical 
re-implementation of V6 UNIX:

https://pdos.csail.mit.edu/6.828/2012/xv6.html

On 04/18/2026 05:32 PM, steve jenkin via TUHS wrote:
> “Imitation is the sincerest form of Flattery”...
>
> I’ve seen mentions on-list of clean reimplementations of Unix, not ‘forks’ - as interesting & important as they have been.
>
> We know that implementing a V7 kernel API was a masters project for Linus,
> taking ~3 years to crank it out, presumably his sole work as a thesis requirement.
>
> For me, the importance of V6 was two-fold:
>
> 	- ken & dmr removed the unnecessary excess, over multiple iterations,
> 		creating the software equivalent of Gordon Bell’s “minimum computer” (eg PDP-11).
> 		“just enough, no more” for a timeshare O/S and software development platform.
>
> 	- they created a “just enough, no more" implementation language in C,
> 		flexible enough to concisely express both kernel & user-land code,
> 		expressive & complete enough to write any systems code,
> 		and produce performant code, able to be instrumented & improved.
>
> Both inventions satisfy the “nothing left to remove” criteria of great Engineering.
>
> That individuals could reimplement the C language or kernel API in a modest time, not just read & understand it,
> shows the level of that original achievement - a definitive O/S for the time [ before networking & graphics terminals ].
>
> The only time (I know of) that the Unix API has been improved on - reduced, simplified & refined:
> 	is "Plan 9” from the same group, which also addressed Security, Networking & Grpahics.
>
> Is the reimplementation question covered by the classic Levenez “Unix Timeline”?
> Not to my eye.
> 	<https://levenez.com/unix/>
>
> 	"This is a simplified diagram of unix history.
> 	There are numerous derivative systems not listed in this chart,
> 		maybe 10 times more! In the recent past, many electronic companies had their own unix releases.
> 	This diagram is only the tip of an iceberg, with a penguin on it ;-)."
>
> Index - single page - lists all covered O/S.
> 	<https://levenez.com/unix/indexunix_a4.pdf>
>
>
> Warren’s page, Timeline of Unix development 1969-1989, is a very clear single page tree of BTL descendants.
> 	<https://www.tuhs.org/unixhist.html>
>
> plus the Graphing Project, BTL descendants
> 	<https://minnie.tuhs.org/Unix_History/index.html>
>
>
> Without Linux, we don’t have a good history of parallel & derivative implementations,
> to show the critical importance of Unix V6 / V7 to the world of computing,
> without which hardware is useless.
>
>
> ‘History’ is infinite, so I’d suggest a time constraint:
>
> 	Either 2000 or 2002, when “Plan 9” was Open Sourced and Inferno was being licensed.
>
>
> I guess I’m thinking of small teams (& single person) and not large commercial efforts, typically forked from BTL,
> which emphasised difference & incompatibility.
>
> There’s been the odd mention of early reimplementations on-list, but I don’t have the Search Skills to extract them.
>
> BSD began as BTL / USG Unix, then diverged with DARPA funding, following a winding path to become free of AT&T copyright material.
> BSD spawned SunOS / Solaris.
>
> AT&T, as well as V7, did PWB/Unix, plus many early variants for different research and production needs, especially real-time for switching.
>
> Research Unix diverged from USL/USG after V7, eventually into Plan 9 and Inferno. It’s well covered in existing Timelines.
>
> Bill Plauger did Idris, standalone and hosted..
>
> Andrew Tanenbaum did Minix, inspiring Linus.
>
> The Mach micro-kernel created its own descendants as a platform to host other O/S, including Darwin / MacOS.
>
> The MKS Toolkit - Mortice Kern Systems of Canada - Unix tools for other platforms, but no kernel…
> I bought it, found it wonderfully helpful, but is it “Unix”?
>   - for users, yes
>   - but not the kernel, it’s not an O/S.
>
> Was “Wind River” and its VxWorks “Unix-like” or something else?
> What about other Real-time O/S and embedded systems?
>
> Does GNU count, as their HURD kernel has never attained the attributes necessary for wide use?
>
> ==============
>
> Then again, what’s even “Unix” in my question?
> There needs to be a selection constraint beyond time.
>
> Even in 1974, every installation modified their kernel, creating many forks and original work,.
>
> Sometimes the work was folded back in, mostly it seems to have been lost, or showed up in other products.
>
> The V7 kernel API, the basis for "Spec 1170 “, then the POSIX standard & now the Single Unix Specification.
> 	<https://opengroupblog.wordpress.com/wp-content/uploads/2016/03/short-version-unix-web.pdf>
>
> MINIX reimplemented the V7 API, creating the model for Linus.
> As the first portable Unix, anything that reimplemented the V7 API is a candidate for my list.
>
> Where does that leave the x86 reimplementation of V6 Unix [ MIT? ], used for teaching?
> That’s definitely a “Heritage Unix” related system.
>
> As would be an ARM V6 variant for teaching. R-Pi’s and knock-offs are prolific and cheap.
>
> What of Unix-like systems who don’t get POSIX Certification?
>
> Home projects, like Linux 0.1, are where interesting systems began,
> by definition, not highly resourced and more interested in the code than ancillary issues.
>
> ==============
> --
> Steve Jenkin, IT Systems and Design
> 0412 786 915 (+61 412 786 915)
> PO Box 38, Kippax ACT 2615, AUSTRALIA
>
> mailto:sjenkin at canb.auug.org.au http://members.tip.net.au/~sjenkin
>
>