TUHS December 2002

tuhs@tuhs.org

21 participants
40 discussions

Re: [TUHS] In V6 exit(): wakeup(&proc[1])) unnecessary?

by Wolfgang Helbig

>>When a process terminates (in sys1.c/exit()), it explicitly wakes up >>the init process. In light of the fact, that every process has a parent, >>this extra wakeup(&proc[1]) seems unnecessary. > >I believe this is meant to handle the case where a process exits >without having called wait() on its children. > >Imagine two processes, A and B, where A is B's parent and A's parent >is some process other than init. B calls exit(), becomes a zombie, >and awakens A. Some time later, A calls exit() without ever having >called wait(). This will cause B to become a child of init, and as B >is a zombie, init should awaken so that B can die. However, A might >have any number of zombie children, and it would be redundant to call >wakeup(&proc[1]) for each one. Therefore, the wakeup call to init is >done up front, whether it is needed or not. This does result in some >unnecessary wakeup calls but on the whole it seems a good tradeoff for >code simplicity. Oh, I see! So the condition to wakeup init would be "Do I have any zombie children"? The code as it is is simpler. But harder to understand. Thank you very much for your help! Greetings Wolfgang Helbig

21 years, 4 months

In V6 exit(): wakeup(&proc[1])) unnecessary?

by Wolfgang Helbig

Hi, encouraged by the profound answers I thankfully recieved to my previous question about the Unix V6 kernel, here is another one: When a process terminates (in sys1.c/exit()), it explicitly wakes up the init process. In light of the fact, that every process has a parent, this extra wakeup(&proc[1]) seems unnecessary. Furthermore, the "goto loop:" at the end of exit() will never be executed. So is this just defensive programming, or did I miss something? Greetings, Wolfgang Helbig

21 years, 4 months

[pups] 11/34

by Robin Birch

Dear All, There is a complete 11/34 for sale on EBay (1 day to go). It comes with a complete V6 with source. Somebody buy it please. This is nothing to do with me, I just saw it tonight. It's in Pontypridd South Wales Cheers Robin -- Robin Birch

21 years, 4 months

RE: [TUHS] re: Raise of interrupt level necessary

by Ian King

Yes, I have v6 running on an 11/34; it's amazing how easy it is to corrupt the filesystem. :-) One of these days, I need to dd another fresh system disk (RK05) off my "sacred copy" of a PUPS image (tediously transferred via 9600 baud serial using Warren's excellent vtserver tools). In any case, why do you ask? - Ian -----Original Message----- From: Wolfgang Helbig [mailto:helbig@Informatik.BA-Stuttgart.DE] Sent: Saturday, December 14, 2002 12:48 AM To: tuhs(a)minnie.tuhs.org Subject: Re: [TUHS] re: Raise of interrupt level necessary [snip] I also removed IPL protection from the return sequence of the trap routine, still works. Testing was done on Bob Supnik's SIMH PDP-11 simulator with only one terminal. By the way, is anyone running V6 on a real PDP-11? [snip]

21 years, 4 months

re: Raise of interrupt level necessary

by Wolfgang Helbig

>Date: Sat, 14 Dec 2002 15:27:40 -0800 Michael Davidson wrote: >If you think that the "rule" is "no ISR can touch user space while >interrupting a kernel process", then it is display() that breaks >the rule. The point is, that this rule leads to somewhat simpler and therefore better code -- breaking the rule calls for some justification. Calling display() from the clock-ISR is the only place where an ISR does not conform to this rule. Greetings Wolfgang Helbig

21 years, 4 months

Re: [TUHS] re: Raise of interrupt level necessary

by Wolfgang Helbig

Dennis, I greatly appreciate your reply. What amazes me most, is the fact that after 20 odd years you still remember those tricky details: >For example, the clock interrupt routine can inspect user >space (for display(), for example, though this is inactive >on the 11/40), and even change it (for profiling). >It does appear that other things (checking for >a direct-from user-mode interrupt) in the clock routine >already guard against these particular problems. To see what happens, I just removed those IPL raising instructions from gword, pword,_savu, _aretu, _copyseg, and _clearseg in m40.s. In fact, it didn't crash the system. At least on my configuration, ISRs don't touch user space while interrupting a kernel process. This rule is broken by the clock ISR, which calls display() unconditionally. Just curious: Is there any other reason beside seeing blinking lights that justifies calling display() 60 times per second and breaking the rule? I also removed IPL protection from the return sequence of the trap routine, still works. Testing was done on Bob Supnik's SIMH PDP-11 simulator with only one terminal. By the way, is anyone running V6 on a real PDP-11? Here is the diff of my modifications to m40.s: # diff m40.s.6 m40.s 44d43 * bis $340,PS 455,456d453 * mov PS,-(sp) * bis $340,PS 472,473d468 * mov PS,-(sp) * bis $340,PS 480d474 * mov (sp)+,PS 485d478 * mov (sp)+,PS 539d531 * bis $340,PS 548d539 * bis $340,PS 594c585 * mov $30340,PS --- . bis $30000,PS 621c612 * mov $30340,PS --- . bis $30000,PS Greetings, Wolfgang Helbig

