Just a small comment. Whoever it was that thought DECtape was a tape was
making a serious mistake. DECtapes are very different from magtapes.

   Johnny

On 2020-07-29 02:21, Clement T Cole wrote:
>
> Cross posting to simh - since much of this has been discussed in the
> last few days there also....
>
> in for penny, in for pound ... here is the history ...  man ... I lived
> this and I'll need a strong drink later tonight after I write it all up.
>
>
> On Tue, Jul 28, 2020 at 7:04 PM Will Senn <will.senn@gmail.com
> <mailto:will.senn@gmail.com>> wrote:
>
>     I recall having to do something with cont.a files, which are not
>     present on these images. So, my questions is, does anyone know of or
>     have an actual 2bsd tape/tape image?
>
> cont.a is a tp-v6 and earlier ism.
>
> DECtape had a directory at the front of the tape (think
> superblock/ilist), but could do cool things and be treated more like a disk.
> When tp was created for a very early version of Unix (I'm not sure
> which, could be V2), Ken/Dennis et al had DECtape units and so the
> original scheme followed the media.   This also meant that the program
> could write files and go back and update the directory, which is a no-no
> with many tape systems.  Then research got a 9-track unit.   So tp was
> changed to calculate how much space was going to be needed, write the
> directory, then the datablocks.  All good ... except...
>
> 9-track could write more files than the directory could take.   So for
> many years, people would use the ar(1) program to take a number of files
> in a directory, create a file called cont.a and then delete the files. 
> Then the tree would be written with tp, when you read it, you reversed
> the ar(1) process.  If you look at the USENIX/Harvard tape on the TUHS
> you'll see this scheme in use.
>
> BTW: tp was written in assembler and all the data structures were using
> PDP-11 binary formats.  Eventually, Harvard wrote stp (super-tp) in C 
> (which is on the USENIX tape Warren has in the archives) that worked
> like the original assembler tp but also put a redundant directory at the
> end of the tape.  Another issue with tp was if the you had a bad block
> in the first few blocks you could not decode the rest of the tape. 
> [There were some other issues with the UNIX tree structure as disks got
> bigger but I'm going to ignore all that - other than to say, tp had
> lived it life].
>
> Enter Mashey and the PWB 1.0 folks (which is based on V6).  Someone in
> USG created cpio (and volcpy) as part of the PWB 1.0.   Like tp it was a
> PDP-11 binary format, but unlike tp, the tape directory is threaded.
> /i.e./ block one describes the first file only and includes the size of
> the following file, then file itself, then a new directory block for the
> next file and again that file (rinse and repeat).  So it improved on tp
> in the directory threading, but was still binary and for a reasons I'll
> leave out had a different user interface.
>
> As part of V7, Ken wrote a new program, tar [you can ask him why].  But
> like cpio he used a threaded tape directory, but unlike cpio it was
> always ASCII and not PDP-11 specific.  Furthermore, the user interface
> was made to parrot tp.  So, certainly, it had the advantage that
> changing tp scripts to use tar was pretty easy i.e. s/tp/tar/     not so
> for coil.  And it was muscle memory compliant.
>
> For PWB 2.0, cpio was updated to allow a -c option to write the header
> in ascii and -s to byte swap the binary.   But the damage had been done ...
>
> Thus began 'tar wars' which was a battle that raged officially over tape
> archive formats, but really was an argument about user interfaces. 
> Since tar was part of Research and the Universities and commercial
> people used it, only USG and the folks inside the Bell System were using
> cpio, as officially none of us had it since PWB was not released to us
> (although thanks to many AT&T employees doing an OYOC year, many schools
> like UCB, MIT and CMU all had the sources to cpio anyway -- for instance
> you'll see it hidden away on Kirk's CD).
>
> I personally had used both, preferred tar for easy of use and ASCII
> directories.  But, note I had written car at Masscomp, but never tpio. 
> This was our trick to use the file scripting list that cpio could do,
> but create tar format tapes - which was handy.  I never wrote tpio which
> would have been cpio format using tp/tar user interface as I did not
> need it.
>
> Roll forward to the /usr/group UNIX standard that Heinz chaired.  We
> ended up not being able to agree on a distribution format, but the ISVs
> were PO because now they could create UNIX programs that might actually
> work across systems, but they had not standard way to distribution.
> Roll forward again to IEEE.  Heinz's committee was officially disbanded
> (story discussed elsewhere) and we were created as IEEE P1003 with Jim
> Issack as Chair. This time the ISV's said we had to have a distribution
> format.  Since *.1 was only an API we were allowed to avoid the user
> interface issue but only examine the on tape format.
>
> It turns out while it seems to have been unintended, Ken's original V7
> implementation has an interesting coding feature/bug which turns out to
> be what clinched the deal.   When Ken creates the directory block for
> each file, he did bcopy of 0's to the buffer before he wrote that data
> that fills it in.  Then when he calculated the checksum for the
> directory header block, he summed the entire block (which because of the
> bcopy was zeros).  This means if you write beyond the end of Ken's
> original header and include that extra data in the chksum, the original
> program will ignore the new information but accept the directory block
> as valid.  i.e. he had built an extension mechanism into the tar on-tape
> format.
>
> cpio's ASCII on tape format was not able to do that as the checksum used
> a sizeof(header struct) in the checksum routine.
>
> USTAR was born ... Ken had written things like the UID/GID as ASCII
> representations of the integer value in the original header.  USTAR
> added the ASCII representation of the username and the group name since
> that was more often portable between systems than the numbers.   There
> were other additions like more room for the pathname new file types
> /etc/.  But the key is that a USTAR tape can be read by the original V7
> (and follow on) tape formats, although may not recognize all the
> filetype or use all of the new information.
>
> A few years later during *.2 discussions, we finally got into the user
> interface stuff and pax(1) was born.  Knowing my hack with car, Keith
> Bostic, Jim McGuiness and I wrote up a description of a program that
> could with both users interfaces scheme.  USENIX provided funding for a
> student to implement it and put the sources out on comp.unix.sources at
> some point.  That proposal was originally accepted at the first tape
> user interface program in *.2 [a few years later after I stopped being
> part of the committee, the USG folks did get an alternate CPIO format
> accepted and cpio as an allowed program.   USENIX paid to have the
> program updated to operate like cpio if it was called that, pure V7 tar
> if called that and if pax, user USTAR].
>
> 'nuf said ... I hope.
>
> Clem
>
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--
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt@softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol