[TUHS] [simh] 2bsd tarball

Wed Jul 29 23:52:58 AEST 2020

When I talk about DECtape in my capacity as the local old fart, i describe
it as "a disk with one track and about 1500 small sectors that spins ve-ry
ve-ry slow-ly..

On Wed, Jul 29, 2020 at 5:58 AM Johnny Billquist <bqt at softjar.se> wrote:

> 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 at gmail.com
> > <mailto:will.senn at 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 at softjar.se             ||  Reading murder books
> pdp is alive!                     ||  tryin' to stay hip" - B. Idol
>
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