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

[Johnny Billquist]

On 2011-05-07 19.00, 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...

True. And I did say that the Unibus first came with the PDP-11. And I 
don't know why DEC decided on having 18 address bits on the Unibus, but 
it can't very well have been for just doing a 2-bit expansion of the 
PDP-11 address space in the future, so it seems more likely that they 
were considering the possibility of using it on 18/36 bit products. But 
that is just guessing on my part. (But really, if it were for just -11 
expansion, why on earth only make room for a 2 bit expansion? When it 
was done, it was a totally open and free decision to make provisions for 
a larger expansion. If that was the case, then I would agree with the 
statement that it was very poor planning.)

Also, the fact that you can run 18-bit data on the Unibus is totally 
lost on a PDP-11, since it never use that, but instead use the two 
additional bits for parity of each data byte.

I don't know much details of the Unibus on the PDP-15 either, but that 
came sooner than the -2020.

Also, not totally relevant, but the 11/40 FE of the KL-10 have a Unibus 
as well, and the machine normally use a dual-ported RP06 which is 
formatted in 18-bit mode, that is also accessed by the FE. As such, 
something non entirely -11-normal is done on the RH11 of the FE...
But it's not really the same as any 18/36 bit machine with a Unibus in 
anyway, so it's more of a curious oddball among -11s.

	Johnny