[TUHS] /dev/drum

[jnc@mercury.lcs.mit.edu (Noel Chiappa)]

    > From: Johnny Billquist

    > Gah. If I were to try and collect every copy made, it would be quite a
    > collection.

Well, just the 'processor handbook's (the little paperback things), I have
about 30. (If you add devices, that probably doubles it.) I think my
collection is complete.


    > So there was a total change in terminology early in the 11/45 life, it
    > would appear. I wonder why. ... I probably would not blame some market
    > droids.

I was joking, but also serious. I really do think it was most likely
marketing-driven. (See below for why I don't think it was engineering-driven,
which leaves....)

I wonder if there's anything in the DEC archives (a big chunk of which are now
at the CHM) which would shed any light? Some of the archives are online there,
e.g.:

  http://www.bitsavers.org/pdf/dec/pdp11/memos/

but it seems to be mostly engineering (although there's some that would be
characterized as marketing).


    > one of the most important differences between segmentation and pages are
    > that with segmentation you only have one contiguous range of memory,
    > described by a base and a length register. This will be a contiguous
    > range of memory both in virtual memory, and in physical memory.

I agree completely (although I extend it to multiple segments, each of which
has the characterstics you describe).

Which is why I think the original DEC nomenclature for the PDP-11's memory
management was more appropriate - the description above is _exactly_ the
functionality provided for each of the 8 'chunks' (to temporarily use a
neutral term) of PDP-11 address space, which don't quack like most other
'pages' (to use the 'if it quacks like a duck' standard).


One query I have comes from the usual goal of 'virtual memory' (which is the
concept most tightly associated with 'pages'), which is to allow a process to
run without all of its pages in physical memory.

I don't know much about PDP-11 DEC OS's, but do any of them do this? (I.e.
allow partial residency.)  If not, that would be ironic (in view of the later
name) - and, I think, evidence that the PDP-11 'chunks' aren't really pages.


BTW, did you know that prior to the -11/45, there was a memory management
device for the PDP-11 which provided 'real' paging, the KT11-B? More here:

  http://gunkies.org/wiki/KT11-B_Paging_Option

I seem to recall some memos in the memo archive that discussed it; I _think_
it mentioned why they decided not to go that way in doing memory management
for the /45, but I forget the details? (Maybe the performance hit of keeping
the page tables in main memory was significant?)_


    > With segmentation you cannot have your virtual memory split up and
    > spread out over physical memory.

Err, Multics did that; the process' address space was split up into many
segments (a true 2-dimensional naming system, with 18 bits of segment number),
which were then split up into pages, for both virtual memory ('not all
resident'), and for physical memory allocation.

Although I suppose one could view that as two separate, sequential steps -
i.e. i) the division into segments, and ii) the division of segments into
pages. In fact, I take this approach in describing the Multics memory system,
since it's easier to understand as two independent things.

    > You can also have "holes" in your memory, with pages that are invalid,
    > yet have pages higher up in your memory .. Something that is impossible
    > with segmentation, since you only have one set of registers for each
    > memory type (at most) in a segmented memory implementation.

You seem to be thinking of segmentation a la Intel 8086, which is a hack they
added to allow use of more memory (although I suspect that PDP-11 chunks were
a hack of a similar flavour).

At the time we are speaking of, the Intel 8086 did not exist (it came along
quite few years later). The systems which supported segmentation, such as
Multics, the Burroughs 5000 and successors, etc had 'real' segmentation, with
a full two-dimensional naming system for memory. (Burroughs 5000 segment
numbers were 10 bits wide.)

    > I mean, when people talk about segmented memory, what most everyone
    > today thinks of is the x86 model, where all of this certainly is true.

It's also (IMNSHO) irrelevant to this. Intel's brain-damage is not the
entirety of computer science (or shouldn't be).

(BTW, later Intel xx86 machines did allow you have to 'holes' in segments, via
the per-segment page tables.)


    > it would be very wrong to call what the PDP-11 have segmentation

The problem is that what PDP-11 memory management does isn't really like
_either_ segmentation, or paging, as practised in other machines. With only 8
chunks, it's not like Multics etc, which have very large address spaces split
up into many segments. (And maybe _that_'s why the name was changed - when
people heard 'segments' they thought 'lots of them'.)

However, it's not like paging on effectively all other systems with paging,
because in them paging's used to provide virtual memory (in the sense of 'the
process runs with pages missing from real memory'), and to make memory
allocation simple by use of fixed-size page frames.

So any name given PDP-11 'chunks' is going to have _some_ problems. It just
thing 'segmentation' (as you defined it at the top) is a better fit than the
alternative...

   Noel