17 Jun
2018
17 Jun
'18
4:14 a.m.
On 2018-06-16 21:00, Clem Cole <clemc(a)ccc.com> wrote:
I think the paper you both are referring to is the "What have we learned
from the PDP-11", by Gordon Bell and Bill Strecker in 1977.
https://gordonbell.azurewebsites.net/Digital/Bell_Strecker_What_we%20_learn…
There is some additional comments in
https://gordonbell.azurewebsites.net/Digital/Bell_Retrospective_PDP11_paper…
below... > On Sat, Jun 16, 2018 at 9:37 AM, Noel
Chiappa
<jnc(a)mercury.lcs.mit.edu> wrote:
Let's start with the UNIBUS. Why does it have only 18 address lines? (I
have
this vague memory of a quote from Gordon Bell admitting that was a mistake,
but I don't recall exactly where I saw it.)
I think it was part of the same
paper where he made the observation that
the greatest mistake an architecture can have is too few address bits. My understanding is that the problem was that UNIBUS
was perceived as an
I/O bus and as I was pointing out, the folks creating it/running the team
did not value it, so in the name of 'cost', more bits was not considered
important.
Hmm. I'm not aware of anyone perceiving the Unibus as an I/O bus. It was
very clearly designed a the system bus for all needs by DEC, and was
used just like that until the 11/70, which introduced a separate memory
bus. In all previous PDP-11s, both memory and peripherals were connected
on the Unibus.
Why it only have 18 bits, I don't know. It might have been a reflection
back on that most things at DEC was either 12 or 18 bits at the time,
and 12 was obviously not going to cut it. But that is pure speculation
on my part.
But, if you read that paper again (the one from Bell), you'll see that
he was pretty much a source for the Unibus as well, and the whole idea
of having it for both memory and peripherals. But that do not tell us
anything about why it got 18 bits. It also, incidentally have 18 data
bits, but that is mostly ignored by all systems. I believe the KS-10
made use of that, though. And maybe the PDP-15. And I suspect the same
would be true for the address bits. But neither system was probably
involved when the Unibus was created, but made fortuitous use of it when
they were designed.
I used to know and work with the late Henk Schalke,
who ran Unibus (HW)
engineering at DEC for many years. Henk was notoriously frugal (we might
even say 'cheap'), so I can imagine that he did not want to spend on
anything that he thought was wasteful. Just like I retold the
Amdahl/Brooks story of the 8-bit byte and Amdahl thinking Brooks was nuts;
I don't know for sure, but I can see that without someone really arguing
with Henk as to why 18 bits was not 'good enough.' I can imagine the
conversation going something like: Someone like me saying: *"Henk, 18 bits
is not going to cut it."* He might have replied something like: *"Bool
sheet *[a dutchman's way of cursing in English], *we already gave you two
more bit than you can address* (actually he'd then probably stop mid
sentence and translate in his head from Dutch to English - which was always
interesting when you argued with him).
Quite possible. :-)
Note: I'm not blaming Henk, just stating that his
thinking was very much
that way, and I suspect he was not not alone. Only someone like Gordon and
the time could have overruled it, and I don't think the problems were
foreseen as Noel notes.
Bell in retrospect thinks that they should have realized this problem,
but it would appear they really did not consider it at the time. Or
maybe just didn't believe in what they predicted.
Johnny
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: bqt(a)softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol