DEC MKA11 and/or M8261/M8163

Eric Smith spacewar at
Mon Feb 16 04:07:15 CST 2015

On Mon, Feb 16, 2015 at 2:28 AM, Paul Birkel <pbirkel at> wrote:
> Need more experience finding Waldo, too!  I see that only C/D have the
> MKA11 in their associated-rack, so that's 1 Mw/CPU in the configuration
> seen.

Mw/CPU isn't an especially useful measure. It's an SMP systems, so
there's some total amount of memory that is shared by all CPUs. None
of it is dedicated to a single CPU, unless the memory control panels
are used to partition the system, which would typically only be done
for diagnostic purposes.  On a commonplace quad-core desktop PC, no
one talks about gigabytes of memory per CPU core, because the memory
is shared between all of the cores, the same as the normal
configuration of the 11/74.

The fact that the MKA11 boxes happen to be in the CPU C and CPU D
cabinets has nothing whatsoever to do with their being associated with
a particular CPU. They had to be in some cabinets, and those were
somewhat arbitrary chosen when the configuration was designed. Putting
one MKA11 in each CPU cabinet would have resulted in the need for much
longer ribbon cables runs for each memory bus, because the MKA11 boxes
are daisy-chained. Putting all the MKA11 boxes in close proximity
significantly reduces the overall cable lengths. This is why the 11/74
is configured with some CPUs in the left side of their cabinets, and
some in the right side, to specifically allow for two adjacent
half-cabinets to contain up to four MKA11 boxes.

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