> On Nov 24, 2015, at 10:55 AM, Johnny Billquist <[email protected]> wrote:
>
> On 2015-11-24 16:43, Paul Koning wrote:
>>
>> ...
>> To elaborate, since this seems to be an area where people get confused:
>
> Thanks. Yes, people do seem to be confused.
>
>> 1. Unibus has 18 bit addresses, with the CSR addresses in the top 4k words
>> and the remaining 128 kW for memory.
>
> 124 kw, but I would hope that everyone guessed that. :-)
Oops.
>
>> 2. In 22 bit PDP-11s with a Unibus, there is a Unibus Map between the CPU
>> bus and the Unibus, which maps Unibus references that ask for memory
>> addresses (i.e., the bottom 31 pages) into 22 bit memory addresses. That
>> allows Unibus DMA to any memory address.
>
> Or between the memory bus and the Unibus, if we talk 11/70.
> (I wouldn't actually know the proper name for whatever it is on an 11/24 or
> 11/44, but CPU bus would suffice, I'd say. On the 11/84 and 11/94 it's the
> PMI bus, which I'm fine with calling the CPU bus here.)
>
> The Unibus map actually covers all 32 address areas, but noone normally do
> DMA to the top "page", which would be the I/O page on an 18-bit system. And
> it can become even more weird on an 11/70 system, since there is actually
> also an I/O space on the memory bus. So better just use the 31 first
> mappings, and ignore the 32nd, unless you are looking to do very specific odd
> things.
It depends on how you count. There are 31 relocation registers in the UBM, for
the 31 pages below 124 kW. The top page (the I/O page) is not all that well
described in the PDP-11/70 handbook, but it implies that it's a hardwired
mapping to the I/O page range of the memory bus, and can be used to reach CSRs
on that side from the Unibus.
Yes, "odd things" indeed.
paul
