24 Nov
2015
24 Nov
'15
9:47 a.m.
On 2015-11-24 16:55, Mouse wrote:
Right.
Well, Paul did talk about the PDP-11 and the Qbus in it's "normal" form.
All VAXen (beyond the uVAX I) use the Qbus only as an I/O bus, while
memory sits on PMI. So then we're back to the same thing as with the
Unibus thingy, where DMA have to go through a remapping, since the
system actually have more address bits than the I/O bus.
Johnny
3. The Q-bus
comes in 2 (or 3?) flavors, the original with 18 bit addresses,$
I wouldn't go that far. I would say the MicroVAX-II counts as "in the
Qbus world", and it has hardware to map Qbus memory space accesses to
memory accesses, rather like the Unibus Map you sketch. DMA is "always
to the memory address specified" only if you never look past the Qbus
view of the world; from the CPU's point of view, Qbus DMA addresses get
remapped before hitting memory.