[TUHS] Accessing the PDP-11/70 MMU registers and the kernel's u area

Noel Chiappa jnc at mercury.lcs.mit.edu
Mon Jun 15 00:46:43 AEST 2020

    > From: Diomidis Spinellis

    > 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 ...

Expressed as 16-bit addreses, on a PDP-11 with mapping disabled, yes.

    > I checked this by looking at /dev/mem. 

I don't know about 2.11, but in other PDP-11 Unixes, /dev/mem gives access to
the actual CPU memory bus (which on a /34, etc, is the 18-bit address UNIBUS;
on a /70 it's a separate 22-bit address bus).  In the /70 memory address
space, the 'I/O page' (which is where the PxR's live) is at the top end of it,
i.e. the registers are at 017772360 (KDSAR0), etc.

    > What am I missing?

PDP-11's have a plethora of address spaces, of different sizes. You need to
always be aware of which one you're working in.

    > 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.

In the kernel virtual address space, yes.

    > Indeed, accessing it through /dev/kmem I get the expected results for
    > e.g. u_comm and u_uid.

Because /dev/kmem gives access to kernel address space for the _current_

    > I have been unable to find it in the machine's physical memory

By far and away the easiest thing, for the _current_ process, is to
use /dev/kmem, which automagally applies the correct mapping.

For other processes, if the process is swapped in, there's some field in the
proc structure which says where in physical memory it us. IIRC, the user
struct and the kernel stack are stored in the very bottom of that.

(This article:


goes into some detail for V6. Not sure how different 2.11 is; I know it uses
one block of kernel address space to map in code overlays, but I don't know
all the details of how it works.)

Anyway, using that, one could read the user area in /dev/mem, at the
appropriate location.

For swapped-out processes, a similar algorithm applies, but you'll
have to look in the swap device (obviously).


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