To quote from Jon’s post: I’d like to add the constraint of things that would have been implementable on the hardware of the late 1970’s, let’s say a PDP11/70 with Datakit or 3Mbps Ethernet or Arpanet; maybe also Apple 2 class bitmap graphics. And quote some other posts: I would argue that quite a bit of that was implementable as early as 6th Edition. I was researching that very topic last Spring [1] and back ported Peter Weinberger’s File System Switch (FSS) from 8th to 6th Edition; the switch itself bloats the kernel by about half a kilobyte. I think it may be one of the few imaginable extensions that do not dilute the incredible bang/buck ratio of the V6 kernel. With that change in place a lot of other things become possible: - a Kilian style procfs - a Weinberger style network FS - a text/file based ioctl - a clean approach to named pipes - a different starting point to sockets Each of these would add to kernel size of course, hence I’m thinking about a split I/D kernel. To some extent it is surprising that the FSS did not happen around 1975, as many ideas around it were 'in the air' at the time (Heinz Lycklama’s peripheral Unix, the Spider network Filestore, Rand ports, Arpanet Unix, etc). With the benefit of hindsight, it isn’t a great code leap from the cdev switch to the FSS - but probably the ex ante conceptual leap was just too big at the time. Paul [1] Code diffs here: https://1587660.websites.xs4all.nl/cgi-bin/9995/vdiff?from=fab15b88a6a0f36b…