how to choose 2-pair or 1-pair RAM?

No, that is not at all how RAM works.

You are looking at each IC on the memory "stick" as separate memory devices. They are not. The individual ICs on the stick form a "bank" of memory and data is stored in and retrieved from the bank all at once, in "registers" which are typically 32-bit or 64-bit wide.

So look at a RAM stick as a bunch of toll-booths across an 8 lane highway. But in this case, the 8 gates are controlled by one lever (the bus clock, in this case). And the rules say the gates can only open when all 8 are ready, and then they all open and close at the exact same time, on the same clock cycle. And only when the "checksum" of the 8bits is correct (not corrupt).

The 8 booths represent the ICs (chips) on the sticks. The first 8 cars (represented by 1s and 0s in a precise order) move into the booths at the exact same time - NEVER one at a time or else that changes the meaning of the data. Then they move out at the same time. Then the next 8 bits are moved in at the same time, then again, out at the same time. Always maintaining the same 1 and 0 order.

If just one of the chips on a stick is bad, then none of the other bits can move in or out because the checksums of the data will not match - the data will be corrupt, and your computer will halt.

You might also envision a troop formation, 8 columns wide. When the Drill Sergeant commands, "Forward March ", they all "step off" at once.

You are correct that a static discharge can damage just one IC on the memory stick, but that renders the entire stick useless, not just that one IC. This is because there are now no longer 8 booths. And without all 8 booths working properly, the other 7 cannot open.

Of course, it is all much more complex than what I described. But that's the basics.

 

thanks Bill for the tech update , just one question , does the "booth" relate to each memory bank - if so can one bank fail but function maintained at reduced capacity by the other bank.

 

Now you are talking about motherboard functions and how the memory controller does its thing. I have never heard of "partial" failure like that but I would think the computer would not make it past POST - power on self-test.

 

or a sudden reduced capacity notification ? - i suppose not likely

 

That is possible when a whole stick fails in such a way that it is no longer detected in the socket. That is not uncommon, but it should be noted that RAM failing is not that common anymore - except from mishandling. Note it is very difficult to find RAM that is NOT warrantied for life.

 

btw OP , the modules are keyed meaning they can only be correctly inserted one way - so take care which way you try to insert the modules or else static will be the least of your problems as smoke billows from your case (of which i vigorously deny first hand experience )