From: bqt@update.uu.se (Johnny Billquist)
Subject: [TUHS] PDP-11 MMU (was: 68000 vs. 8086)
Date: Fri, 1 Jul 2016 00:17:05 +0200 [thread overview]
Message-ID: <e65515f8-137c-d149-a1a2-2a54f7ec9c72@update.uu.se> (raw)
In-Reply-To: <mailman.31.1467314550.30583.tuhs@minnie.tuhs.org>
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On 2016-06-30 21:22, Clem Cole <clemc at ccc.com> wrote:
>
> but when Moto came out with a memory management chip it had some
>> > severe flaws that made paging and fault recovery impossible, while the
>> > equivalent features available on the 8086 line were tolerable.
> Different issues...
>
> When the 68000 came out there was a base/limit register chip available,
> who's number I forget (Moto offered to Apple for no additional cost if they
> would use it in the Mac but sadly they did not). This chip was similar
> to the 11/70 MMU, as that's what Les and Nick were used to using (they used
> a 11/70 running Unix V6 has the development box and had been before the
> what would become the 68000 -- another set of great stories from Les, Nick
> and Tom Gunter).
Clem, I think pretty much all you are writing is correct, except that I
don't get your reference to the PDP-11 MMU.
The MMU of the PDP-11 is not some base/limit register thing. It's a
paged memory, with a flat address space. Admittedly, you only have 8
pages, but I think it's just plain incorrect to call it something else.
(Even though noone I know of ever wrote a demand-paged memory system for
a PDP-11, there is no technical reason preventing you from doing it.
Just that with 8 pages, and load more physical memory than virtual, it
just didn't give much of any benifits.)
Johnny
--
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
next parent reply other threads:[~2016-06-30 22:17 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <mailman.31.1467314550.30583.tuhs@minnie.tuhs.org>
2016-06-30 22:17 ` Johnny Billquist [this message]
2016-07-01 0:01 ` Clem Cole
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