Path: utzoo!utgpu!watmath!att!tut.cis.ohio-state.edu!bloom-beacon!usc!cs.utexas.edu!uunet!ncrlnk!ncr-sd!tw-rnd!johnl From: johnl@tw-rnd.SanDiego.NCR.COM (John Lindwall) Newsgroups: comp.sys.atari.st Subject: Re: Multitasking on the ST Message-ID: <483@tw-rnd.SanDiego.NCR.COM> Date: 14 Aug 89 23:00:05 GMT References: <8908021826.AA05333@ucbvax.Berkeley.EDU> <15627@watdragon.waterloo.edu> <652@opal.tubopal.UUCP> <471@tw-rnd.SanDiego.NCR.COM> <1066@philmds.UUCP> <482@tw-rnd.SanDiego.NCR.COM> <1071@philmds.UUCP> Reply-To: johnl@tw-rnd.SanDiego.NCR.COM (John Lindwall) Organization: NCR Distributed Systems Laboratory Lines: 93 In article <1071@philmds.UUCP> leo@philmds.UUCP (Leo de Wit) writes: >In article <482@tw-rnd.SanDiego.NCR.COM> johnl@tw-rnd.SanDiego.NCR.COM () writes: >|In article <1066@philmds.UUCP> leo@philmds.UUCP (Leo de Wit) writes: > [] >|>Why should memory protection be a hotter item when parallel processes >|>are involved? >| >|Because of the potential for a single user program to cause the termination >|of all the processes in the system. > >This is equally well possible in the current 'one-process-running-at-a-time' >scheme. Note that there are already accessory-based editors, batch >modem transfer programs, TSR print spoolers for the ST right now. > This point is well taken, but does not negate the point that process protection is desirable (in my mind). >| [My (John Lindwall's) example of a single process crashing the machine] > >I think you'll need a bit more than just an MMU for a secure system. >S'pose your nasty process alters some system vector (applying patch 271 >to SAFEDOS). A pity that the last bug was not yet removed... My point >is, that a MMU is a probably undispensable ingredient IN AN OTHERWISE >EXCELLENT SYSTEM. That safe system (which undoubtedly will have a >notion of privileges, users) is what you should start with in the first >place. > An MMU can protect pages of memory that the system considers special. The system vectors could be marked as un-writable by user level processes. An attempt to modify the protected pages could be trapped and prevented. I agree that the capabilities of an MMU are best utilized when designed into a system as opposed to grafted in as an afterthought. Current users of Commodore's 68020/68030 boards have an MMU in the system which has minimal benefit to the system, because AmigaDOS was not designed to support an MMU. In fact the only use I see for it (in the Amiga at present) is in copying the OS ROM into fast ram for a speed gain. True use of the MMU comes when running an OS designed to use it (like Unix when that becomes available). I am using the Amiga as an example here not because I believe it is the ULTIMATE SYSTEM that Atari should emulate. I feel ST users and developers can benefit from examining the problems and shortcomings that the Amiga is seeing now that MMU's are being phased in as standard equipment. >B.T.W. what do you do when a thunderstorm is coming up, but your >raytracer has yet to finish its last hour of calculations? Use a TMU :-) ? > Well, here in Sunny California (tm) we don't get many of those! :) :) :) >|I'm all for system robustness for ANY system. My point is that when you >|introduce multitasking, memory protection is more important due to the >|potential to disrupt other processes. > >I heard this one before and I still won't believe it, unless a proper >argument is given. > So I assume (if you were using a multi-tasking system) that you would prefer NOT to have process protection? I do not see the logic in this. >|>[about VM] >| >|Sounds Good! But now you're wandering into the area of virtual memory and >|that's not what our original discussion was about. Thanks for the reply. > >You bought an MMU but you don't want VM? Gee, that would be the first >reason I would buy an MMU for (and not for protection). As long as the >filesystems used are single-user, not write protectable, I don't care >much for safe core. > Yes, VM is nice. In my day-to-day Amiga experience I do not run out of memory much. I DO experience agonizing crashes which kill all my processes. From this experience I developed the priority of process protection being more important. If VM were available, I'm sure I would enjoy it and make use of it. Thank you for this enjoyable discussion. I hope it can continue on the amiable and informative level that has been maintained all along. Looking forward to your reply. ---------------------------------------------------------------------- John Lindwall johnl@tw-rnd.SanDiego.NCR.COM "Above opinions are my own, not my employer's" Health is merely the slowest possible rate at which one can die. -- ---------------------------------------------------------------------- John Lindwall johnl@tw-rnd.SanDiego.NCR.COM "Above opinions are my own, not my employer's" Health is merely the slowest possible rate at which one can die.