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DIMM Configurations

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.

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