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[173.48.168.163]) by smtp.gmail.com with ESMTPSA id s81-v6sm8181588qke.47.2018.06.16.17.15.15 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 16 Jun 2018 17:15:15 -0700 (PDT) Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (1.0) From: Clem cole X-Mailer: iPhone Mail (15F79) In-Reply-To: <86dcd19d-f805-f338-f190-ab38d1ac82c1@update.uu.se> Date: Sat, 16 Jun 2018 20:15:14 -0400 Content-Transfer-Encoding: quoted-printable Message-Id: <20B7C3F5-2E44-41AB-91E4-510451428C83@ccc.com> References: <86dcd19d-f805-f338-f190-ab38d1ac82c1@update.uu.se> To: Johnny Billquist Subject: Re: [TUHS] core X-BeenThere: tuhs@minnie.tuhs.org X-Mailman-Version: 2.1.20 Precedence: list List-Id: The Unix Heritage Society mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: tuhs@minnie.tuhs.org Errors-To: tuhs-bounces@minnie.tuhs.org Sender: "TUHS" Hmm. I think you are trying to put to fine a point on it. Having had this c= onversation with a number of folks who were there, you=E2=80=99re right that= the ability to memory on the Unibus at the lower end was clearly there but I= =E2=80=99ll take Dave Cane and Henk=E2=80=99s word for it as calling it an I= O bus who were the primary HW folks behind a lot of it. Btw it=E2=80=99s re= placement, Dave=E2=80=99s BI, was clearly designed with IO as the primary po= int. It was supposed to be open sourced in today=E2=80=99s parlance but DEC c= losed it at the end. The whole reason was to have an io bus the 3rd parties= could build io boards around. Plus, By then DEC started doing split trans= actions on the memory bus ala the SMI which was supposed to be private. Afte= r the BI mess, And then By the time of Alpha BI morphed into PCI which was= made open and of course is now Intel=E2=80=99s PCIe. But that said, even t= oday we need to make large memory windows available thru it for things like m= essaging and GPUs - so the differences can get really squishy. OPA2 has a f= ull MMU and can do a lot of cache protocols just because the message HW real= ly looks to memory a whole lot like a CPU core. And I expect future GPUs to= work the same way.=20 And at this point (on die) the differences between a memory and io bus are o= ften driven by power and die size. The Memory bus folks are often willing t= o pay more to keep the memory heiarchary a bit more sane. IO busses will of= ten let the SW deal with consistency in return for massive scale. =20 Sent from my PDP-7 Running UNIX V0 expect things to be almost but not quite.= =20 > On Jun 16, 2018, at 6:14 PM, Johnny Billquist wrote: >=20 >> On 2018-06-16 21:00, Clem Cole wrote: >> below... > On Sat, Jun 16, 2018 at 9:37 AM, Noel Chiappa=20 > wrote: >>>=20 >>> 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 mista= ke, >>> but I don't recall exactly where I saw it.) >> =E2=80=8BI think it was part of the same paper where he made the observat= ion that >> the greatest mistake an architecture can have is too few address bits.=E2= =80=8B >=20 > I think the paper you both are referring to is the "What have we learned f= rom the PDP-11", by Gordon Bell and Bill Strecker in 1977. >=20 > https://gordonbell.azurewebsites.net/Digital/Bell_Strecker_What_we%20_lear= ned_fm_PDP-11c%207511.pdf >=20 > There is some additional comments in https://gordonbell.azurewebsites.net/= Digital/Bell_Retrospective_PDP11_paper_c1998.htm >=20 >> 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. >=20 > Hmm. I'm not aware of anyone perceiving the Unibus as an I/O bus. It was v= ery clearly designed a the system bus for all needs by DEC, and was used jus= t like that until the 11/70, which introduced a separate memory bus. In all p= revious PDP-11s, both memory and peripherals were connected on the Unibus. >=20 > Why it only have 18 bits, I don't know. It might have been a reflection ba= ck on that most things at DEC was either 12 or 18 bits at the time, and 12 w= as obviously not going to cut it. But that is pure speculation on my part. >=20 > But, if you read that paper again (the one from Bell), you'll see that he w= as 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 w= hy it got 18 bits. It also, incidentally have 18 data bits, but that is most= ly 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 m= ade fortuitous use of it when they were designed. >=20 >> 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 mig= ht >> 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 bi= ts >> is not going to cut it."* He might have replied something like: *"Boo= l >> 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 alwa= ys >> interesting when you argued with him). >=20 > Quite possible. :-) >=20 >> 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 a= nd >> the time could have overruled it, and I don't think the problems were >> foreseen as Noel notes. >=20 > 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 d= idn't believe in what they predicted. >=20 > Johnny >=20 > --=20 > Johnny Billquist || "I'm on a bus > || on a psychedelic trip > email: bqt@softjar.se || Reading murder books > pdp is alive! || tryin' to stay hip" - B. Idol