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 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 > > 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 > > > > _._,_._,_ > > ------------------------------------------------------------------------ > > Groups.io Links: > > > > You receive all messages sent to this group. > > > > View/Reply Online (#62) | Reply > To > > Group > > > > > | Reply To Sender > > > > > | Mute This Topic | New Topic > > > > > > Your Subscription | Contact > > Group Owner | Unsubscribe > > [bqt@softjar.se > ] > > > > _._,_._,_ > > -- > 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 >