21 years, 4 months

qbus scsi controllers

by Robertdkeys＠aol.com

I know it causes one to salivate like Pavlov's dogs, even thinking about them, but, anyone know of any qbus scsi controllers that might someday come available at anything like reasonable sorts of guildern de realme? (running and ducking for cover....(:+]].....) Just thought I would check. Thanks Bob

21 years, 4 months

Re: [TUHS] flaky webster or sigma WQESD4 and RQD11 controllers on KA650???

by Robertdkeys＠aol.com

The particular webster wqesd4 I have looks like about an '86 or so board, from chip dates, and the main chip that is socketed is marked with "ENGR-SAMPLE"...doh.... Maybe I do have a very early board that won't run with a faster cpu. The rom is a part no. 080469-01 REV 01, if that means anything to anyone, as marked on its paper label. The later sigma board does not have the problem. Oh, well, if I have to run it with a KA630 at slo-poke gait, so be it....(:+\\... There are two diodes, one looks like an ordinary silicon rectifier and one looks like a zener of some sort, possibly, off pin 19 of the 74ls240 at position V2 on the board. Those come off the J6 pin 10 of 10, and are in parallel. Thanks Bob

21 years, 4 months

flaky webster or sigma WQESD4 and RQD11 controllers on KA650???

by John Holden

Early ROM revisions on the Webster MSCP controllers were too slow for the KA650 and would often loose interrupts (amoungst other things). Signals such as NPR and BR grants were polled from a pseudo microcode loop which took too long. The RS232 interface doesn't use standard receivers. Check for a 5 volt zener on the input of an 74LS240 (from the 10 pin i/o connector) and add a schottky in parallel (a 1N5818 would do) if one isn't there

21 years, 4 months

flaky webster or sigma WQESD4 and RQD11 controllers on KA650???

by norman＠nose.cs.utoronto.ca

If you can read this, the 4-port Wombat (Webster/Sigma ESDI controller) in the KA650 system through which this mail must pass on its way out is working fine, as it has since I installed it in that system a year and a half ago. According to my notes, I had no trouble communicating with the onboard diagnostic monitor (with the board in another KA650 reserved for system testing). I used the monitor quite a bit, starting it many times, first because it took me a long time to realize that the terminator had been left installed backwards in one of the drives (so the drive wasn't visible to the controller no matter what I did to the cables). I don't think I tried the serial port on the back of the Wombat board; instead I uttered the magic commands to load and run the Wombat communication program on the VAX itself. I'd expect this to be more timing-sensitive than the hardware port. In any case it worked fine. Once the controller could see the disks (and I'd done some formatting and some testing with the diagnostics), I tested the Wombat under my oddball version of UNIX, and found what may be an MSCP implementation botch. A UNIBUS/QBUS MSCP storage port gets only one interrupt vector; it is supposed to set one of two flags to indicate whether it is interrupting because there were no command slots and it freed one, or because there were no messages waiting to be read and it supplied one. The Wombat appeared often to be tardy in setting the `message waiting' flag; my port driver often received an interrupt, checked the flags, found nothing to do, and of course dismissed the interrupt without realizing that the controller had in fact reported an I/O completion. (And of course if I halted the system and took a crash dump I found the flag set. It took some real-time tracing to uncover the problem.) It is easy to work around this--instead of relying on the flag, one can go look at the message descriptors to see if there's a new message-- so that is what my driver now does. I have a vague memory that the original Ultrix MSCP driver, whence the current BSD one is probably descended and the current Ultrix one more distantly, already did it that way. Silly me, I programmed from the protocol spec rather than just blindly copying someone else's unexplained code ... None of this explains why Bob Keys is having trouble, but it does suggest to me that the Wombat firmware might have other subtle bugs that are tickled by particular systems, perhaps even timing-dependent problems. (I haven't checked, but it could well be that the interrupt- flag bug would be invisible to a KA630 because it took a little longer to get to that part of the interrupt routine.) I have two Wombat boards with slightly different firmware. (One is labelled RQD11-EC and its diagnostic monitor calls itself WOMBAT; the other is branded by SpectraLogic, I forget what model it calls itself, but the monitor calls itself SLEUTH.) My notes don't say whether they both displayed the interrupt botch. I do know that communications with the diagnostic monitor worked fine with both. The RQD11-EC claimed to have firmware (or perhaps diagnostic monitor) version 2.38. Norman Wilson Toronto ON

21 years, 4 months

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TUHS December 2002