Dell Precision T7500 (Early 2009) - Manual (Page 18)

In addition, the dual-processor riser features six additional memory slots. The slots are numbered DIMM1 through DIMM6. DIMM1 is
furthest from the processor.
Memory Population Rules
Your computer requires DIMMs within a channel to be populated starting with the DIMMs farthest from the processor first. This
means the DIMM slots 1, 2 and 3 must be populated before DIMM slots 4, 5 and 6. In addition, when populating a Quad-rank DIMM
with a Single- or Dual-rank DIMM in the same channel, the Quad-rank DIMM must be populated farthest from the CPU.
To maximize available memory bandwidth, DIMMs within a configuration should generally be spread across as many channels as
possible before populating multiple DIMMs per channel. The population guidelines below help to achieve this.
Single CPU configurations (6 DIMM slots on MB)
If configuration contains DIMMs of all the same size, populate in the following order: DIMM1, DIMM2, DIMM3, DIMM4, DIMM5,
DIMM6
If configuration contains DIMMs of mixed sizes, populate the larger DIMMs first. For example, for a 4GB configuration
consisting of one 2GB DIMM and two 1GB DIMMs, the population would be DIMM1=2GB, DIMM2=1GB, DIMM3=1GB,
DIMM4=empty, DIMM5=empty, DIMM6=empty.
Dual CPU configurations (6 DIMM slots on MB plus 6 DIMM slots on Riser)
If configuration contains DIMMs of all the same size, populate in the following order: MB_DIMM1, Riser_DIMM1, MB_DIMM2,
Riser_DIMM2, MB_DIMM3, Riser_DIMM3, MB_DIMM4, Riser_DIMM4, MB_DIMM5, Riser_DIMM5, MB_DIMM6, Riser_DIMM6.
If configuration contains DIMMs of mixed sizes, populate the larger DIMMs in the dual-processor riser.