Qbus split I&D?

Johnny Billquist bqt at update.uu.se
Wed Mar 18 12:29:52 CDT 2015

On 2015-03-18 17:57, John Wilson wrote:
> On Wed, Mar 18, 2015 at 11:16:02AM -0400, Noel Chiappa wrote:
>> The UNIBUS map will not respond to UNIBUS cycles in the address range
>> assigned to the UNIBUS memory.
> Just to state the obvious:  I assume the whole point of this "Unibus fence"
> concept is really for things that *aren't* memory.  Mixing in some Unibus
> memory that's slower than PMI etc. and needs to use real addresses for DMA
> (vs. mapped ones) seems like a giant hassle for just 6.5% more RAM, so I
> figure it's really for bus windows or ROM boards or frame buffers or any
> oddball device that looks like memory and is too big to fit in the I/O page.

(And it's not giving you more RAM. PMI memory already gives you 4M minus 
the I/O page)


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