Marcus Watts wrote:
Various wrote:
From: Otto Moerbeek <[EMAIL PROTECTED]>
To: Ted Mittelstaedt <[EMAIL PROTECTED]>
...
What's more, iirc the MMU of the pdp11 isn't what we call a MMU today,
it could not even do paging.
The pdp-11 mmu could handle program relocation, segmentation (after
a fashion) and memory protection. I'm not sure what more you
could expect from an mmu. What you mean by "paging" is
probably "demand paging", which means the ability to run a program
without requiring that it be entirely resident. The key
feature you need for that is a guarantee that any instruction fault
caused by missing memory can be either restarted or continued.
In most architectures that's a question of cpu design not mmu.
True. But it's mostly a combination of MMU and CPU.
The MMU needs to either abort or trap the offending instruction, and the
CPU needs to know how much side effects had been done so that they can
be undone before a restart.
The PDP-11 MMU can either abort the instruction, or do a trap after the
instruction completes. The CPU have a register telling of register
modifications done, as well as the pre-fetch PC. In additions to this,
the MMU have both an expansion direction, a modified bit, and an
accessed bit. And that is in addition to the protection field and size
field of the page (and the address relocation). So I can't really
imagine anything that you cannot do with the PDP-11 MMU. Heck, there is
even the funny "bypass cache" bit. Useful for multiprocessor systems...
Since the PDP-11 have a different page table for I- and D-space, you can
even have execute-only pages.
In the case of the pdp-11 that's mostly a moot point. The pdp-11 only
provides for mapping the 64k of memory space into into 8 segments
(addressable on 64-byte "clicks") and there's just not much win to
demand paging 8 "pages". (actually 6 x 8 pages; there was kernel,
user, and supervisor mode, & each had separate instruction and data
spaces, but supervisor mode was rarely used in Unix environments, and
only a few large user mode programs ran using split I/D space.)
2.11BSD uses supervisor mode for the networking parts of the kernel. And
the kernel is all I/D-space, and a bunch of programs are as well. The
development is still contiuing. :-)
For
what it's worth, though, I *think* it was possible to restart most
instructions on the /45 and /70, which were the "big" machines and the
primary target of most later pdp-11 work.
I don't think there is a single instruction that you can't restart.
Some small, older machines missed a few registers needed for proper
restarts however, and on those things were a bit more of a gamble if you
wanted to go that path.
In fact, some use was made
of this feature -- automatic stack growth. If you look through ancient
Unix source, you'll find interesting bits of kernel code that manage
this.
There's actually a cheesy way to do demand paging with microprocessors
that don't support demand paging (such as the original 68000--another
"16 bit" machine). The way to do this is to run two processors in parallel
but skewed by one instruction. If the first one does a bad memory fetch,
then the second one will not have fetched the instruction causing the
fault so contains restartable machine state. Masscomp sold a machine
like this once.
Didn't the first Apollos do this?
Johnny
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: [EMAIL PROTECTED] || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
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