DIMM Configurations
Single-sided and double-sided DIMMs
Each DRAM chip on a DIMM provides either 4 bits or 8 bits of a 64-bit data word. Chips that
provide 4 bits are called x4 (by 4), and chips that provide 8 bits are called x8 (by 8). It takes eight
x8 chips or sixteen x4 chips to make a 64-bit word, so at least eight chips are located on one or both
sides of a DIMM. However, a standard DIMM has enough room to hold a ninth chip on each side.
The ninth chip is used to store 4 bits or 8 bits of Error Correction Code, or ECC (see "Parity and ECC
DIMMs" sidebar on next page).
An ECC DIMM with all nine DRAM chips on one side is called single-sided, and an ECC DIMM with
nine DRAM chips on each side is called double-sided (Figure 7). A single-sided x8 ECC DIMM and a
double-sided x4 ECC DIMM each create a single block of 72 bits (64 bits plus 8 ECC bits). In both
cases, a single chip-select signal from the chipset is used to activate all the chips on the DIMM. In
contrast, a double-sided x8 DIMM (bottom illustration) requires two chip-select signals to access two
72-bit blocks on two sets of DRAM chips.
Single-rank, dual-rank, and quad-rank DIMMs
In addition to single-sided and double-sided configurations, DIMMs are classified as single-rank or
dual-rank. A memory rank is defined as an area or block of 64-bits created by using some or all of
the DRAM chips on a DIMM. For an ECC DIMM, a memory rank is a block of 72 data bits (64 bits
plus 8 ECC bits).
A single-rank ECC DIMM (x4 or x8) uses all of its DRAM chips to create a single block of 72 bits, and
all the chips are activated by one chip-select signal from the chipset (top two illustrations in Figure 7).
A dual-rank ECC DIMM produces two 72-bit blocks from two sets of DRAM chips on the DIMM,
requiring two chip-select signals. The chip-select signals are staggered so that both sets of DRAM
chips do not contend for the memory bus at the same time. Quad-rank DIMMs with ECC produces
four 72-bit blocks from four sets of DRAM chips on the DIMM, requiring four chip-select signals. Like
the dual-rank DIMMs, the memory controller staggers the chip-select signals to prevent the four sets of
DRAM chips from contending for the memory bus at the same time.
Figure 7. Single-sided and double-sided DDR SDRAM DIMMs and corresponding DIMM rank
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