Organization
Most DIMMs are built using "x4" (by 4) memory chips or "x8" (by 8) memory chips with 9 chips per side. "x4" or "x8" refer to the data width of the DRAM chips in bits.
In the case of the "x4"-registered DIMMs, the data width per side is 36 bits; therefore, the memory controller (which requires 72 bits) needs to address both sides at the same time to read or write the data it needs. In this case, the two-sided module is single-ranked.
For "x8"-registered DIMMs, each side is 72 bits wide, so the memory controller only addresses one side at a time (the two-sided module is dual-ranked).
