Jerome Fine replies: Assuming that your internet search and math are correct, you do have a 1 MByte memory board. I don't have the time at the moment to check it out, but I tend to believe you are correct since I have never run across a 4 MByte memory board from National Semiconductor. YES!! It will work in a PDP-11/83, BUT as only ordinary memory, NOT as PMI memory. With your memory board, the CPU is in the first slot and the memory is usually in the next slot(s), but the latter is NOT a requirement. You are correct that a PDP-11/83 will allow PMI memory. However, ALL KDJ11-Bx boards are able to use PMI memory since the CPU module is what activates PMI memory provided the PMI memory board is placed in the FIRST memory slot(s) and FOLLOWED by the KDJ11-Bx module. IN ADDITION, ALL of the slots which hold the memory and CPU MUST be ABCD slots. In a BA23, the first 3 slots are ABCD followed by 5 ABAB slots. In a BA123, the first 4 slots are ABCD followed by 8 ABAB slots. In BOTH the BA23 and the BA123, the ABAB slots are serpentine with regard to the bus grant. Ask if you have a question! I will keep you in mind if I ever dispose of my PMI memory. Any other questions? 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.