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

Rich Alderson RichA at
Fri Dec 4 12:59:15 CST 2015

From: Eric Smith
Sent: Thursday, December 03, 2015 9:57 PM

> On Thu, Dec 3, 2015 at 12:29 AM, Pontus Pihlgren <pontus at> 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.

SAIL did indeed run WAITS, which officially forked from the PDP-10 monitor at
4S72 (Level 4 Monitor, Summer 1972 release), but which began diverging when it
was still the PDP-6 Monitor.[1] It supports asymmetric multiprocessing, based
initially on the 1055 code[2], though diverging immediately because of the
differences between a KA-10 and a Model 166 processor (PDP-6 CPU).

Things get more complicated with the introduction of the KL-10 processor.
Prior to this, SAIL used non-DEC disks and their own file system, similar but
not identical to the DEC Level D[3]; with the introduction of the Massbus,
they moved to RP06 and RP07 but kept the SAIL file system.  However, they did
not adopt the Tops-10 drivers for the Massbus; instead, they modified the
TOPS-20 drivers (as of release 5.1) to interoperate with a Tops-10 style
system call regimen.  At the same time, they made the KL-10 the master in the
three-processor system.

Because they started with the PDP-6 and continued development until ~1990
(including porting to the Foonly F2 at CCRMA and the KL-10 at Livermore),
there was no such thing as a WAITS install tape (or suite).  That made getting
WAITS running on a system at the museum a long and winding road, an adventure
in dissertation level research, and that in turn is why I know so much about
the internals and history of the operating system.


[1] "Tops-10" was simply a renaming of an operating system which began on the
    PDP-6 in 1964 and continued in an uninterrupted line of development up
    through the final release, Tops-10 v7.04 (1988), and maintenance (v7.05,

[2] The PDP-10 monitor, when introduced, came in 5 variants, of which 2 seem
    to have been vaporware (to use an anachronistic term).  These went from a
    single-user monitor (10/10) up to a timeshared swapping monitor with disks
    (10/50).  This was the version to which AMP ("10/55" " was introduced, and
    really the only version supported beyond initial release.

[3] For example, project-programmer numbers are SIXBIT rather than numeric,
    even when they look numeric.  In a DEC file system, the Master File
    Directory is [1,1] and in the system that is represented by the octal
    value 000001000001; in the WAITS file system, the MFD is [1,1] with an
    internal representation of 000021000021.

Rich Alderson
Vintage Computing Sr. Systems Engineer
Living Computer Museum
2245 1st Avenue S
Seattle, WA 98134

mailto:RichA at

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