[Simh] pdp11 i/o addressing

[Bob Eager]

On Fri, 16 Feb 2018 14:51:27 -0500
Clem Cole <clemc at ccc.com> wrote:

> 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.

When I teach virtual memory, I start by talking about the 'old days'
and overlays. I remember Overlay Description Language on the PDP-11!