< Alright, so what we have is the last 4KW used up for stack < space, register mapping, and IO mapping. I would guess the and also boot proms. Default boot on PDP11 is 173000Q. The first page is where all the vectors for the interrupts and traps are. < MMU... do I have it right? Even on a machine with a full < 22bits of address lines on the backplane, like an 11/73, the < CPU still only has a 16bit address space. The faq doesn't make < it terribly clear what happens if you want to open up any < arbitrary window. Think of the 4mb (q22) as linear physical space. At any time the CPU/MMU can allocate contigious or scattered blocks of that space as logical physical space. To do a "long jump" a local jump to a system space is done, memory management code is run and then a jump to the now available code (in logical space) is done. One of the background jobs would be to swap out old segments that are unused to make room for current processes and swap them back if the non current process should wake up. The cpu is always in logical 16bit space but the windows (multiple) can be moved around. An ascii graphic would show the CPU 16bit space as several blocks mapped to multiple blocks scattered in a larger space. The cpu literally never leaves the range of a 16bit space but instead trade out chunks of it for different ones out of an available pool. You never actually jump put of logical space only shuffle what physical memory is part of that space. Allison