Putting two memory controllers on a 780?

[Dave Martindale]

There are two types of memory controllers for 780's.  The MS780-C (the older
one) does not do interleaving internally, and should capable of considerable
speedup by having a second MS780C installed and run interleaved with the
first.  Not only do you get twice the available memory bandwidth for
accesses to consecutive words, but the memory controller contains a 4-deep
buffer for requests which is enabled only when interleaving is turned on.
Thus it can handle short bursts of even higher rates without appearing
busy to a device trying to access it.  Interleaving must be turned on
before you load any of unix into memory; if you don't, the memory
in the second controller won't even be found unless the first controller
has a full 4Mb on it.

The newer controller, the MS780-E, has been installed on VAXES shipped since
June or July or so, and does interleaving internally.  It seems an all-around
better controller.  It stores data internally as 32-bit words, so it can accept
a 32-bit longword write without doing the read-alter-rewrite cycle that the
older controller must do (the old controller can do only quadword writes
without a read).  It fits in the same space as an old controller but takes
up to 16Mb of memory.  It seems uniformly faster than the old controller;
for many operations the memory operates as fast as the SBI can handle the
data.  And you don't need to buy a CPU expansion cabinet to put the second
controller in.  But you have to figure out what to do with the old
controller that is replaced.

At Waterloo, we bought one new controller, put it in one 780 replacing the
old controller, and then installed the old controller as an addition to
another 780, giving it two old controllers interleaved.  Thus both end
up with more memory bandwidth.  (you can't interleave old and new controllers).