Odd "endianness" [was Re: RE: Base 64 posts to the list]

Rich Alderson RichA at livingcomputers.org
Wed Dec 14 13:11:24 CST 2016

From: Lars Brinkhoff
Sent: Wednesday, December 14, 2016 3:54 AM

> Noel Chiappa wrote:

>>> 9-track tapes on the PDP-10 used one of the following encodings:

>> What about 7-track, any idea? I would assume 6 x 6-bit tape frames per
>> 36-bit word, but that's just a guess.

> A reasonable guess, and one I'd make too.  But I don't know either.

According to the monitor calls manuals for both Tops-10 and TOPS-20, as
well as the manual specifically on tape labels etc., that's a very good
guess, and a correct one.

> Supposedly, many ITS backups were made to 7-track tapes.  That became a
> problem when the old tape drives started to fail, and available new tape
> drives were only 9-track.

The same issue occurred at SAIL for the early WAITS backups prior to the
introduction of a KL-10 into the system (which grew from a PDP-6 to a
PDP-10/PDP-6 to a KL-10/KA-10/166, then shrank to a KL-10/KA-10 down to
a KL-10 only).  The KL-10 had TU-78 drives on an RH20.

AIUI, the older backup tapes were refrangled onto new 9-track media before
the KA-10 and its drives were retired.


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

mailto:RichA at LivingComputerMuseum.org


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