Before I go into the detail, let me first say that this bug likely is as old as V5.5 (I only have looked at the V5.6 code on a friend's system) which is when extended device drivers were first introduced. So, we can say that the bug is about 10 years old and qualifies for this list since V5.5 of RT-11 was released in 1989. The bug in RT-11 occurs when RT-11 has had a SYSGEN to allow extended device drivers and a device driver which is extended (allows more than 8 devices - in my case, I was using a DU: MSCP device driver which allowed 64 partitions). In a test case, the following sequence produces a crash: SRUN KEX.SAV/LEVEL:n/NAME:KEXn for n = 1 => 6 after each do a CTRL/B to return to the background Then do: ABORT KEXn for n = 1 => 6 UNLOAD KEXn for n = 1 => 6 I tried this under RT11XM on V5.6 with an RL02 being the boot device and DU: being auto-installed and not referenced the whole time. In the actual situation, DU: was being referenced on a magneto optical disk drive to obtain specified files, but the DU: device driver was not LOADed since there was insufficient space left in low memory. So, although there are probably a number of work arounds, the real problem is that UNLOAD has a bug and does not work correctly in these circumstances. UNLOAD in KMOVLY has a bug, as far as I can understand. In fact, possibly more than one. But, for now, just one that I can handle. It would seem, in addition, that co-ordination between different portions of the monitor may not have been done very successfully since the USR does have the correct code to handle the situation (a "Beq" instruction) while UNLOAD seems to want to ignore the problem. Of course, if the USR had not handled the problem correctly, there are likely going to be many more occasions when the bug would have occurred, so in the USR it was caught and the code is correct. The exact description of the bug and owner tables may not be correct. If so, please refer to the SSM. But the essential nature of the bug is, I believe, correctly described. THE PROBLEM, from what I can understand, is that in UNLOAD, a system job MAY "own" a specific device (done via a LOAD command). There is a two word owner table entry for each device which has 4 bits allowed for each of the possible 8 drives normally associated (DU0: => DU7:) with each device driver. When a SYSGEN is done which includes extended device drivers, that owner table entry of two words is too small and is used to "point" to a 16 word (maximum size) table within the device driver ONLY when the device driver is LOADed (I presume that a .FETCH may also allocate the same 16 words, but they would not be used). SO IF THE DEVICE DRIVER IS INSTALLed, BUT NOT LOADed, the two word owner table entry can't "point" anywhere and the pointer word is set at a default of zero. In the USR, when that word is picked up, a "Beq" is used to detect that the device driver has not yet been LOADed and no owner specific code is executed. BUT, UNLOAD in KMOVLY does not have that instruction ("Beq") and merrily goes and assumes that the extended device driver owner table entry (in the case of an extended device driver SYSGENed system PLUS an actual extended device driver such as DU:) is at the location starting at zero. In the process of disconnecting the system job from a device driver, the first 16 words in low memory are assumed to contain the owner table entry AND the 8 vectors there (00 => 34 - which obviously includes the EMT vector) can be "MODIFIED". Which results in crashes in RT-11. If anyone is truly interested in this bug, but does not understand what I have stated - the explanation I have given is only a small portion of all the detail, please inquire further. Since this bug has not been encountered before - or if anyone has but was unable to track it down - then likely the situation does not occur very often. When DU: is resident, obviously DU: has been LOADed. But I suspect that an extended LD: which is not LOADed could also cause the same problem. The simple solution that I was told about a number of years ago (when I had not yet been informed that it was UNLOAD which was causing the problem) is to not do the UNLOAD, but to instead do a BOOT which, of course, does an UNLOAD of everything. Sincerely yours, Jerome Fine