Dell Precision T5500 (Early 2009) - Manual (Page 4)

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)
l If configuration contains DIMMs of all the same size, populate in the following order: DIMM1, DIMM2, DIMM3, DIMM4, DIMM5, DIMM6
l 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 3 DIMM slots on Riser)
l 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, MB_DIMM5, MB_DIMM6.
l If configuration contains DIMMs of mixed sizes, populate the larger DIMMs in the Riser.
NOTE: If any DIMMs are >30mm tall (possible early 16GB DIMMs), they must be installed on the system board only.