[Info-vax] Copying VMS SaveSet Under Windows

[Johnny Billquist]

On 2010-11-21 20:50, Michael Kraemer wrote:
> JF Mezei schrieb:
>> About ANSI tapes:
>>
>> Question: does the tape drive itself signal end of file and end of tape
>> ? Or just end of tape ?
>>
>> or does the tape automatically skip inter block gaps and return block
>> after block, with only end of tape being signaled ?
>
> there's nothing magic about ANSI tapes.
>  From the tape drive's (and UNIX') point of view,
> the files on tape are still sacks of bytes,
> separated by single tape marks, and a double tape
> mark signals the EOT.

Um. Now that was a lie if I ever saw one. :-)
Tapes are made of records. For each record, you either read the whole 
record, or parts are lost, as the next read will read the next record.

This is in the basic design of a tape. Unix tries to hide this, but it 
can't really. And noone actually wants Unix byte streap wrapping of 
tapes, which is why everyone always uses the raw tape devices, and not 
the tape block devices...

In addition to the records, you then also have file marks, which 
indicates the end of one file. On "traditional" tapes, a double tape 
mark with no data records in between marked the logical end of tape.
With ANSI tapes, you instead have a special file header that marks the 
logical end of tape, and two file marks after each other does not signal 
the leot.

> It's just that some OS's (e.g. VMS) wraps
> each tape file in an extra header
> and an extra trailer file containing additional
> file info (name, date, recl etc), and the whole
> tape is assigned a volume label recorded somewhere
> in the first header. It's not rocket science,
> but of course requires some extra work to teach a Unix system
> to read that information - if anybody would be interested.
> I did such things about twenty years ago to facilitate
> data exchange with the non-DEC world. DEC Unices
> (Ultrix, OSF/1) have the "ltf" utility on board,
> but iirc it has limited capabilities (i.e. no
> multivolume support etc.)

Indeed. There is nothing magic about tapes, and it is easy enough to 
read and process tapes even under Unix. But you gotta do it a record at 
a time, and not as a byte stream.
And yes, the ANSI stuff is just some more data to create some form of 
structure of the contents on the tape. Easy to write a program of your 
own to handle that, if you want to.

But the ANSI information is sucked in just like any other information, 
but a Unix program. If you write a program to read tapes, it will read 
tapes. The ANSI "meta"-data is nothing special, or hidden. It will come 
as data, just like everything else.

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol