[Info-vax] OT: obscure PDP11 OSes (even more dinosaury)

[Don North]

On 6/20/2015 8:02 PM, David Froble wrote:
> Stephen Hoffman wrote:
>> On 2015-06-20 20:29:30 +0000, Hans Vlems said:
>>
>>> My question was whether the 11/20 had support for 18 bit addresses. I thought
>>> that the 11/20 was the only model limited to a 2^16 address space.
>>
>> The <http://gunkies.org/wiki/PDP-11> and <http://hampage.hu/pdp-11/main.html>
>> pages indicate not; that there were other boxes that used 16 bits of the 18
>> bit UNIBUS.
>>
>>  From the Things DECfolks Can't Unsee department: the new Dunkin Donuts coffee
>> shop color scheme looks like that of a PDP-11.
>>
>>
>
> The first PDP-11 I used was an 11/40.  I don't think it had more than 16 bits of
> address space.  Could be wrong.  I seem to recall that the 11/40 could run RSTS,
> but the 11/20 could not.  Top of the line at that time was the PDP-11/45.

The PDP-11/20, PDP-11/05 (aka PDP-11/10), and PDP-11/04 were MMU-less UNIBUS 
machines so they only supported a maximum of 56KB of memory. The UNIBUSes on 
these machines were 18bit address to be compatible with the UNIBUS, but of the 
full 256KB UNIBUS address space the low 56KB of memory could be addressed, and 
the upper 8KB of address space was the I/O page.

The LSI-11 (aka PDP-11/03) was an MMU-less QBUS machine and it supported a 
maximum memory of 60KB with a 4KB I/O page.

All the other PDP-11 models had integrated MMUs (or the option for an MMU) and 
supported either 256KB (18b UNIBUS) or 4MB (22b memory bus with a UNIBUS map) 
memory capability.

Every PDP-11 however just had a 16b virtual address and supported a maximum of 
64KB of mapped address space (or 64KB Ispace + 64KB Dspace for higher end models).