Jerome Fine replies: In general, there should never be a reason to alter the jumpers. Since 4 MBytes is the maximum allowed for the PDP-11, the hardware would almost always be set to use all of the memory. There is probably a bank of jumpers for the starting address and a second for the ending address. The starting address will probably be empty - for zero. The ending address could be full or empty. I suggest that if you can't find a manual for the settings to just use the board as is. 99.9% of the time, I would expect the setting to reflect use of all of the memory - if that is what you wish - and I can't really understand any other option for 99.9% of the cases.