Aastii
VIP Member
Will break it down:
When you look at your motherboard, depending on the motherboard, you will probably have 2 different colours of DIMM slots in 2 banks of 2, so you will have a total of 4 slots, of 2 different colours. If you look closely at the board, it will say somewhere near the DIMM slots, usually in between the DIMM slots and CPU socket
DIMM A_1
DIMM A_2
DIMM B_1
DIMM B_2
This shows which banks work together, so if you have 2 stick of memory, putting them in A and A will make the memory work in dual channel, putting them in A and B will make them run in single channel.
Dual channel is better than single because it doubles the memory bandwidth, so you use the full speed of the memory. But there are limitations to when you can use it.
If you mix speeds, so for example DDR2 667MHz and DDR2 800MHz, the speed of the 800MHz stick will be underclocked to 667MHz. This is because to work in dual channel, the memory must be at the same speed, so to get them the same there would be 2 options - overclock the 667MHz, which could cause instability, damage to the DIMM and void warranty, or underclock the 800MHz, which will hit performance of that 1 stick of memory, but will keep stability, and give overall better performance.
Same DRAM technology is sort of a silly thing to say, that means you must use only DDR, or only DDR2, or only DDR3 memory, but it is impossible to run different standards together because of the layout of the pins, you can't put DDR memory in a DDR3 motherboard for example, it just won't fit.
Same bus width is the same as saying they must be the same speed. Double sided or single sided DIMMs means that there are banks of chips on just 1 side, or on both sides of the stick of memory.
You can mix brands, and you can mix timings, but remember that this will decrease the likelyhood that the DIMMs will work together correctly in dual channel (the chances that they are matched). Mismatched sets of memory will cause problems that you would usually associate with faulty memory, when in fact the memory isn't faulty, it is just that the DIMMs aren't happy working together.
I left the "must be sme capacity" to last, because that is sort of a grey area. You can use a 512MB and 256MB stick together, but only 256MB of the 512MB stick will be used, so you will have the equivelant of 2 x 256MB sticks, for a total of 512MB working in dual channel.
However, some motherboards of the time used a technology that Intel introduced called Flex mode, which if you use different sized DIMMs, it would use the full capacity. To explain how:
Take for example a 1GB (1024MB) and a 512MB stick of memory that you wish to use together in dual channel. Like it would do with normal dual channel, it would take the smallest capacity (512MB) and use that much from the largest stick of memory, so would have:
512MB stick + 1GB stick - 512MB
512MB + (1024 - 512MB)
512MB + 512MB = 1024MB
So you would have a total of 1GB working in dual channel, with 512MB of the 1GB stick left over doing nothing.
What it does with that when you have a motherboard that supports Flex mode is run that excess memory in single channel, so it will use the total 1536MB
1024MB Dual channel + 512MB single channel = total 1536MB used
