A memory rank is defined as an area or block of 64-bits (72-bits for ECC) created by using some or all of the DRAM chips on a DIMM. 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 memory controller chipset. 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.