This is a feature of your processor. Prior to Venice Revision E
processors, the B1 and B2 slots are on the "primary" 64 bit bus.
The A1 and A2 slots are only to be used when running in dual
channel mode. If you have two matched sticks, you could place
them in A1 and B1, for example. If the sticks do not match,
install them in B1 and B2, which is the "single channel" bus.
Venice corrects this fixation with the B bus, which the earlier
processors had.
Before Venice (like Clawhammer, Newcastle, Winchester)
1 stick - place in B1 or B2.
2 sticks - place in B1 and B2, for single channel mode.
If sticks are matched, place in A1&B1 or try A2&B2
3 sticks - invalid configuration
4 sticks - must be two matched pairs. If the sticks do not
match, some DIMMs will be ignored, or the computer
will not post. Many users have trouble, even when
they follow the rules to the letter. Set memory
speed to DDR333 first, and then install the second
pair. It might POST for you.
Venice or later:
1,2,3, or 4 sticks work. A and B slots have equal qualities
(no more "B is king").
If sticks do not match, processor memory controller runs in
single channel mode.
If 2 or 4 sticks are installed in matched pairs, in slot
pairs A1&B1 and A2&B2, the BIOS should use dual channel mode.
With 3 sticks, only single channel mode is possible.
The Asus BIOS still does not work properly with respect to
all of these options. You may get a surprise, depending on
what mix of DIMMs you try. Consult this document, as it
explains the memory controller from AMD's perspective -
this is full of technical jargon, and is hard to read.
http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/26094.PDF
Having to explain all this stuff to customers, must drive
Asus phone support crazy
Paul
