[TUHS] PDP-11 MMU (was: 68000 vs. 8086)

Johnny Billquist bqt at update.uu.se
Fri Jul 1 08:17:05 AEST 2016

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 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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