With equal amounts of memory in both slots, memory can be "interleaved" so that successive chunks of memory alternate slots. That way, memory accesses get distributed to both slots almost perfectly evenly, allowing their bandwidth to combine. With uneven amounts, memory cannot be interleaved and has to be mapped first to one stick and then to the other. A program that is accessing a large contiguous chunk of memory will find almost all of its accesses going to one stick, and there will be no bandwidth combination.
The question of RAM compatibility is extremely complicated, so
fitting RAM to motherboard is chancy if the RAM is not certified.
To understand the complexity, here are some of the factors you should take
under consideration when fitting RAM to motherboard.
Memory technology
DDR, DDR2, DDR2, DDR3, DDR4 (they won't even fit in the same slots).
CAS latency
How many clock cycles the memory module will delay in returning data requested by the CPU.
Voltage
Higher voltage is usually faster, but low voltage generally means less stress to the CPU memory controller.
Timings
Given through a series of numbers, for example 4-4-4-8,
indicate the number of clock cycles that it takes the memory to perform a certain operation.
In the BIOS timings are usually detected automatically, but some BIOS allow
for timings to be preset as part of over-clocking.
Denoted usually by CL-tRCD-tRP-tRAS-CMD, where:
- CL: CAS Latency. The time it takes between a command having been sent to the memory and when it begins to reply to it. It is the time it takes between the processor asking for some data from the memory and then returning it.
- tRCD: RAS to CAS Delay. The time it takes between the activation of the line (RAS) and the column (CAS) where the data are stored in the matrix.
- tRP: RAS Precharge. The time it takes between disabling the access to a line of data and the beginning of the access to another line of data.
- tRAS: Active to Precharge Delay. How long the memory has to wait until the next access to the memory can be initiated.
- CMD: Command Rate. The time it takes between the memory chip having been activated and when the first command may be sent to the memory. Sometimes this value is not announced. It usually is T1 (1 clock cycle) or T2 (2 clock cycles).
RAM frequency
The signalling rate what words of data can be transferred into or out of the memory. This is part of the RAM label, for example DDR3-2400.
RAM Density
Denotes the on-chip organization.
Low density modules have 100% compatibility with all systems and chipsets.
High density modules only have 10% compatibility and are very slow.
RAM controller
Controllers are limited in the amount of RAM they can address,
thus limiting the motherboard size of the RAM stick.
Chipset technology
The motherboard chipset affects using memory, an example is regarding
multi-channel configurations.
I think we can already conclude that for a RAM stick to be compatible with
a motherboard, both must agree on a very large number of parameters.
Any incompatibility on any one parameter will make a given RAM unusable
on the motherboard.
Some motherboards will accept RAM whose settings are in a certain range,
rather than one specified amount,
but others will give error or even fail to detect RAM that does not suit.
RAM bought from the manufacturer of the computer is guaranteed compatible,
but may be costly. When looking for third-party RAM, it is usually
hit-and-miss, since the motherboard specs are never fully published.
I have several times used the
Crucial Advisor tool, or even better, the Crucial System Scanner,
found on this page.
In my experience their results are always correct, and the RAM that is
proposed by Crucial has a very competitive price.
References :
Best Answer
This is to help with insertion. From www.kingston.com:
"Curved edge DDR4 modules feature a curved edge to help with insertion and alleviate stress on the PCB during memory installation."