From the 2.11 BSD sources I understand that the PDP-11/70 MMU address and data registers, KDSA and KDSD, start at 0172360 and 0172320 respectively [1]. Yet, when I read the register contents I don't get what I would expect to see: increasing by 0200 memory values for KDSA and the same constant value for KDSD [2]. I checked this by looking at /dev/mem. # od -o /dev/mem 0172360 | head -1 0172360 000002 000016 001403 012700 000400 000402 012700 000200 # od /dev/mem 0172320 | head -1 0172320 101016 005064 000026 005067 175456 016467 000006 175430 I get the same results when I examine the memory through SIMH: sim> examine 172360 172360: 000002 sim> examine 172362 172362: 000016 sim> examine 172364 172364: 001403 sim> examine 172320 172320: 101016 sim> examine 172322 172322: 005064 The MMU kernel instruction registers, KISA and KISD, contain similarly nonsensical values as do the registers located at a different memory location (077320, 0772360) indicated in another source [3]. What am I missing? My goal is to access from the console the kernel's u area. According to mem(4) and the symbols in /unix, this should be at address 0140000. Indeed, accessing it through /dev/kmem I get the expected results for e.g. u_comm and u_uid. However, I have been unable to find it in the machine's physical memory, hence my question regarding the MMU's operation. [1] https://github.com/RetroBSD/2.11BSD/blob/master/usr/sys/pdpstand/M.s#L346 [2] https://github.com/RetroBSD/2.11BSD/blob/master/usr/sys/pdpstand/M.s#L247 [3] https://gunkies.org/wiki/PDP-11_Memory_Management Diomidis