A hybrid DDR5 DIMM device includes a PCB board with a host interface through one of two DDR5 sub-channels, and a plurality of DDR4 or slow DDR5 SDRAM chips on the PCB coupled to this single channel DDR5 host interface. An embodiment processing system includes a host CPU to access one or more pairs of hybrid DDR5 DIMM devices for 4×DDR5 memory capacities (4DPC), a first or second hybrid DDR5 DIMM including a plurality of half-speed SDRAM chips, and a first or second DDR5 sub-channel coupled the host with slow SRAM chips on DIMM. Mounting same data-buffer and RCD chips on hybrid DIMM to a server motherboard can double available DDR4 DIMMs' speed to DDR5 speed rate. Pairs of hybrid DDR5 DIMM devices cascaded one-by-one can aggregate more DDR5 DIMM devices to expand memory capacities at double speed of DDR4 or DDR5 SDRAM chips, beyond current DDR5 speed limit 6400 MT/s.
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