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[ih] NCP and TCP implementations

I wrote both code and microcode for the C/30 IMP. We started by
implementing a straight emulation of the Honeywell 316 architecture and
instruction set so that we could port the existing IMP code pretty much
untouched. Once that was working, we then conducted extensive performance
testing to determine where the code was spending its time, and moved a
number of expensive (from a CPU perspective) operations, such as queue
handing and process scheduling, to the microcode. That also allowed us to
make the queue handing instructions "atomic", which helped
eliminate possible race conditions without requiring software semaphores
and the like. We were able to really improve the IMP's forward capacity on
the fast path though the use of microcode optimization. In the end, the
C/30 IMP code looked very different from the original 316 code base, even
though it was still in assembly language (what we now called "C/30
assembly"). We had talked about porting the code base to C and compiling
that down to C/30 assembly, but that never ended up happening.


On Mon, Mar 16, 2020 at 4:09 PM Paul Ruizendaal via Internet-history <
internet-history at elists.isoc.org> wrote:

> Dear all,
> The C/30 and C/70 were discussed on this list in October 2017. I think I
> recall from then that the MBB project got started because BBN had not done
> a CPU project in a while, Al Nemeth had some ideas and ARPA needed a
> replacement for the 316/516.
> This is an excerpt from a message I posted back then:
> ====
> The MBB processor:
> The MBB processor is documented in this paper (available from the ACM
> library, unfortunately behind a paywall):
> M. F. Kraley, R. D. Rettberg, P. Herman, R. D. Bressler, and A. Lake,
> ?Design of a User-Microprogrammable Building Block? in Proceedings of the
> 13th Annual Microprogramming Workshop, IEEE, New York, 1980.
> It is an interesting read and I can certainly recommend it; it also
> discusses some aspects of the C/30 and C/70 configurations.
> The MBB processor seems to have been word (not byte) addressable, with 20
> bit addresses and data paths. It is highly reminiscent of the Alto, with
> I/O device controllers partially implemented in microcode. It is also
> somewhat reminiscent of the TI990 and the later Sparc in that it had 1024
> registers with a visible window of 16 registers. It is unique in that the
> processor had two optional daughter boards to customise the system: (i) a
> board to assist with macro-instruction decoding, (ii) an MMU board.
> The C/30 version seems to have had a macro-instruction daughter board, but
> with addresses going straight through. When used as an IMP, some 30% of
> microcycles seem to have gone on I/O processing and the remaining 70% on
> executing H316 code.
> The C/70 version had both daughter boards. The MMU board divided the 1MW
> address space into 128 pages of 4KW, and had protection & dirty bits per
> page. It could hold page tables for up to 8 tasks. 128 pages by 4KW is only
> 19 bits, perhaps the MMU board used 1 bit to simulate byte accesses.
> Apparently, there was also a ?switch? (IMP?) version of the C/70, without
> the MMU board and running a minimal OS (but using the ?C? microcode &
> board).
> The C/70 seems to have implemented a load/store type architecture with 40
> basic instructions, each offering one of 19 addressing modes in an
> orthogonal setup. The 19 addressing modes were designed around typical C
> data access operations. Next to that there were 44 specialised instructions.
> Procedure calls were very fast, as specialised instructions existed to
> switch to a new register bank as part of the call, with spilling to main
> memory upon deep recursion. Apparently it was possible for C code to ?call'
> into microcode, and this may be how system calls were done.
> ====
> According to the paper mentioned above, it is all documented in detail in
> the "MBB Microprogrammer's Handbook" BBN Report No. 4268, Feb. 1980.
> Unfortunately, this document does not seem to be available on the DTIC
> website. Maybe this document still exists in the BBN (and successor)
> library. As the topic comes up from time to time, maybe it could be added
> to Bitsavers?
> BBN-UNIX for the C/70:
> This was a port of the V7 based Unix that BBN had running on the PDP11,
> with various BSD-type extensions. It had the Gurwitz TCP/IP stack
> integrated, and accessible via the VAX-TCP API (which was compatible with
> the earlier NCP Unix API). It also had some of the earlier IPC features
> (Haverty?s await() and capac() calls, ports, etc.). That TCP stack is
> available for study on TUHS
> https://minnie.tuhs.org/cgi-bin/utree.pl?file=BBN-Vax-TCP/bbnnet (note
> how the code has #ifdef?s for VAX and MBB).
> Again from the discussion three years ago, I think this Unix version and
> the C/30-C/70 were maintained into the late nineties by BBN.
> There was a commercial version of the C/70 (called the C/60) done by/for
> the BBN computer division - see for instance this advert:
> https://books.google.nl/books?id=2LaqX2JB6_UC&pg=PA18&lpg=PA18&dq=BBN+C/60&source=bl&ots=Udp4oJD-zO&sig=ACfU3U39YuIjEJ6g1SGYgMqHsil-v6FzdQ&hl=nl&sa=X&ved=2ahUKEwjKtIHh45_oAhWCjKQKHdZoD_EQ6AEwBHoECAgQAQ#v=onepage&q=BBN%20C%2F60&f=false
> <
> https://books.google.nl/books?id=2LaqX2JB6_UC&pg=PA18&lpg=PA18&dq=BBN+C/60&source=bl&ots=Udp4oJD-zO&sig=ACfU3U39YuIjEJ6g1SGYgMqHsil-v6FzdQ&hl=nl&sa=X&ved=2ahUKEwjKtIHh45_oAhWCjKQKHdZoD_EQ6AEwBHoECAgQAQ#v=onepage&q=BBN
> C/60&f=false>
> Paul
> > On Mar 16, 2020, at 7:28 PM, Noel Chiappa via Internet-history <
> internet-history at elists.isoc.org> wrote:
> >
> >
> >> From: Bernie Cosell
> >
> >> The TIP was a 316 IMP with the terminal handling code in the upper 16K
> >> of memory.  The C/30 was an implementation of the 316 on the MBB
> >
> > Any idea/recollection of why the 316 TIP hardware was ditched and
> replaced
> > with the C/30 for the TAC? Just to have more modern/maintainable
> hardware? (I
> > recall the 316 was made of a lot of tiny cards.)
> >
> >> The MBB was "microprogrammable" and so we built the C/70 ... Alex/Dave:
> >> was any of the development of the C70 written up?
> >
> > I definitely read a document about the MBB. I have this vague memory
> that it was
> > hardcopy, but I doubt I'll be able to locate it anytime soon.
> >
> > ISTR that the MBB took a daughtercard, which implemented extra
> functionality,
> > and the C/70 took its own; I don't recall exactly what the C/70 one did,
> > though.
> >
> >    Noel
> >
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