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

[Rich Alderson]

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...

-- 
Rich Alderson                                   news at alderson.users.panix.com
    the russet leaves of an autumn oak/inspire once again the failed poet/
    to take up his pen/and essay to place his meagre words upon the page...