I believe that the PDP-11 ISA was defined at a time when DEC was still using
random logic rather than a control store (which came pretty soon
thereafter). Given a random logic design it's efficient to organize the ISA
encoding to maximize its regularity. Probably also of some benefit to
compilers in a memory-constrained environment?
I'm not sure at what point in time we can say "lots of processors" had moved
to a control store based implementation. Certainly the IBM System/360 was
there in the mid-60's. HP was there by the late 60's.
-----Original Message-----
From: TUHS <tuhs-bounces(a)minnie.tuhs.org> On Behalf Of Larry McVoy
Sent: Monday, November 29, 2021 10:18 PM
To: Clem Cole <clemc(a)ccc.com>
Cc: TUHS main list <tuhs(a)minnie.tuhs.org>; Eugene Miya <eugene(a)soe.ucsc.edu>
Subject: Re: [TUHS] A New History of Modern Computing - my thoughts
On Sun, Nov 28, 2021 at 05:12:44PM -0800, Larry McVoy wrote:
> I remember Ken Witte (my TA for the PDP-11 class) trying to get me to
> see how easy it was to read the octal. If I remember correctly (and I
> probably don't, this was ~40 years ago), the instructions were divided
> into fields, so instruction, operand, operand and it was all regular,
> so you could see that this was some form of an add or whatever, it got
> the values from these registers and put it in that register.
I've looked it up and it is pretty much as Ken described. The weird thing
is that there is no need to do it like the PDP-11 did it, you could use
random numbers for each instruction and lots of processors did pretty much
that. The PDP-11 didn't, it was very uniform to the point that Ken's
ability to read octal made perfect sense. I was never that good but a
little google and reading and I can see how he got there.
...
--lm
For DEC memo’s on designing the PDP-11 see bitsavers:
http://www.bitsavers.org/pdf/dec/pdp11/memos/
(thank you Bitsavers! I love that archive)
Ad van de Goor (author of a few of the memo’s) was my MSc thesis professor. I recall him saying in the early 80’s that in his view the PDP-11 should have been an 18-bit machine; he reasoned that even in the late 60’s it was obvious that 16-bits of address space was not enough for the lifespan of the design.
---
For those who want to experiment with FPGA’s and ancient ISA’s, here is my plain Verilog code for the TI 9995 chip, which has an instruction set that is highly reminiscent of the PDP-11:
https://gitlab.com/pnru/cortex/-/tree/master
The actual CPU code (TMS99095.v) is less than 1000 lines of code.
Paul
Eugene Miya visited by last week and accidentally left his copy of the
book here so I decided to read it before he came back to pick it up.
My overall impression is that while it contained a lot of information,
it wasn't presented in a manner that I found interesting. I don't know
the intended target audience, but it's not me.
A good part of it is that my interest is in the evolution of technology.
I think that a more accurate title for the book would be "A New History
of the Business of Modern Computing". The book was thorough in covering
the number of each type of machine sold and how much money was made, but
that's only of passing interest to me. Were it me I would have just
summarized all that in a table and used the space to tell some engaging
anecdotes.
There were a number of things that I felt the book glossed over or missed
completely.
One is that I didn't think that they gave sufficient credit to the symbiosis
between C and the PDP-11 instruction set and the degree to which the PDP-11
was enormously influential.
Another is that I felt that the book didn't give computer graphics adequate
treatment. I realize that it was primarily in the workstation market segment
which was not as large as some of the other segments, but in my opinion the
development of the technology was hugely important as it eventually became
commodified and highly profitable.
Probably due to my personal involvement I felt that the book missed some
important steps along the path toward open source. In particular, it used
the IPO of Red Hat as the seminal moment while not even mentioning the role
of Cygnus. My opinion is that Cygnus was a huge icebreaker in the adoption
of open source by the business world, and that the Red Hat IPO was just the
culmination.
I also didn't feel that there was any message or takeaways for readers. I
didn't get any "based on all this I should go and do that" sort of feeling.
If the purpose of the book was to present a dry history then it pretty much
did it's job. Obviously the authors had to pick and choose what to write
about and I would have made some different choices. But, not my book.
Jon
