TUHS

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34 participants
6476 discussions

Re: [TUHS] A New History of Modern Computing - my thoughts

by jnc＠mercury.lcs.mit.edu

> The ++ operator appears to have been. One would expect that most people on this list would have read "The Development of the C Language", by Dennis Ritchie, which makes perfectly clear (at 'More History') that the PDP-11 had nothing to do with it: Thompson went a step further by inventing the ++ and -- operators, which increment or decrement; their prefix or postfix position determines whether the alteration occurs before or after noting the value of the operand. They were not in the earliest versions of B, but appeared along the way. People often guess that they were created to use the auto-increment and auto-decrement address modes provided by the DEC PDP-11 on which C and Unix first became popular. This is historically impossible, since there was no PDP-11 when B was developed. https://www.bell-labs.com/usr/dmr/www/chist.html thereby alleviating the need for Ken to chime in (although they do allow a very efficient implementation of it). Too much to hope for, I guess. Noel

3 years, 5 months

Re: [TUHS] PL/I stuff - was: Book Recommendation

by jnc＠mercury.lcs.mit.edu

> From: "Charles H. Sauer"k <sauer(a)technologists.com> > I haven't done anything with 9 ktrack tapes for a long time ... > I don't recall problems reading any of them. ... > IMNSHO, it all depends on the brand/formulation of the tape. I've been > going through old audio tapes and digitizing them The vintage computer community has considerable experience with old tapes; in fact Chuck Guzis has a business reading them (which often includes converting old file formats to something modern software can grok). We originally depended heavily on the work of the vintage audio community, who pioneered working with old tapes, including the discovert of 'baking' them to improve their mechanical playability. ("the binder used to adhere the magnetic material to the backing ... becomes unstable" - playing such a tape will transfer much of the magnetic material to the head, destroying the tape's contents.) It's amazing how bad a tape can be, and still be readable. I had a couple of dump tapes of the CSR PWB1 machine at MIT, which I had thoughtlessly stored in my (at one period damp) basement, and they were covered in mold - and not just on the edges! Chuck had to build a special fixture to clean off the mold, but we read most of the first tape. (I had thoughtfully ade a second copy, which read perfectly.) Then I had to work out what the format was - it turned out that even though the machine had a V6 filesystem, my tape was a 'dd' of a BSD4.1c filesystem (for reasons I eventually worked out, but won't bore you all with). Dave Bridgham managed to mount that under Linux, and transform it into a TAR file. That was the source of many old treasures, including the V6 NCP UNIX. Noel

3 years, 5 months

PL/I stuff - was: Book Recommendation

by Douglas McIlroy

> What, if any, features does PL/I have that are not realized in a modern language? Here are a few dredged from the mental cranny where they have mouldered for 50+ years. 1. Assignment by name. If A and B are structs (not official PL/I terminology), then A + B A = B copies similarly named fields of B to corresponding fields in A. 2. Both binary and decimal data with arithmetic rounded to any specified precision. 3. Bit strings of arbitrary length, with bitwise Boolean operations plus substr and catenation. 4. A named array is normally passed by reference, as in F(A). But if the argument is not a bare name, as in F((A)), it is passed by value. 5. IO by name. On input this behaves like assignment from a constant, with appropriate type conversion. 6. A SORT statement. 7. Astonishingly complete set of implicit data conversions. E.g. if X is floating-point and S is a string, the assignment X = S works when S = "2" and raises an exception (not PL/I terminology) when S = "A". My 1967 contribution to ACM collected algorithms exploited 3 and 4. I don't know another language in which that algorithm is as succinct. Doug

3 years, 5 months

Re: [TUHS] 9-track tape (was PL/I stuff)

by Paul Winalski

DEC's VAX/VMS group got a customer bug report that was accompanied by a 9-track tape containing the programs and data necessary to reproduce the problem. When the engineer mounted the tape, it contained completely different data. He tried a different tape drive and this time he got the expected data. It turned out that the customer had reused the tape and recorded the reproducer at 1600 bpi on top of previous data recorded at 800 bpi. If the tape was mounted such that the drive didn't see the PE burst, it could still read the NRZI-encoded 800 bpi data. -Paul W.

3 years, 5 months

Re: [TUHS] Book Recommendation

by Douglas McIlroy

The following remark stirred old memories. Apologies for straying off the path of TUHS. > I have gotten the impression that [PL/I] was a language that was beloved by no one. As I was a designer of PL/I, an implementer of EPL (the preliminary PL/I compiler used to build Multics), and author of the first PL/I program to appear in the ACM Collected Algorithms, it's a bit hard to admit that PL/I was "insignificant". I'm proud, though, of having conceived the SIGNAL statement, which pioneered exception handling, and the USES and SETS attributes, which unfortunately sank into oblivion. I also spurred Bud Lawson to invent -> for pointer-chasing. The former notation C(B(A)) became A->B->C. This was PL/I's gift to C. After the ACM program I never wrote another line of PL/I. Gratification finally came forty years on when I met a retired programmer who, unaware of my PL/I connection, volunteered that she had loved PL/I above all other programming languages. Doug

3 years, 5 months

IBM Unix

by Ron Natalie

I remember getting an early RT without being given the root password. Now there hadn’t been too many Unix boxes I wasn’t able to break into. The Rt you could turn the key to the wrench symbol and get into the maintenance menus. Selecting to view some document which was displayed with more and that you could bang a root shell out of. > On Nov 24, 2021, at 14:42, Clem Cole <clemc(a)ccc.com> wrote: > > > > >> On Wed, Nov 24, 2021 at 1:43 PM Larry McVoy <lm(a)mcvoy.com> wrote: >> SMIT - just say no. > There were a number of things IBM did well with AIX so I'm not quite so glib knocking everything from them. > >

