pdp11/84 PMI memory: What is the problem with Q bus?

Chris Zach cz at alembic.crystel.com
Fri Apr 24 14:16:15 CDT 2020

> Oh, right. Still, it sounds from reports here like regular DMA doesn't work
> in QBUS mode either - and technically that table entry might mean than since
> it doesn't work for the QBUS _at all_, that includes no block mode.

Regular Q bus DMA seems to be working fine. I can see the performance 
difference pretty easily when looking at 1,2,4,8,16 word transfers, 
anything else I should be checking?

> I actually have -B/-C boards, I should plug one in in QBUS mode, and get my
> QSIC prototype working again (it somehow random failed during the last year,
> and I've been too lazy to debug it), and write a little program to DMA blocks
> in and out, and see what happens to the data. If I get really energetic I
> could throw a 'scope on the bus and look at bus cycles and see if they look
> OK.

I can also bring up an RL02 with the RLV12 (also does DMA) and see how 
an authentic DEC appliance works with this memory card. But really I'm 
not seeing any speed increase of PMI memory vs. non-PMI memory with the 
exception that the initial memory check runs a good bit faster with PMI 
over without.

And that might make sense: PMI memory responds to the Q bus just like 
normal memory. So from a Q bus device perspective it's just boring old 
memory and thus no speed improvement. What might speed up is if I did 
memory to memory (VM0:) copies, but with only 2mb running at the moment 
there's not a lot of time to check for performance differences.

Maybe it can't properly handle things like 18 bit DMA cards like the RXV12.

Let's find out: What's the worst that can happen?

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