Registered RAM has buffer chips that increase the amount of RAM a
computer can have. You can never use registered memory in mainboards
that require unbuffered memory or vice versa.
Both are generally found in server-type hardware, but unbuffered ECC
works in desktop/gaming hardware. In general, although you can use
unbuffered ECC in mainboards that do not support ECC, the ECC
functionality will often be ignored. To my knowledge, only boards for
unbuffered RAM from Asus, Supermicro and Tyan actually use ECC
functionality when given ECC RAM. The rest of the boards for unbuffered
RAM will use ECC RAM as non-ECC RAM.
There is also "fully buffered" RAM (DDR2 only for now, "FBDIMM" or
"FB-DIMM") which is like registered RAM, but unlike normal registered
RAM which only buffers the control and address lines, FBDIMMs also
buffer the data lines. There is no practical advantage from this for
now as the capacity is not bigger than normal registered RAM and
although it makes mainboard design easier in theory in practice I
haven't seen socket 771 boards cheaper than Socket F or socket 604. And
the buffer chips take substancial amounts of extra power/heat. Being an
Intel concept, FBDIMMS are of course incompatible with everything else.
Here is a breakup of what you need where:
- AMD socket 754: unbuffered DDR, ECC or non-ECC
- AMD socket 939: unbuffered DDR, ECC or non-ECC
- AMD socket 940: registered DDR, ECC
- AMD socket AM2: unbuffered DDR2, ECC or non-ECC
- AMD socket F: registered DDR2, ECC
- Intel socket 478: unbuffered DDR, ECC or non-ECC
- Intel socket 775 (Core2Duo etc.): unbuffered DDR2, ECC or non-ECC
- Intel Xeon Woodcrest (51x0) and Dempsey (50x0) socket 771: FBDIMM DDR2, ECC
- Previous generation Xeon socket 603/604: registered, can be DDR or DDR2 (depends on chipset/board), ECC
In the AMD world, it is determined by the CPU socket type whether you
need registered or unbuffered RAM. In the Intel world, it is the
chipset and hence the board, that is why some socket 603/604 boards
require registered and some require unbuffered. Existing socket 771
chipsets all require FBDIMM.
As mentioned earlier, all boards using registered or FBDIMM memory will
also make use of ECC. Boards for unbuffered RAM might or might not and
generally only boards from Asus, Supermicro or Tyan will use ECC. The
rest will ignore ECC but generally work.
Please feel free to email me with questions. The pages on wikipedia are
also up-to-date with more information on this. Keep in mind there is
never an actual guarantee that a give piece of memory works with a
given mainboard or CPU.