Weekly Classic Computer Trivia Question (20141205)

Jerome H. Fine jhfinedp3k at compsys.to
Sat Dec 6 18:36:15 CST 2014

 >Rich Alderson wrote:

>From: Jerome H. Fine
>Sent: Friday, December 05, 2014 3:05 PM
>>>tony duell wrote:
>>>>Approximately how many DECtapes will fit on a single one terabyte drive?
>>>All that were ever made?
>>If I remember correctly, a TU-58 tape holds 512 blocks of 512 bytes
>>or 262,144 bytes in total or 2 ** 18 bytes
>Jerome, "DECtape" without a following "II" refers to *real* DECtape, the
>3/4" medium used on DEC's 555, TU-55 and TU-56 DECtape drives.  These were
>formatted with 578 blocks of 256 18-bit words (even for the PDP-11, which
>ignored bits 0 and 1) or 1102 blocks of 129 12-bit words (= 86 18-bit words)
>depending on which architecture the drives were connected to.
>These were 4" reels, not cassettes.
If my arithmetic is still correct, all of the effective capacities of
the DECtape usage were less than 300,000 bytes.  Please
correct me if I am wrong.

And regardless of all of the questions AND  INTERPRETATIONS
of the actual capacity of a one terabyte drive, the final capacity of a
such a drive after a Low Level Format would be over 3 million tapes
or the same answer I gave last time.  Since the TU-55 tape has only
about 13% more capacity than a TU-58 tape, that answer was

Note that I assume ABOUT 25% reduction in capacity between
the unformatted and formatted drive.

So again, it seems likely (probably more so for DECtape) that DEC
produced less than 3 million tapes which means that Tony Duell's
answer is correct.

As for all the chatter that I seemed to have produced over

terabyte = 2  **  40
terabyte = 10  **  12

at least I defined my assumption and everyone knew the standard being
used.  As for the reason for the different standards, even they are a 
aspect of the marketing lies which are often used to hype and market
hardware and software.  So I suggest that we all just accept that both
of the standards are valid as long as they are clearly defined since at
present memory and hard drives seem to use different definitions that
have been accepted for a number of years.  I would point out that one
reason that memory may have stayed with 2  **  30 is that memory is
usually produced and sold in multiples of 2  **  30 these days whereas
hard drives are all over the map and almost never such a simple multiple.

Please don't use the previous paragraph to start the conflict all over

Jerome Fine

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