DDR1 SDRAM module has only one. Also, each which dimm slots to use has eight RAM chips, but the lower one has an unoccupied space for the ninth chip.
A DIMM or dual in-line memory module comprises a series of dynamic random-access memory integrated circuits. Intel P5-based Pentium processors began to gain market share. While the contacts on SIMMs on both sides are redundant, DIMMs have separate electrical contacts on each side of the module.
Another difference is that standard SIMMs have a 32-bit data path, while standard DIMMs have a 64-bit data path. Since Intel’s Pentium, many processors have a 64-bit bus width, requiring SIMMs installed in matched pairs in order to populate the data bus. The processor would then access the two SIMMs in parallel.
DIMMs were introduced to eliminate this disadvantage. Variants of DIMM slots support DDR, DDR2, DDR3 and DDR4 RAM. 278-pin DIMM, used for HP high density SDRAM.