wikipedia article

Johnny Billquist bqt at
Tue Jun 13 10:02:21 UTC 2006

Otto Moerbeek wrote:
> On Mon, 12 Jun 2006, Ted Mittelstaedt wrote:
>>>-----Original Message-----
>>>From: John Nemeth [mailto:jnemeth at]
>>>Sent: Monday, June 12, 2006 1:15 PM
>>>To: Ted Mittelstaedt; Nikolas Britton; Ted Unangst
>>>Cc: Hamorszky Balazs; misc at; freebsd-questions at;
>>>netbsd-users at
>>>Subject: RE: wikipedia article
>>>On Nov 1,  6:11pm, "Ted Mittelstaedt" wrote:
>>>} Prior to the release of the 80386 the Intel processors didn't have
>>>} memory protection which was a requirement of any processor running
>>>} the BSD kernel.
>>>    This is not entirely true.  The 80286 had memory protection.
>>>However, its memory protection was completely based on segments (i.e.
>>>it could not do paging).
>>Oh, yeah, your right about that.  Me bad.
>>>Also, it was only a 16 bit processor.
>>What was the bit size of the CPU's originally used to write UNIX in Bell

The PDP-7 was/is an 18-bit machine.

> What's more, iirc the MMU of the pdp11 isn't what we call a MMU today,
> it could not even do paging.

You're wrong. You could easily do paging on a PDP-11, if you wanted to. 
The main reasons this wasn't done are two.
1) Each page is 8K. At the time, that was considered way too large pages 
for a demand page system.
2) The address space is only 64 per process, which means you only have 8 
pages. Not only is that perhaps a little little for meaningful paging 
(most programs tend to refer to all 8 pages most of the time). The main 
memory on a PDP-11 is furthermore 4 meg, so having a lot of processes 
full memory space in physical memory at the same time is not a problem.

The PDP-11 MMU is a beatiful MMU. Nothing like the crap Intel spits out. ;-)


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

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