segmented memory models

[Johnny Billquist]

Patrick Finnegan <pat at computer-refuge.org> wrote:

 > On Monday 04 August 2008, Sean Conner wrote:
 > > > It was thus said that the Great Jim Brain once stated:
 > >> > > Tony Duell wrote:
 > >>> > > >No it doesn't, given that a PDP11 address to a program is always
 > >>> > > > 16 bits. The 18 or 22 bit phuysicall addresses were created by
 > >>> > > > the MMU.
 > >> > >
 > >> > > Did an MMU exist for the 8086?
 > > >
 > > >   If such a think existed, it would have been an external circuit,
 > > > and would have been very hard to support since the 8086 did not
 > > > include support for restartable instructions (same situation on the
 > > > 68000).
 >
 > The PDP-11 doesn't support restartable instructions either.  You don't
 > need restartable instructions to support an MMU, only to support
 > virtual memory type operations.  For example, my Z80-based Altos 8000
 > has a bank-switching MMU that could operate the same way as an MMU on a
 > 808[68] would.

Actually, some PDP-11 models do support restartable instructions. Just 
not all of them. It requires the MMR3 register, which tells what 
modifications have been done to different registers before the 
instruction was aborted, so that you can back out of that, and then 
restart the instruction.
Some PDP-11 models' MMU would also allow you to implement a virtual 
memory system, if you wanted to. It's just that noone did, and perhaps 
for good reasons. With only 8 pages, and way more physical memory than 
virtual, there isn't really much point in implementing a virtual memory 
system. But on an 11/70, it is definitely doable.

Another example of the hardware designers implementing something that 
they didn't know was needed, but which could be done without much extra 
effort, so they did it, just in case someone would want to play around 
with it. But I think we've already established that the PDP-11 isn't 
brain damaged. :-)
(Oh, and I agree with Tony Duells sentiment about the Intel brain 
damage. It's plain and simple just a question of not doing something one 
way when it was common knowledge at the time that that was the way to do 
it. NIH or just sheer lack of clues don't matter. It's brain dead all 
the same.)

	Johnny