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

[Johnny Billquist]

On 2015-06-26 16:02, Bob Koehler wrote:
> In article <5586381E.9040801 at spam.com>, Don North <idontwant at spam.com> writes:
>>
>> 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.
>
>     I think you missed the 11/34.  IIRC no MMU, only UNIBUS mapping
>     registers.  I/O space was 18 bit, but RAM was 16.

Actually, the 11/34 did not have any Unibus mapping registers, as those 
only exist on machines with 22-bit physical memory addressing. And the 
11/34 was only 18 bits. And as far as I can remember, the 11/34 could 
not be had without an MMU, but I might remember that wrong...

That said, it would not make much sense on any machine to have Unibus 
mapping registers while not have an MMU. That would mean you could DMA 
to memory you could never access from the CPU...

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
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