I am thinking of adding some RAM to my PC, and I was reading my
motherboard manual about RAM. It says you should try to get 2 chips
from the same manufacturer if you are going to put them in a pair of
DDR slots. How important is it that they are from the same
manufacturer?
There are two requirements.
1) There is what the motherboard really needs technically.
2) There is what the BIOS feels it needs, to make dual channel work.
Sometimes those aren't the same thing. For example, there was one
AMD motherboard, where the BIOS was checking the PCB revision field
stored in the SPD EEPROM on the DIMM. Which is a stupid thing to do,
and totally unnecessary. A guy bought two DIMMs that looked like
they matched, and yet the BIOS rejected them as not matching. That
doesn't happen too often.
The most constrained motherboards, are the ones that operate in 128 bit
mode. That is where DIMMs in separate channels, receive exactly the
same address and command information. For such motherboards to work
in dual channel, they have restrictions on DIMM population. For example,
using two DIMMs or four DIMMs, is all that is allowed. They don't allow
three DIMMs in those boards.
In such a case, the rows, columns, banks, and ranks of the DIMMs have
to match. That is all that is needed worst case. The color of the DIMMs
doesn't matter. The brand doesn't matter. The chips have to match, in
the above stated way, so that when they receive identical control info,
they respond in exactly the same way.
Some motherboards have a more relaxed requirement. Those motherboards
feed different information to each channel. In those cases, all that
needs to match, is the total memory quantity on each channel. For example,
on one of those motherboards, you could stick two 512MB DIMMs on
one channel, and a single 1GB DIMM on the alternate channel, and
it would run dual channel.
Rows, columns, and banks, are the three internal dimensions of a
memory chip. A memory array is addressed with rows and columns.
Inside a memory chip, there might be four banks (four separate
arrays). To address a chip, the row number, column number, and
bank number are fed to the chip.
Memory chips work side by side, to make a wider structure. If you
stick (8) 8 bit wide chips next to one another, you get a 64 bit
wide array. That happens to be the width of the memory bus, connected
to the DIMM.
If there are sixteen chips on a DIMM, eight are in one array and eight
are in a second array. For purposes of unique terminology, these
are referred to as two "ranks". An imprecise way to think of it,
is "double sided" = "dual rank". So you want the "sidedness" of the
memory DIMMs to match. You aren't matching, for example, if one
channel has a single sided 512MB DIMM and the other one has a
double sided 512MB DIMM. Those would actually need different
row, column, and bank numbers to work.
So, insisting on using exactly the same memory product (same
color, model number blah blah blah), helps ensure that the
rows, columns, banks, and ranks match. It also helps in cases
where the BIOS is poorly designed, and the BIOS is being a bit
too picky about what is a matching pair of DIMMs.
The user manual may give hints about how picky the architecture is.
If you mention the motherboard in question, I can give more
info if needed.
Paul
