[Info-vax] Y3K for PDP-11 Operating Systems

[G Cornelius]

Richard B. Gilbert wrote:
> I have no PDP-11.  I had one at work many long years ago.  I don't miss it.

You must never have worked on an -8!

A 22-bit PDP-11 with RSX-11M+ (I/D space, supervisor mode libraries)
was a huge improvement over the sad little 18-bit beasts (physical
address, of course). And memory resident overlays at least meant
that you did not have to wait for more than a single system call's
overhead for additional segments of your overlaid program to be
accessed. But, oh, the task builder overlay description language
files you had to manage!

The 18-bit physical address space was a broken concept from the
beginning: "Hey, we have all this 18/36 bit stuff already built
for the 10- and 20-series, let's give the 11's two entire bits
of extended physical address!"  To go to 22 bits your device
drivers still had to go through the 18 bit atrocity, with the
extra two bits tucked into the CSR somewhere, just so they could
address a set of "Unibus mapping registers" to extend the map
from 18 bits to 22 bits. In retrospect it all seems to have been
rather poor planning.

If in fact your code fit into the address space and did not need
32 bit arithmetic, it could be fast.  The first time I started
working in a group that had VAXen I noticed two 750's sitting
off to the side because they had not been able to handle the
application load.  Their replacement: a pair of 11/84's.
Granted, this was specialized in that the language interpreter
had to do a lot of single byte operations, resulting in the
initial versions, on 750's at least, being rather inefficient.
Sometimes extra bits just mean extra overhead.

George