Speaking of 11/73s

Jerome Fine replies: FIRST, you require a Qbus backplane with as many slots as the memory and CPU will occupy. In general, I have heard that you should NOT mix boards with different sizes of PMI memory - Since I have two MSV11-JE boards (2 MByte PMI memory - the maximum DEC made was a 2 MByte memory board - I am told that Chrislen made a 4 MByte board - anyone have it at a reasonable price, especially if you are not using it anymore), that is not a problem. The DEC MSV11-JD was a 1 MByte memory board. These two boards were to be used ONLY for the Qbus. On an 11/84 system, DEC managed to require the use of either the MSV11-JB (1 MByte) or the MSV11-JC (2 Mbyte). I have been told to NEVER mix these boards between an 11/83 and an 11/84 system. Don't know why. Now, assuming you have 2 (or 1) MSV11-JE memory, you MUST place it in an ABCD slot above the CPU board which must be the KDJ11-BA,B,C (M8190-"blank",YB) which is the quad version of the 11/73. PMI will not work with the KDJ11-AA,B,C which is the dual version of the 11/73 (M8192) or at least if it does (I have never bothered to try), it will operate as ordinary Qbus memory regardless of where it is placed in the backplane. I also them place the 11/73 CPU board as the very next board right after the PMI memory and the CPU board MUST also occupy an ABCD slot. If you are using a BA23 with 3 ABCD slots (all BA23s that I know of have exactly 3 ABCD slots), the the first 2 slots MUST be the PMI memory and the 3rd slot MUST be the quad 11/73 CPU. If you have only one PMI memory board, make it the first slot and the CPU the 2nd slot. I presume (I have never had 3 * 1 MByte boards of PMI memory) that on a BA123 with 4 ABCD slots, it is possible to have 3 PMI memory boards at the top and the CPU in the 4th slot. If you place the PMI memory under the CPU, it still works as normal Qbus memory. Someone (either Allison or Megan Gentry) once explained how PMI memory works, but I forget. I have never tried to use one board above and one board below - has anyone ever done so? Based on a VERY limited set of benchmarks, PMI memory seems to speed up the system (CPU wise) by about 20%. Moving to a KDJ11-EF (11/83) seems to add another 13% for a total of about 33% between an 11/73 at 15 MHz without PMI memory and an 11/83 with PMI memory. So if you want to increase the speed, the major boast comes from PMI memory and works with all quad 11/73 that I have tested PMI memory with; just place it on top with memory and CPU underneath, all in ABCD slots. I have been told that PMI memory will not function as PMI memory with an 11/53. The increase in speed is purely a hardware function and does not depend on the OS or anything else, although I have seen RT-11 able to "know" that PMI memory is or is not present - have not ever bothered to find out how. Sincerely yours, Jerome Fine