Triprocessor PDP-10 [Was: Re: [multicians] Emacs humor]

[Eric Smith]

On Thu, Dec 3, 2015 at 12:29 AM, Pontus Pihlgren <pontus at update.uu.se> wrote:
[about KL10/KA10/PDP-6 tri-processor
> Wow, that's impressive. How was it done? Was it done with DEC or was it
> a local "hack"?

Prior to the 1091 and 20xx, all PDP-10 processors used essentially the
same memory bus, and the memory boxes were multiported. The necessary
hardware configuration might not have been quite as simple as just
cabling the three dissimilar processors to the memory boxes, but it
probably wasn't too terribly complicated.

Getting standard DEC software to run on such a configuration would
have required quite a bit of work. DEC supported asymmetric
multiprocessing on the KA10 (DECsystem-1055) and KI10
(DECsystem-1077), and possibly on the KL10 (DECsystem-1088). Symmetric
multiprocessing (SMP) wasn't available in TOPS-10 until some time
after the KL10 was available, and for SMP only multi-KL10 systems were
supported. I think SAIL ran the WAITS operating system, rather than a
DEC OS, though WAITS probably started out as a fork of an early DEC
PDP-10 "Monitor". ("Monitor" was the name of the OS before it became
TOPS-10.)

My understanding is that the SAIL tri-processor configuration was
asymmetric multiprocessing.  (Not just asymmetric in that the CPUs
were different, but also in how I/O devices were configured on them,
and which CPU the operating system mostly ran on.) However, I wasn't
there and only heard about the system second-hand at best.