flash (or ide) storage for unibus 11?

Paul Koning paulkoning at comcast.net
Tue Nov 24 11:39:38 CST 2015

> On Nov 24, 2015, at 10:55 AM, Johnny Billquist <bqt at Update.UU.SE> 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. :-)


>> 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.


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