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

[Josh Dersch]

Hi all --

Been working on an MSCP implementation for Joerg Hoppe's Unibone project,
and it's working well on PDP-11 systems; less well as of late on the VAX.
I've been looking to tidy up a few dark corners in the MSCP spec and one
thing that's left is bus adapter purges -- I have a pretty good grasp of
Unibus mechanics these days but I'm not quite understanding the reasoning
behind this.  Here's what the Storage System Unibus Port Description
(AA-L621A-TK) document says:

"To support such higher-level protocol functions as transfer restarts,
compares, etc., the host memory interface must allow repeated access to a
given host memory location for  both reads and writes.  On purely Unibus
systems such as 11/44, this requirement is trivially met with no
participation by the host CPU.

On systems with bus adapters such as the 11/780, the repeated access
requirement means that the relevant adapter channel may have to be purged,
requiring the active cooperation of the host CPU.  The port signals its
desire for an adapter channel purge by interrupting the host.  The host
writes zeroes to the SA register to indicate purge completion."

This is also discussed, from the bus adapter point of view, in the
technical documentation for the bus adapter itself.   (See
http://www.vaxhaven.com/images/2/29/EK-DW780-TD-001.pdf)  It hasn't been
particularly enlightening to me, but I will admit to not having read every
page of this and the DW780 doc -- maybe I missed something :).

I understand the mechanism here; in essence it's:
1) MSCP controller decides a purge is necessary after a DMA transfer and
requests one by setting a value in a reserved slot in the communications
area
2) Host system (MSCP driver) sees the special value, and issues a purge
command to the bus adapter.
3) Host system then clears the value in the communications area
4) MSCP controller continues on its merry way.

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.)

Anyone have any experience with this?

Thanks!
Josh