Beekerc, The R900 memory population rules are as follows; All systems will include 4 Memory Risers  Supported configuration is a minimum of one DIMM in each riser  Memory must be populated beginning with DIMM Slot 1  Additional memory must be added sequentially beginning with Slot 2  DIMMs in the same number sockets in risers 1 & 2 or risers 3 & 4 must be the same  Identically numbered FBDIMM sockets for both memory boards must be populated with FBDIMMs identical in terms of timing, technology, and size. Example: DIMM A1 and B1 must be identical.  FBDIMMs installed in different socket positions (numbers) on a memory board can be different for dual-channel operation. Example: DIMMs A1 and B1 can be different from DIMMs A2 and B2.  Additional memory can be added by installing identical pairs of DIMMs in the lowest numbered available slots.  So to answer your question the pair would be riser A1 and B1, not 1 and 2 on riser A. Let me know if this clarifies things.
I have an R900 with 128GB of RAM (4GB DDR2 667 FBDIMM  x 32 sticks, split between four risers).  I'm retiring some 2950's which have 8GB sticks of the same type (DDR2 667 FBDIMM).  According to the hardware guide, the R900 memory is arranged in 16 banks of pairs.  What i want to know is as i start replacing the 4GB sticks with 8GB sticks, what is the mapping strategy? let's say i just want to swap in one pair of RAM.  is the pair Riser A, position 1 and 2, or is the pair Riser A, position 1 and Riser B position 1?  then for the next pair, can i keep installing in Riser A and Riser B or does the next pair have to go into Riser C and Riser D  Thanks in advance beekerc
R900 memory upgrade - memory paring placement