I quite agree - the PDP 11/70 was quite a high end 16 bit machine, but it was the machine that I was talking about and the one I would most like to revisit (although I wouldn't turn down an 11/40 if somebody offered me a working one). I don't think I would contemplate putting Plan9 on a machine with no MMU or a 64K physical memory limit.
My first reasonable multi-user, multi-tasking computer system (back in the early 80s) was home made 6809 machine with 6829 MMU and eventually 1MB of ram, running OS-9/6809. It initially ran with 64K for programs and and the rest of memory was a big ram disk - because what else could you do with such a ridiculous amount of memory. It did pretty well at providing a personal Unix like environment, although counldn't reproduce the fork() semantics and there was no memory protection, and the memory contraints meant always running the C compiler one pass at a time.. But we eventually ported 'Level 2' OS-9 which could use a mapping ram/MMU, and with that I had a quite robust multi-user system, with up to 64K available per process, and 64K available for the kernel. I was able to get most Unix programs running on it (except for a few with big tables that compiled to larger than 64K) and no longer had to worry about exiting the editor before doing a compile. Most of the core system utilities were written in assembly language - so the equivalent of 'ls' for example, required no more than a 256 byte memory allocation. And all executables were loaded read-only and re-entrant (shared text) which helped. The only real Achilles heal was the 6809 had no illegal instruction trapping, so executing data could occasionally result in an unrecoverable freeze.. I never liked the 68K version os OS-9 quite as much. Because of the larger address space it used the MMU for protection only, with no address translation - so the kernel was mapped into the same address space as the user programs but just not accessible in user mode. It just didn't seem as elegant. Anyway, thats why I don't see 64K per process as necessarily being inadequate for a lean operating system, although it would be easy enough to write extravagant code that would not run in 64K, or a design that relied on a large virtual address space - especially if you were used to relying on virtual memory. I just don't know if how small Plan9 can go, and unless someone has already explored those limits, I suppose rather than speculating i'll just have to plan on a little experimentation when I get a bit of spare time. Regards, Digby On Mon, 8 Oct 2018 at 19:13, Nils M Holm <n...@t3x.org> wrote: > On 2018-10-08T15:29:02+1100, Digby R.S. Tarvin wrote: > > A native Inferno port would certainly be a lot easier, but I think you > > might be a bit pessimistic about would can fit into a 64K address space > > machine. The 11/70 certainly managed to run a very respectable V7 Unix > > supporting 20-30 simultaneous active users in its day, [...] > > The 11/70 was a completely different beast than, say, an 11/03. > The 70 had a backplane with 22 address lines, a MMU, and up to > 4M bytes of memory. So while its processes were limited to > 64K+64K bytes, I would not consider it to be a typical 16-bit > machine. > > -- > Nils M Holm < n m h @ t 3 x . o r g > www.t3x.org > >
