Seeking enlightenment regarding Bus Adapter Purges on Unibus VAXen (and their ilk)

Matt Burke matt at
Fri Sep 6 03:32:12 CDT 2019

On 06/09/2019 05:22, Josh Dersch via cctalk wrote:
> What I do not understand is (a) why such purges are necessary, and (b) how
> the MSCP controller knows when one should occur.  The Port Description doc
> hints that it has to do with repeated access to a given area of memory.
> The DW780 documentation hints that it needs to happen after *any* block
> transfer.  (See pg. 2-58 of the document linked above.)
I think point 3 and 4 on page 2-59 are critical here. The memory
accesses via the buffered data path must be to consecutive increasing
addresses. If a non-consecutive access is required (transfer restart,
compare etc) then the data path must be purged first. In the case of a
write this will flush any residual bytes to memory and for a read it
will ensure that fresh data is loaded to the buffer on the next access.

At least that's my interpretation from reading

I'm not sure if the adapter needs to request the purge at the end of
each block transfer. The driver may do this anyway but I'll have to
check the source listings.


More information about the cctech mailing list