[Simh] pdp11 i/o addressing

[Clem Cole]

curmudgeon warning below.....

On Fri, Feb 16, 2018 at 11:06 AM, Ethan Dicks <ethan.dicks at gmail.com> wrote:

>
> I started on a VAX with 2MB of physical memory in a 16MB physical
> address space but with 4GB virtual addresses.  Switching over to the
> PDP-11 was odd from that.
>

​Sigh... I fear that is a fault of your education.

If you ask many (younger) programmers what VM was designed to solve
(particularly those that never memory constrained systems such as you get
in 8, 12, 16 or 18 bit processors), they will tell you 'So can have more
addressable memory.'​  The problem is said programmers never experienced or
learned about overlays.   Conceptually, a PDP-11 can allow a lot more than
the 64Ks physical limit by 'swapping out' and 'overlaying parts' and
calling subroutines through 'thunks' [which to quote my old friend Paul
Hilfinger from page 427 of his book:  *"an onomatopoetic reference to the
sound made by a pointer as it moves instantaneously up and down the stack"*].
A process has be allowed to be larger that 64K, but only 64K (128K on
seperate I/D systems) in the set up memory maps at a time.    If you need
to call a subroutine to (optionally) bring in the routines and its data
into memory if it is not already there, and then set up the map to point
the routine in question.

BTW: If you play with BSD 2.11 or the like, it uses overlays to allow
programs to grow in size.   This was needed as people started to try to
move features from 4BSD and later back to the PDP-11.   At this point, I
believe you must have what was sometimes referred too as '17th address bit
- i.e. I/D space which gives you 128K bytes of mapped in memory at a time.
  But you can (with care) let you programs grow.

The point is that VM is a mechanism* to automatically manage overlays*.
 The implementation of this management gets easier if there are more
address bits than physical address bit, but that is the key item that is
happening.

Sadly, since people stopped learning about overlay management, the context
of what it was doing under the covers was lost.

Clem


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