Quote:
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Originally Posted by initialsBB
DIMMs for Mac Pro must fit these specifications:
667 MHz, FB-DIMMs
72-bit wide, 240-pin modules
36 devices maximum per DIMM
Error-correcting code (ECC)
i know i made a mistake above saying it was 32 modules but, duh, it's 36... question remains the same however.
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On a "normal" stick of RAM, you have 8 memory chips ("devices"), each transmitting one byte for a total of 64 bits. On ECC memory, there's 9 memory chips making them 72 bit wide. The extra byte is used to check the validity of the other 8 chips' data.
In order to double capacity without resorting to higher-capacity chips, the factory can put 8 (or 9) chips on each side of the memory stick, thereby cramming 16 (or 18) chips on a stick.
In servers, with their usually larger requirements for memory, you can also find memory sticks that have two memory chips on top of each other, a bit like a double-decker bus, thereby facilitating 32 chips on a stick of RAM (or 36 with ECC).
The memory controller obviously needs to be capable of "reading" the 16 extra chips, or you'll waste half the capacity of the stick of RAM... I am not aware of sticks that are stacked 4 chips high on both sides, but they might be out there (or arrive at some point in the future) and they won't work properly in your new Mac Pro.
The number of rows of chips (8 or 9 chips in a row) is called "rank".
The Mac Pro supports single- dual- and quad-rank memory modules.