3 May
2018
3 May
'18
5:39 p.m.
That would be a pretty ugly way to look at the world.
'Beauty is in the eye of the beholder', and all that! :-)
Not to mention that one segment silently slides over
into the next one
if it's more than 8K.
Again, precedent; IIRC, on the GE-645 Multics, segments were limited to 2^N-1 pages,
precisely because otherwise incrementing an inter-segment pointer could march off
the end of one, and into the next! (The -645 was implemented as a 'bag on the
side'
of the non-segmented -635, so things like this were somewhat inevitable.)
wouldn't you say that the "chunks" on a
PDP-11 are invisible to the
user? Unless you are the kernel of course. Or run without protection.
No, in MERT 'segments' (called that) were a basic system primitive, which
users had access to. (Very cool system! I really need to get moving on trying
to recover that...)
*Demand* paging is definitely a separate concept from
virtual memory.
Hmmm. I understand your definitions, and like breaking things up into 'virtual
addressing' (which I prefer as the term, see below), 'non-residence' or
'demand loaded', and 'paging' (breaking into smallish, equal-sized
chunks),
but the problem with using "virtual memory" as a term for the first is that to
most people, that term already has a meaning - the combination of all three.
(I have painful memories of this sort of thing - the term 'locator' was
invented after we gave up trying to convince people one could have a network
architecture in which not all packets contained addresses. That caused a major
'does not compute' fault in most people's brains! And 'locator'
has since been
perverted from its original definition. But I digress.)
There is no real connection between virtual memory and
memory
protection. One can exist with or without the other.
Virtual addressing and memory protection; yes, no connection. (Although the
former will often give you the latter - if process A can't see, or name,
process B's memory, it can't damage it.)
Might have been just some internal early attempt that
never got out of DEC?
Could be; something similar seems to have happened to the 'RK11-B':
http://gunkies.org/wiki/RK11_disk_controller
> I don't have any problem with several
different page sizes, _if it
> engineering sense to support them_.
So, would you then say that such machines do not have
pages, but have
segments?
Or where do you draw the line? Is it some function of how many different
sized pages there can be before you would call it segments? ;-)
No, the number doesn't make a difference (to me). I'm trying to work out what
the key difference is; in part, it's that segments are first-class objects
which are visible to the user; paging is almost always hidden under the
sheets.
But not always; some OS's allow processes to share pages, or to map file pages
into address spaces, etc. Which does make it complex to separate the two..
Noel