New subject: Unix & Memory Management Units (MMU)

9 Dec 2016

On 2016-12-08 18:18, Paul Ruizendaal &lt;pnr(a)planet.nl&gt; wrote:
...

DEC's Custom Special Systems (CSS) group .. build a simple base/limi
 register device, soon after the 11/20 was released.> ... So an early
 version of after the original 11/20 port from the PDP-7 had this
 however.....  Oh, right, I'd forgotten about that: the KS-11 - I've
previously enquired to
 see if anyone had _any_ documentation for this, but so far, nada.  I was looking
for that a few years back. Dennis Ritchie's home pages have
 some info on this (https://www.bell-labs.com/usr/dmr/www/odd.html)
 At the bottom of that page he writes:

 ""Back around 1970-71, Unix on the PDP-11/20 ran on hardware that not only did
not support virtual memory, but didn't support any kind of hardware memory mapping or
protection, for example against writing over the kernel. This was a pain, because we were
using the machine for multiple users. When anyone was working on a program, it was
considered a courtesy to yell "A.OUT?" before trying it, to warn others to save
whatever they were editing.
 [..snip..]
 We knew the PDP-11/45, which did support memory mapping and protection for the kernel and
other processes, was coming, but not instantly; in anticipation, we arranged with Digital
Special Systems to buy a PDP-11/20 with KS-11 add-on. This was an extra system unit bolted
to the processor that made it distinguish kernel from user mode, and provided a classical
PDP-10 style "hi-seg" "low-seg" memory mapping unit. I seem to recall
that maybe 6 of these had been made when we ordered it.""

 My hypothesis is that the very first versions of Unix were using a memory scheme similar
to that used in the later LSX and MX derivatives: the kernel resides in lower memory and
the user program in upper memory; each process switch implied a swap. Disclaimer: I have
not studied the V0 source to verify this hypothesis.

 When this topic had my interest I looked for (but did not find) information on what
""the classical PDP-10 style "hi-seg" "low-seg" memory
mapping unit"" was. Here my hypothesis would be that in kernel mode mapping was
off, and that in user mode there were two segments, each with a base and limit into
physical memory -- and that this setup has an echo in how the later KL-11 MMU was used.

 Does anyone have an idea what PDP-10 MMU Dennis may have been referring to? 
If you read the Wikipedia page about the PDP-10, you'd find the answer.

Basically, kernel mode uses physical memory. User mode have a low 
segment and a high segment. This is decided by the high bit of the address.
For each segment, there is a base register and a length register.
Commonly, the low segment stored read/write data, while the high segment 
could be shared between processes, and have readonly data/code.

But you could play in other ways with it, if you wanted to.

Essex MUD, as far as I remember, had the game data in a shared hiseg, 
which could be written by the program.

So your guessing is pretty good. Not sure I'd say this is similar to how 
the later PDP-11 MMU works, though. But I can see someone making the 
comparison, since the PDP-11 pages can vary in size, within limits.

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt(a)softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

Re: [TUHS] Unix & Memory Management Units (MMU)