[ih] bytes [Re: "network unix"]

[Steve Bunch]

> From: Brian E Carpenter <brian.e.carpenter at gmail.com <mailto:brian.e.carpenter at gmail.com>>
> On 11/10/2016 08:32, Noel Chiappa wrote:
>>> From: "John Levine" <johnl at iecc.com <mailto:johnl at iecc.com>>
>> 
>>> There was a competing 16 bit word addressed design by the designer of
>>> the PDP-8, which after DEC rejected it became the DG Nova.
>> 
>> I hear this repeated a lot, but I'm not sure it's accurate. That competing
>> design has surfaced (Google "PDP-X"), and it's not very much like the Nova.
> 
> I jumped from programming a PDP-8 to programming an Imlac PDS-1, which
> struck me at the time as being remarkably like a 16-bit PDP-8. The core
> was word-addressed. There was a separate graphics processor which also took
> 16-bit instructions, but vectors were defined in 8-bit bytes. There's no
> resemblance to the PDP-X description.

In 1972-73 as a new grad student I took a computer architecture class at the University of Illinois from professor Michael Faiman.  Michael spent a significant amount of time on a gate-by-gate, equation-by-equation analysis of the PDP-8 (possibly 8i, can’t recall).  At the Center for Advanced Computation we had an Imlac PDS-1 that was essentially idle, so I took it over.  It had a delicate core memory (very power-supply voltage sensitive) and RC-delay based UART timing.  Both had to be routinely adjusted, so I found myself looking through the prints of the logic design.  

The Imlac PDS-1 WAS the PDP-8 we’d studied, for all practical purposes, stretched to 16 bits and enhanced for graphics.  There was an extra bit of opcode so room for new instructions, extra bits for address, and of course the graphics “processor” *, the raison d’etre of the machine, was added.  But the similarity was unmistakeable, the signals and registers even had the same names.  Our EE, Jim Bailey, knew the Imlac guys and told me that the lead designer of the Imlac PDS-1 was indeed the ex-DEC engineer who had done the same PDP-8 that we’d studied.

The PDS-1 was used for the Network Graphics Protocol Level 0 interpreter mentioned in RFC 472 and 549, written in IMOL (Imlac Machine-Oriented Language), using a compiler constructed by Smokey Wallace that ran on a PDP-10 (I used BBN Tenex).  The NGP-0 interpreter was used to access the UC Santa Barbara OLS (On-Line System) graphics system remotely and in other experiments with John Pickens (recently deceased) of the UCSB Computer Systems Laboratory, experiment with computer generated holograms, display cloud simulation data, and other projects.

The NGP0 interpreter was embedded in an emulator for a standard TTY terminal.  We also used the program to gather statistics on typical Telnet keyboard usage.  Those stats were available when deciding things like buffer sizes for the NCP of Network UNIX a couple of years later (I wrote the mbuf code), which in our environment was heavily used for Telnet.  (I’ve been corresponding with Paul Ruizendaal on Network UNIX.)

I salvaged the CPU logic book when the PDS-1 was scrapped around 1980 and still have it, along with the original SRI mouse and 5-key keyset.

Steve


* The graphics “processor” was actually just a few added registers and logic and a graphics/cpu state bit, and that bit then was an extra input to key equations.  Each instruction cycle was either a graphics instruction or a CPU instruction.  It took extra effort to avoid creating graphics programs that shut out regular instruction processing long enough to lose interrupts.

> I believe the Imlac founders had jumped ship from DEC in 1968.
> 
> (The first protocol I designed and implemented was for booting and driving
> PDS-1s from an IBM 1800, in 1971, at CERN.)
> 
>   Brian
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