Hopefully, this request is clear enough to be understood. Both the software and the hardware portion of the questions are independently important, so please answer one aspect even if you can't help with the other. Over the past 30 years of using PDP-11 software (RT-11 over 95%) and hardware, I have never had occasion to use a Unibus system with more than 256 KB of memory (such as a PDP-11/34). I would appreciate help in understanding the Unibus Map hardware which (if I understand its purpose correctly) is to convert 18 bit addresses given to a Unibus controller into 22 bit addresses for real physical memory for systems like a PDP-11/84 and perform DMA from / to the hard drive. Also helpful would be an explanation of the related software used under RT-11 along with exactly where the Unibus Map hardware is located on a real DEC system (on the CPU board I presume) since the identical CPU board is used for both the Qbus and the Unibus with both the PDP-11/84 and the PDP-11/94. ------------------------------------------------------------------------- DO NOT READ THE FOLLOWING if you don't use RT-11!!!!!!! By way of context, I have successfully modified the HD0: device driver originally written by John Wilson which obviously executes ONLY under E11. Last week, John mentioned in a private e-mail that it is possible to direct the HD0: "controller" (all references to hardware in quotes refers to E11 software) to ignore the "Unibus Map hardware". Since I execute under E11 using Qbus emulation in order to stay totally compatible with the real DEC PDP-11/23, PDP-11/73 and PDP-11/83 systems (all Qbus and all with 4 MB of physical memory) which I have available for comparison testing, user buffers in physical memory above 256 KB are often a situation which can't be avoided. Consequently, it had always been a high priority to have a 22 bit device driver for HD0: under E11 BECAUSE HD0: is TWICE as fast as MSCP (DUX.SYS device driver). In addition, I have also been able to write code that avoids the overhead of using the HD0: device driver which makes HD0: FOUR times as fast as MSCP and also allows for direct access of 2 TB of disk space via a 32 bit block number. Since I have an immediate application for a data base of 32 GB (I have a disk drive of 160 GB, so 32 GB is only 20% of its capacity - although under RT-11 15 years ago in 1992, 32 GB would have been only a dream), the code which I have written will actually be used quite soon. However, I would very much like the HD0: device driver to be able to execute under both "Qbus" and "Unibus" hardware. And while I can test the code for a "Qbus" and for a "Unibus" without a "Unibus Map", I am not clear about what is required for a "Unibus" with a "Unibus Map" such as a PDP-11/84 running under RT-11. Megan are you still watching the list? Allison, if you can't answer, can anyone but Megan help? Is anyone else familiar enough with the "Unibus Map" that you can suggest what RT-11 actually needs to do with the "Unibus Map"? A solution is to check the "hardware" configuration and refuse to allow the HD0: device driver to LOAD or .Fetch if the modified version is not executing under "Qbus hardware". By the way, if anyone who is thinking of a controller for the Qbus which is able to use SATA drives, I would be happy to modify an RT-11 device driver to an HD0: type device that is able to handle drives up to 2 TB in the same manner that DU(X).SYS can handle drives up to 8 GB. Anyone interested?? Sincerely yours, Jerome Fine -- If you attempted to send a reply and the original e-mail address has been discontinued due a high volume of junk e-mail, then the semi-permanent e-mail address can be obtained by replacing the four characters preceding the 'at' with the four digits of the current year.