meerkat said:
The T180 looks to be some kind of desktop. The info here shows four slots.
http://www.crucial.com/store/listparts.aspx?model=Aspire+T180
Memory matching can be at two levels. You would need to know more
about the chipset, to know what options might be supported.
The tightest matching, is matching the pairs of DIMMs. For example,
if you wanted a total of 1GB of memory, you would buy two 512MB DIMMs.
The DIMMs in that case, have to match "rows, columns, banks, and ranks".
In terms of memory chips, each memory chip has a size, like 32M x 8, and
in that 32M address space, it takes a certain number of row and column address
bits, to address the memory. Inside the RAM chip, there are multiple
"slabs" of equal size, stacked along the Z axis, and that third dimension of
addressing is called banks. Finally, at the DIMM level, there are ranks (and
to save time, a sloppy incorrect definition would be single and double
sided DIMMs are single rank and double rank respectively).
So, if the chipset was an older one, and needed matching at that level,
you don't have to match the DDR2-800 or the 5-5-5 part, but if the
existing module was 512MB and single sided (8 chips), you'd want the
second DIMM to be 512MB and single sided (8 chips) as well. When there are
timing differences, like one stick is DDR2-533 4-4-4-12 and the second stick is
DDR2-800 5-5-5-12, the BIOS picks the highest common denominator for the
sticks, so the pair would both run at DDR2-533 4-4-4-12 perhaps. The extra
speed of the DDR2-800 stick might not get used, because the DDR2-533 might
hold it back, practically speaking.
Newer chipsets try to match the quantity of RAM on a per channel basis. For
example, you could have a 512MB and a 256MB on one channel, and a 256MB and
a 512MB on the other channel. The "DIMMs across from one another" are not
matched. What the chipset does in that case, is feed the channels independent
commands, or else it disables certain interleaving or page opening/closing
options, to make it work. Generally speaking, this doesn't really influence
the end user's RAM selection process that much, because for the most part,
it still means at least matching the sticks on a size basis. But what it
could mean, which is significant, is you could mix a 512MB single sided DIMM
with a 512MB double sided DIMM, and the chipset could still run in dual
channel mode.
Finally, if you fail to match the sticks well enough to enable dual channel
mode, all is not lost. Most modern chipsets will run in virtual single channel
mode, accessing one stick at a time. The user still sees the same total amount
of memory, and the only impact, is a reduction in memory bandwidth. Some
people, who did a poor job selecting memory, are blissfully unaware they even
made a mistake. So if you get it wrong, the operation of the computer is
seamless, and the computer won't make a big stink about it.
There are some older chipsets, where the motherboard will be quite upset
if the DIMMs don't match, and the computer may fail to post. But it sounds
like your computer is pretty new, so I'm guessing that is not going to happen.
So to know exactly how picky the computer is going to be, you'd want
to find out what chipset is used. You could use a utility like CPUZ,
or Everest, to list the chipset number. Based on the number, then you
could look for more info. Some chipsets have more info available for
them than others - and in a pinch, you can always track down a retail
motherboard's downloadable user manual, to get more help with how to
match memory.
But at the very least, even without any more info about the computer,
I'd try to match the sides and number of chips on the module. If the
existing DDR2 stick is 512MB and double sided, I'd try to find
another double sided DIMM with the same number of chips on it,
as a partner for the existing DIMM. Then pray that the BIOS does
not have a problem handling the selection of a compromise clock
speed and timings. That is because, it might be a lot of work, to find
a stick that matches size and ranks, as well as clocks and timings.
I'm really surprised the user manual for the computer, doesn't say
a few words about the memory. Many motherboard manuals usually
have a short sparse section on population rules for memory.
HTH,
Paul
