[TUHS] /dev/drum

Johnny Billquist 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:
>    http://ana-3.lcs.mit.edu/~jnc/tech/pdp11/jpg/tmp/PDP11145ProcHbook72pg6-1.jpg

Cool. Thanks. That is a very different terminology. The MMU is not even 
called the MMU, but the "Memory Segmentation Unit".
Very interesting.

>      > 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:
>    http://minnie.tuhs.org/cgi-bin/utree.pl?file=V6/usr/sys/conf/m45.s
> 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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