PDP-11's (LSI-11's) were originally limited to 32k 16-bit words (64kb) but were exanded to 18bit addressing allowing up to 256Kb. The PC is only 16 bit though s

PDP-11's (LSI-11's) were originally limited to 32k 16-bit words (64kb) but were exanded to 18bit addressing allowing up to 256Kb. The PC is only 16 bit though so 64k of code directly addressable at any one time (ignoring the reserved area for memory mapped hardware). The MMU was pretty simple so easy enough to access the extra memory without relying on the OS (simpler under RT-11 than RSTS/E). I still have shelves of DEC documentation (and a few PDP-11's).