UK PDP11 wanted
jnc at mercury.lcs.mit.edu
Tue Apr 14 16:16:18 CDT 2015
> From: Johnny Billquist
Sure; glad you liked it.
>> But the QBUS part of _any_ QBUS backplane is not 'directional'
> there is still a question on whether the CPU just pass all signals
> through that you need.
Perhaps I'm confused, but the only QBUS signals which are not 'broadcast'
(i.e. sent down a wired-OR bi-directional bus transmission line to all cards)
are the grant lines. The CPU doesn't pass them through, it originates them -
but memory cards ignore those lines anyway?
> With PMI memory, it obviously works somewhat different, since the CPU
> sources signals in just one direction (hence the reason the PMI memory
> have to be before the CPU)
Actually, the PMI signals are all (I'm pretty sure, but have not checked) just
like most other QBUS signals - 'broadcast' bi-directionally down a
transmission line. The 'PMI memory before the CPU' thing in 11/83's is simply
an artifact of the way 'standard' Q/CD backplanes are wired on the CD side
(used for PMI) - see that second post for details.
In the 11/84 backplane, the PMI memory lines _are_ bussed to all the slots,
just like the QBUS lines, which is why 11/84 PMI memory cards go after the
CPU. (So you could in fact have an empty slot between the CPU and the first
PMI memory card in an 11/84, and it should still work.)
> [the CPU] also interacts/intercepts with all other memory accesses to
> the PMI memory.
My impression is that the CPU does not get involved in DMA access to PMI
In an 11/83, I gather the DMA devices just do ordinary QBUS memory cycles to
it (see the MSV11-J manual, pg. 1-1, middle). In an 11/84, with UNIBUS
adapter, IIRC the UNIBUS adapter does PMI cycles direct to the memory (but
don't quote me on that, I haven't checked the 11/84 documentation, although
the MSV11-J manual indicates it does - see pg. 1-1, bottom).
> Which is yet another reason why the PMI memory sits before the CPU. If
> you put PMI memory after the CPU, it will instead work as normal Qbus
I'm pretty sure that whether PMI memory works as PMI memory or QBUS memory
depends only on what it hears on the busses, and nothing else. If it sees a
PMI cycle for it, it does a PMI cycle; if it sees a QBUS cycle for it, it does
a QBUS cycle.*1 So in an 11/84, where it's on the bus after the CPU, it does
PMI cycles with the CPU because the CPU does a PMI cycle.
(1: Note that the MSV11-JB and -JC variants don't do QBUS properly, and are
PMI only, and so can't be used in an 11/83. I seem to recall hearing it's
just QBUS block mode transfers that they don't do properly?)
Which all does raise an interesting question: if an M8190 is in a system with
a _mix_ of PMI and regular QBUS memory, do accesses to both memories work
fine? (E.g. if it's in a Q/CD backplane with a PMI board before it, and a
regular QBUS memory card after it.)
Something to try, I guess - I have a working Q22/CD box, and plenty of cards
- should just plug them in and try it!
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