Newsgroups: alt.folklore.computers
Subject: Re: What is so greate about the PDP 10?
From: don@zl2tnm.gen.nz (Don Stokes)
Date: 13 Dec 92 04:35:14 GMT
Distribution: world
Organization: The Wolery

lasner@watsun.cc.columbia.edu (Charles Lasner) writes:
> But this is the problem of "all your eggs in one basket" since there were plans
> to make the TT-10 and the Jupiter, and all in-between.  Moreover, the plan
> to make the VAX largely turns its back on a sizeable software investment in
> the -10/20 stuff.  So, what do you tell the stockholders:

You seem to think the VAX was a completely standalone effort.  It wasn't.
The 11/780 was basically a replacement for the pdp11/70, which had taken
the pdp11 line to about as large as it could practically get (as the 11/74
effort had largely proven).  It was always intended that users of large
pdp11 systems would migrate to VAX, not users of the -10 line.  (This is
1977 I'm talking about; not the lead-up to the cancelling of Jupiter.)

I don't think the VAX would have been as successful if the migration from
the pdp11 was there.  RSX code had been known to just run on the VAX. 
RSTS code took a little more persuasion, but it wasn't hard (and the VAX
BASIC compiler has a few bits of compatibility stuff for RSTS syscalls). 
Other languages under RSTS were generally ports of RSX code and just worked. 
Having ported quite a lot of code from RSTS/E to VMS, I can report that it's
not too difficult, and most of the problems were to do with differences in
the third party "database" software libraries on the two platforms.

I know of one outfit who were running an 11/70 with RSX, and were intending
to migrate to a VAX.  Then the system disk on the 11/70 died.  They worked
over a weekend to recompile the system onto the VAX (very few changes 
required), mounted the 11/70's data disks on the VAX and continued as if
nothing had happened.  There's a fair chance that just renaming the .TSKs
to .EXE and running under VAX RSX would have worked too, but they didn't
for some reason (the target VAX may have been one without compatibility
mode -- this was only a few years ago).

While the VAX was intended as a follow on from the pdp11 line, the
architecture allowed for a potentially bigger machine than the -10
architecture did, and the only limiting factor in building bigger and
bigger VAXes was the technology available.  Compare this to the fact the
20xx line were already requiring tweaks to the architecture to support
bigger address spaces.  The pdp11 line had already been through these kinds
of tweaks to get the physical address space up (16, 18 and 22 bit physical
addresses, each requiring different memory management code) and a couple of
half-hearted attempts to increase virtual memory (split I&D, supervisor
mode), and I can quite understand why DEC didn't really want to carry this
on into the 80s with the -10 line.

Remember that the -10 was a bunch of architectural and engineering tradeoffs
that were valid in the 60s, whereas the VAX's tradeoffs were valid in the 
70s.  Neither are valid now.

I suspect that when there was demand for a bigger VAX, DEC found themselves
facing the prospect of supporting two "big" machines, and had to make the
choice between them.  

> I find it gratifying in recent years to find many of the -11 supporters seeing
> their favorite models disappearing, such as the 11/70, etc., as they see the
> -11 relegated to an also-ran product, destined to be scrapped entirely shortly.
> In fact, I find it *more* gratifying to see the VAX being edged out in much the
> same way as the Alpha takes over.

The 11/70 is 17 years old for chrissakes.  Sure we get nostalgic about 'em.
It reigned as the most powerful genuine pdp11 until the 11/84, and even then
could outrun an 11/84 quite comfortably on real applications after you
factored in the I/O; it could be argued that it wasn't completely obsoleted
until 1990 with the release of the 11/94 (which *still* only has Unibus for
I/O).  15 years is a bloody long time for one model, and there's still more
of 'em chugging along just fine than any kind of -10.

I'm not sure when DEC stopped supplying 11/70s, but I'd guess that it was
after the 11/84 was released in '84/'85.  

As for VAXes: it's been known for a while that the VAX is going to lose the
MIPS/$ race, but current VAXes are in the bunch with the not-quite-bleeding-
edge RISCs at this time even so.  DEC has learned a thing or two from the
previous "upgrade" debacles; Alphas will run VMS just fine (or even unix if
you really must 8-).  No-one's being left out in the cold like they were in
the -10 to VAX "migration".

> Will I come back here in 6 years to hear about how the Alphans are being
> edged out of the company as the Betans take over?

I doubt it.  Apart from obvious losers, major architectures last a long
time at DEC, eg the pdp11 at 22 years (and still going strong), the PDP-8
at 25? 26? years, although now discontinued.  The -6/-10 lasted nearly 20
years, and the VAX will still be kicking on its 20th birthday in 1997. 
The Alpha is *not* an obvious loser.  It'll be around well into the next
century, unless DEC goes down the tubes, in which case there won't be any
Betans.  There's also no competition within DEC at this stage -- the VAX
crew have been co-operating closely with the Alpha lot -- the NVAX+ chip
in the VAX 7000 & 10000 is pin compatible with the 21064 for example.

--
Don Stokes, ZL2TNM (DS555)                        don@zl2tnm.gen.nz (home)
Network Manager, Computing Services Centre            don@vuw.ac.nz (work)
Victoria University of Wellington, New Zealand              +64-4-495-5052