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

[Richard B. Gilbert]

On 5/7/2011 9:00 PM, Rich Alderson wrote:
> Johnny Billquist<bqt at update.uu.se>  writes:
>
>> On 2011-05-06 12.08, G Cornelius wrote:
>>> Richard B. Gilbert wrote:
>
>>> 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.
>
>> Well. The 18-bit thingy was not totally for the PDP-11. The Unibus was
>> first done for the PDP-11, true, but the PDP-11 at that time was only 16
>> bits (there was no MMU). However, the Unibus was also used in some
>> 18/36-bit products, where those two extra bits made sense. And sharing a
>> bus design between several machines also makes sense (and both address
>> and data can be 18 bits).
>
> There wasn't any Unibus in the PDP-10 world until the introduction of the 2020,
> which used the Unibus for I/O interfaces.  That was 1978, nearly a decade after
> the PDP-11 brought Unibus into being...
>

Please be a little more careful about your attributions.  The above text 
does not contain anything that I wrote!