User guide

and so on. By installing identical RAM modules
in the matching A & B slots (i.e. A1 & B1, A2 &
B2...), you enable interleaved access to RAM as
two banks (A and B). Basically the benefit is to
reduce wait states and contention when accessing
main memory. This provides potentially higher
bandwidth (throughput) on the memory bus. An
analogy might be of a box with two openings
(one on each side) versus one with only a single
opening. If someone were putting items into or
removing them from one opening, another per-
son could still have access via the second open-
ing. This is a crude simplification but you get the
general idea. By interleaving memory the two
partner DIMMs can be accessed a one really big
fat wide DIMM.
For increased performance it is better to config-
ure a PCI-based Power Macintosh computer for
memory interleaving rather than installing mem-
ory in a non-interleaved configuration. This
means that you will get better performance if you
configure your system with two 16 MB DIMMs
rather than one 32 MB DIMM. This applies to all
other combinations of same-sized DIMMs. My
128-Meg DIMMs partner up to form 256-Meg
DIMMs through the magic of interleaving.
The actual performance will vary from computer
to computer. In general, though, a Power
Macintosh with a PowerPC 604 microprocessor,
such as the Power Macintosh 8500 or 9500 series
computer, or any clone that has the same board in
it, gets anywhere from a 5% to 15% boost in per-
formance. The average is about an 8% increase in
performance speed. On a Power Macintosh with
a PowerPC 601 microprocessor, such as the
Power Macintosh 7500 series, you may get only a
slightly better performance gain by using memo-
ry interleaving rather than non-interleaved
DIMMs. Some third-party benchmarking appli-
cations may report exaggerated performance dif-
ferences between interleaved and non-inter-
leaved computers.
The question which comes to mind is, how do
you populate DIMMs in your PCI-based Power
Macintosh Computer to maximize performance
using memory interleaving. If you have an odd
number of DIMMS, where should you place the
odd DIMM to get the best performance from
memory interleaving? Interleaving is accom-
plished by 'pairing' two DIMMs in corresponding
slots. That is, one DIMM in A1, and another
DIMM in B1 will set the machine up to use mem-
ory interleaving. What about the DIMMs that
don’t have identical partners?
If you have an odd number of DIMMs, the
matched pairs will run the memory interleaved.
The odd DIMM will then run non-interleaved.
For the interleaving to be most effective, the
DIMMs must be the same size and speed, (usual-
ly, should be of the same manufacturer, but not
necessary). In reference to the memory address-
ing, the A1/B1 will be the lower addresses, going
up to the A6/B6 being the highest address.
In relation to performance, it really does not mat-
ter where the DIMMs are placed. The software is
intelligent enough to figure out which banks are
being used, and is able to "stitch" the memory
together as required.
Note: Memory interleaving is only available in
the Power Macintosh 7500, 8500, and 9500 series
computers. The Power Macintosh 7200 uses a dif-
ferent memory controller which does not support
interleaving.