bqt at update.uu.se
Sat Apr 28 20:41:37 AEST 2018
On 2018-04-28 02:19, Noel Chiappa wrote:
> > From: Johnny Billquist
> > For 1972 I only found the 11/40 handbook.
> I have a spare copy of the '72 /45 handbook; send me your address, and I'll
> send it along. (Every PDP-11 fan should have a copy of every edition of every
> model's handbooks... :-)
Gah. If I were to try and collect every copy made, it would be quite a
collection. Thanks for the offer. If you really want to get rid of one,
and is willing to send to Switzerland, then I'll take it in a private
mail. But you don't really have to.
> In the meantime, I'm too lazy to scan the whole thing, but here's the first
> page of Chapter 6 from the '72:
Cool. Thanks. That is a very different terminology. The MMU is not even
called the MMU, but the "Memory Segmentation Unit".
> > went though the 1972 Maintenance Reference Manual for the 11/45. That
> > one also says "page". :-)
> There are a few remnant relics of the 'segment' phase, e.g. here:
> which has this comment:
> / turn on segmentation
> Also, if you look at the end, you'll see SSR0, SSR1 etc (as per the '72
> handbook), instead of the later SR0, SR1, etc.
So there was a total change in terminology early in the 11/45 life, it
would appear. I wonder why.
I can only speculate, but I probably would not blame some market droids.
Trying to think about this, I feel that 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. With segmentation you cannot
have your virtual memory split up and spread out over physical memory.
You can also, pretty much, arbitrarily point where in physical memory
your virtual memory starts and ends.
With pages, this is obviously not the case anymore. The PDP-11 do have
the ability to have pages start at close to arbitrary addresses in
physical memory, but you can certainly have your virtual memory spread
out over different places in physical memory. Your virtual memory is
also not just described by a base and length, as you have 8 pages,
starting at fixed virtual memory addresses. You can also have "holes" in
your memory, with pages that are invalid, yet have pages higher up in
your virtual memory which are valid. 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.
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.
So, by this definition, it would be very wrong to call what the PDP-11
have segmentation. And I would suspect that to be the reason why DEC
changed their terminology. Segmentation simply started to become an
established term, and it did not match what the PDP-11 did, so the
documentation had to change to better describe what it was.
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: bqt at softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
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