Pertec Tape Drive Interface Musings

[Johnny Billquist]

On 2015-06-10 17:05, Dennis Boone wrote:
>   > This hypothetical interface + matching software would be intended for
>   > archiving old tapes and/or making new copies from archived file
>   > (i.e., to make new boot media for bringup of an old computer). Key
>   > features would include preservation of block sizes (even if varying
>   > arbitrarily) and file marks. I'm not sure if there's already a good
>   > file format for that, and I have a dim memory of previously reading a
>   > lament about common archival methods failing to preserve blocking.
>
> The E11/SimH .tap formats are dead simple, and relatively complete as
> far as capturing the arrangement of the bits on tape.  They retain block
> size, actual data, file marks, and have a provision for indicating
> errors encountered when reading the tape.  There was a discussion
> recently (simh list?) about standardizing the behavior of the error
> marks.  The format doesn't have a mechanism for preserving metadata.  In
> the SimH variant, record lengths must be even.  In the E11 variant, they
> on't have to be.  Otherwise, the two are equivalent.
>
> Using dd to read tapes to disk discards the block size information.

One problem with "preserving errors" is that actual tapes do not have 
any indication that you have a tape error. In fact, many times you can 
recover a tape block by repeatedly read it. Eventually you might read it 
without errors. Not to mention how many tapes I have "fixed" by just 
folding the tape in the opposite direction where there have been a 
bend/kink in it. Easy fix.

I guess it could possibly be useful to indicate a bad block on a tape, 
in order to preserve the numbering of all the blocks, and constantly 
giving a read error when used in a simulator. But it's kindof a weird 
reflection of a physical error into a virtual one.

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
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