Newsgroups: vmsnet.pdp-11
Subject: Re:  Unix on PDP-11's
From: <sms@wlv.iipo.gtegsc.com>
Date: Mon, 14 Sep 92 21:32:50 -0700
Organization: The Internet
Return-Path: <pat+@transarc.com>

> From: John_Wilson@mts.rpi.edu

> Just a quick reality check here -- what's the point of running Unix
> on a PDP-11??!!

	Same reason that some folks climb mountains, some collect
	old cars, and still others have to work with what's available
	at the moment.

> If you're extremely lucky and have an 11/94 or something,
> the best you can hope for is that you won't even notice that you're on
> a PDP-11;

	You've missed one of the main points - you're _supposed_
	to notice that you're on a PDP-11.  If i wanted "fast" i'd
	take the "easy" way out like so many others.

> more likely, the 11's slow speed will remind you continually
> of Unix's inefficiencies (HOW can an OS which is as stripped-down as the
> Unix kernel have trouble fitting on a 124KW machine?!).

	This probably isn't the place to have a religious discussion
	about "efficiency of Unix".  PDP-11s not being multi-mips
	machines i accept, but that's nothing to do with "efficiency".

	Stripped down?  I should say not.  Granted there's no NFS
	around (just as well, i don't particularily care for it at times),
	but just about every other niceity from 4.3BSD is present and
	accounted for.  Ever try putting 4.3BSD into a 256kb machine?

	Typical kernel and networking sizes:

	size /unix
text	data	bss	dec	hex
57152	7152	39808	104112	196b0	total text: 105984
	overlays: 7424,8128,7744,7680,8000,7680,2176

text	data	bss	dec	hex
59520	3006	37730	100256	187a0

	So just between the two there's ~251kb of memory, add in a disc
	buffer cache between 64 and 128kb, namei cache, etc, etc and
	around 320kb is needed before the first user program can be
	loaded.    Last i checked you can't put 320kb of memory on
	a 18bit PDP-11 (to tie this whole thing back into the original
	subject line).

> quite reasonable for used ones) or something with a similar performance:
> power consumption:size ratio;  the only difference you'll notice is that
> it works better/faster/cheaper.

	The point was missed, that perhaps "faster" and "cheaper" aren't
	the goals in all this.  The "better" is open to debate - depends
	what metric of "goodness" is being used.

> The only point I see in having a PDP-11 (and I have six) is for doing things
> which it does well, like letting a

	That's certainly 1 point, but hardly the _only_ point.  Another
	point might be to "do something that was said couldn't be done".

> under RT;  try that under Unix -- but why would you bother?  device control
> isn't Unix's bag), or running realistic timesharing under RSX or RSTS;  our

	Depends which devices and exactly what about them is being controlled.

	I can't comment on RSTS (except that it looked kinda neat), but
	the last time i had a brush with RSX it was not any more
	"realistic" than Unix when it came to timesharing.  Now if you'd
	mentioned "realtime" rather than timesharing as a strong point for
	RSX that'd be a different story.

> high school used to run 5-6 users all day long under RSTS on our 11/34a,
> sure it was slow but it sounds like we would have been lucky to be running
> at all under Unix.

	Good thing you weren't running RSX-11 - you might get 5 or 6 users
	logged in but everyone'd end up waiting in the MRL for the guy before
	them to finish something.

> you're running it on.  PDP-11's have many strengths, which some OS's go
> to great lengths to exploit, but Unix strikes at the 11's weak points instead.

	I disagree, but then that's to be expected.  The only "weak point",
	and again it depends on what one's goals are, is a small address
	space - that just means one has to be a bit more careful/thrifty
	(can't afford 80kb buffers on the stack, etc).

> PDP-11's are a good place to mess with the 11's neat hardware (ever seen
> a refreshing vector display on a generic Unix box?), or mess with the 11's

	Haven't had one (or the urge) to play with.  So?

> neat OSes (address windows, asynch I/O, vector ownership/jam lists (boy
> could MS-DOS use that!), dynamically loadable/unloadable device drivers etc.),

	Had address windows back on a V7 system - didn't really use 'em all
	that much, but that does point out another point about Unix on
	the PDP-11:  you can work over the kernel and _add_ all that neat
	stuff.  If a primary goal is to do work assuming those neat features
	are already present, then perhaps Unix on a PDP-11 isn't your cup
	of tea.

> or mess with the 11's neat instruction set (even the 68K/VAX/Z8000 attempted
> clones seemed to miss the point of complete symmetry).  A SPARCstation is

	Total agreement on that point.  The 11 has a super instruction set,
	and there's just enough assembly in the kernel (and libc.a routines)
	that continued fluency with the instruction set is a good idea when
	hacking on a PDP-11 Unix.

	Steven Schultz