Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

["Digby R.S. Tarvin" <digbyt42@gmail.com>]

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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 <nmh@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
>
>

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