[TUHS] [simh] 2bsd tarball
Johnny Billquist
bqt at softjar.se
Thu Jul 30 00:30:11 AEST 2020
Yes. That is a much better way to look at it.
Johnny
On 2020-07-29 15:52, John Cowan wrote:
> 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
> <mailto: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>
> > <mailto: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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> [bqt at softjar.se <mailto:bqt at softjar.se>]
> >
> > _._,_._,_
>
> --
> Johnny Billquist || "I'm on a bus
> || on a psychedelic trip
> email: bqt at softjar.se <mailto:bqt at softjar.se> ||
> Reading murder books
> pdp is alive! || tryin' to stay hip" - B. Idol
>
--
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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