[TUHS] core

Johnny Billquist bqt at update.uu.se
Sun Jun 17 08:14:00 AEST 2018

On 2018-06-16 21:00, Clem Cole <clemc at ccc.com> wrote:
> below... > On Sat, Jun 16, 2018 at 9:37 AM, Noel Chiappa 
<jnc at 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.​

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.


There is some additional comments in 

>   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 Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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