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The Blueprints

The last two pages should have provided at least a perfunctory understanding of what Registered DIMMs are doing, the next step is to look at how it is done in real hardware. For illustration purposes, we have a data sheet from Micron showing the PCB and the different units (U1-U22). You may notice that the drawing shows relatively small chips, the reason being that the latest JEDEC specs call for BGA memory components to be used on Registered DDR DIMMs.

Mechanical drawing of a Micron Registered DDR Module (courtesy of Micron)

The memory discreets are rather self-explanatory here, in addition, there are the two registers (U7 and U16) and the PLL (U6). This means that that, for every physical bank on the module, one register is needed but that the PLL is shared between the two banks.

In real life, we have four different modules on the butcher block today, that is, the DIMMs supplied by AMD together with their reference system made by Legacy. Second, we have the only Registered ECC DDR400 modules currently qualified by AMD, supplied by Kingston Technology, third, we have some Samsung modules and fourth, we have some high performance DDR400 Registered ECC DIMMs from Mushkin. Neither the Mushkin nor the Legacy DIMMs conform to the latest JEDEC specs since they are using TSOP components and support faster timings than what JEDEC has specified for Registered DDR400 Modules

             

Left / top: Mushkin Registered ECC DDR400 2:3:2 DIMM, bottom: "Legacy" Module (2.5:3:3). Center: Samsung Original Registered ECC DDR400 3:3:3, Right: Kingston TwinPack Registered HyperX modules (the modules used by Kingston, hidden under the heat spreaders designed by "your's truly", are the same as the Samsung Modules)

In order to paint a correct picture, the different latencies need to be taken into account as well. For the record, JEDEC specifies the latencies for Registered ECC DDR400 modules as 3:3:3, as we were told by Kingston when we asked for the reason for the high latencies used.

One issue that is often overlooked in the context of Registered Components is the enormous work that the registers need to accomplish and which translates into equally enormous power consumption power consumption and even though the latest generation of those "Memory Drivers and Transceivers" are much more behaved than some of the older units, they will still run extremely hot. The same holds for clock generators or PLLs

      

Coose-up and personal with the components on either module, click for larger image.

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