3 years, 5 months

Re: [TUHS] Book Recommendation

by Norman Wilson

Larry McVoy: Say hi to Barry for me, I knew him back in the UUCP days, he was always pleasant. ==== And for me. Knew him back in my DECUS days. Also tell him that since he runs the World, it's all his fault. Norman Wilson Toronto ON

3 years, 5 months

Two anecdotes

by Alan Glasser

Here are two anecdotes that Doug suggested I share with TUHS (I am new to TUHS, having joined just last month). *First*: *The creation of access(2).* [Marc Rochkind documented a version of this on page 56 of his book *Advanced Unix Programming* (1985, First Edition) discussing link(2). The footnote on that page says "Alan L. Glasser and I used this scheme to break into Dennis Ritchie and Ken Thompson's system back in 1973 or 1974."] Doug pointed out that the timing was probably later, as access(2) was not in the Sixth Edition release, but probably right after the release (after May 1975?). It arose from a discussion I was having with Marc, with whom I worked on SCCS and other PWB tools. We were discussing some mechanism that would require moving directories (actually, simple renaming) in a shell procedure. I told Marc that only root could make links to directories or unlink directories, but he told me that he has renamed directories with the mv command. I said then mv must be setuid to root, so we looked, and, of course, it was. I then looked at the source code for mv and quickly saw that there was no attempt to check permission on the real uid. So I told Marc it would allow anyone to become root. He wanted to see it in action, so I logged into research (I don’t remember what our organization's shared login was). No one in our organization had root access on research. Marc and I didn't have root access on our organization's machines; Dick Haight et. al. didn't share that privilege (Dick was the manager of the super-users). I think the actual sequence of commands was: cd / cp etc/passwd tmp ed tmp/passwd 1s/^root:[^:]*:/root::/ w q mv etc etc2 mv tmp etc su mv etc tmp mv etc2 etc mv etc/as2 etc/.as2 {logout, hangup and wonder} The last bit was a test to see what was noticed about what I did. Marc and I talked for a while about it and discussed if we had any need to be root on our local machines, but couldn't think of any pressing need, but knowing we could was a bit of a comfort. After a short time, Marc suggested logging back in to see what, if anything, had been done. /bin/mv had lost setuid to root /etc/as2 was restored /etc/.as2 was nonexistent And the next day, the motd on research mentioned that there's a new syscall: access. And mv(1) now used it. *Second*: Our organization was one (out of possibly others) subject of Ken's *codenih* that he documented in his Turing Award article in CACM. As previously described, root access was closely guarded in the PWB organization and, according to Doug, freely available in research. Ken had given us a login that was shared by PWB development and we had given Ken a login to our systems. We had no root access on research and Ken had no root access on our systems. Our C compiler guy, Rich Graveman, who kept in close contact with Dennis and was always getting us the latest compiler to install, had gone to MH and came back with a tape of a new compiler. Rich, being a careful fellow, did a size on c0, c1, c2 on the files from the tape and did the same on the running compiler pieces in /lib. Lo and behold, he discovered that the new compiler from Dennis was smaller than the old compiler even though it had a whole new feature (I think it was union). So Rich did nm's on the two different c0's and discovered a name "codenih" in the old compiler that wasn't in the new one from Dennis. He logged into research, cd'ed to /usr/ken and did an ls -ld codenih, followed by a cd codenih. Was he surprised! Then he went back to a local machine and tried to login as root/codenih, which, of course, worked. He was even more surprised and told a number of colleagues, myself included. (I logged into research and printed out the source in /usr/ken/codenih. I was super impressed.) I think you misunderstood the codenih bit. As Ken had given us a (shared among a few of us) login, we had given him one. And Dick Haight refused him root access. And no one in PY had root access on research. So much for denying Ken root access on the PWB systems. Ken "infected" the PWB C compiler with codenih, which gave him free rein. I don't know how or when he first installed it, but I suspect he was aware of any extant security holes (e.g., the mv setuid root) to allow him to replace the compiler the first time. I don't know if the PWB crowd was the impetus for Ken writing codenih or if it was something he had used on others prior to us or if he ever used it on anyone else. Needless to say, Dick Haight was beside himself. I just thought it was a great feat of programming and was thrilled when he described it in CACM. Alan

3 years, 5 months

Re: [TUHS] Recreation of the PDP-7 UNIX TMG compiler compiler

by Douglas McIlroy

> I was deeply motivated by TMG. The good news is that you could say what > you wanted and it just did it. The bad news was the error handling. > Because it was recursive if you made a syntax error the program > backtracked and tried again. And again. And again. And eventually was > back at the first character of the input with nowhere to go. So it > issued one of its two messages -- "Syntax Error". This is somewhat of a caricature. If one's compilation strategy were to build an abstract syntax tree of the whole source program before emitting any object code, then things would go as Steve describes. In reality, TMG wasn't used this way. For one thing, ASTs needed too much memory. In TMG one could emit code while parsing. Typically code was generated on a statement-by-statement basis. This limited the backtracking, so even a naive "syntax error" diagnostic could be localized to the statement level. Long-distance connections, such as the branch labels in a nest of if statements, could nevertheless be realized recursively. Thus, in actual use, a TMG "grammar" was partly a parsing program and partly abstract specification. The balance of viewpoints was left to the discretion of the grammar's author. Yacc swung the pendulum toward the abstract. Doug

3 years, 5 months

> Who asked Ken for grep?

by Douglas McIlroy

I can say unequivocally that I asked Ken for grep, and the next day it was in /bin. But ... I understand now that grep already existed in /usr/ken/bin. My request was the last straw that tipped the balance from private to public. Hopefully Ken can answer whether he made it originally for himself or for Lee. Doug

3 years, 5 months

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