18 Mar
2015
18 Mar
'15
6:45 a.m.
On 2015-03-17 22:45, John Wilson wrote:
But thinking about how it must work hurts my head.
It's emulating Unibus
memory at the same time that it's emulating the Unibus map -- i.e. CPU
accesses (which should be relocated through the onboard PARs) are coming
over the same bus as DMA (which should be relocated using the Unibus Map).
How does it know which is which? Does it need to tap into each model of
CPU somehow (like how a Microverter gets at MMR3)? Or is it something
simple like, BBSY is never asserted by the CPU (not obvious from docs --
the CPU doesn't need to negotiate to become a master but it still is one
when it's accessing memory) so if it's on, this is DMA? And what if there's
a cache, like the KK11A, that doesn't know about the outboard PARs?
Not sure it's that hard. On the bus, I believe it is visible if this is
an NPR transaction or not. The Unibus map only applies to NPR transactions.
Addresses originating from the CPU should get the full 22 bit address
from the MMU.
I was thinking about the Enable/34 again the other day. As others have
mentioned, there is the question (if I remember right) how the PAR
registers could be 16 bits (required for 22 bit addressing) when the CPU
PAR registers only hold 12 bits, and only 12 bits comes out.
One possible other answer might be that the CPU had been modified, so
that the internal PAR registers were disabled. I found a post that
possibly suggested something like this on the net.
See
http://gopher.quux.org:70/Archives/usenet-a-news/FA.unix-wizards/81.07.09_u…,
where the mention that the Enable/34 gives you new PAR registers, but
keep the PDR registers in the CPU.
But this is still way too little to actually know for sure how things works.
Johnny