In many of the replies regarding RAM & mixing different modules (for instance, PC100 with PC133, or PC2100 with PC2700), the answer is usually that your system will only run at the speed of the slower module. I don't understand this...if I setup my mobo to run the memory at 133mhz, isn't that the speed it will run the memory at? I mean, if I install a stick of PC100 & the board is setup to run at 133, that stick won't override the mobo setting & clock back to 100, it will try to run the memory at 133mhz. If the memory can handle that speed, it will work, if not, there'll be problems...am I wrong?

YES, you are creating a bottle neck by using PC100 memory in the above scenario, therefore bus signaling will slow down to accomodate that throughput.........

Please explain if you can...I've heard this said many times before but no one ever explains the theory behind it...if the PC100 can handle the 133mhz speed, how does that create a bottleneck?

IF the pc100 can handle the speed and IF you set the settings and timings on the pc100 to handle that speed, then you are overclocking the memory and there is no bottleneck. of course, you also end up with charcoal memory not long after, assuming your computer bothers to boot up in the first place. Unlike high performance dram, you cant do much with sdram without risking the ram. As for the bottlenck, having a fsb of 133 means that it is capable of running at 133, however, if your ram isnot, then your bus will not run at 133 because it has to wait for its previous signal to clock before it reads another. If your memory is to slow, what happens is that the bus can only travel as slow as the memory allows it too. This of your bus as a high speed highway with a 133 mph limit. If you have to stop for red traffic lights every 5 minutes, chances are you will never reach 133 miles per hour. If the lights are 20 minutes apart and timed so that you never hit one though, then you'll be cruisin.

"if the PC100 can handle the 133mhz speed"

The pc100 cannot handle the 133 speed. If it could it would be sold as 133. As x86 said the computer adjust the speed to handle what ever you have installed at the lowest speed. This is the function of electronics design in the pc. I assume the designers did this to keep the computer from crashing if there was a mismatch.

I disagree...I have PC100 running at 133 right now...it's been like that for a LONG time & it hasn't "turned to charcoal" yet

I run PC100 with 112MHz in my old P2B board and in a neighbours computer a 128MB stick of PC100 worked at 133MHz with CL2 settings for about a years. It's still fine but we did a major upgrade with 512MB CL2 PC133 and a geforce TI4200 graphiccard lately.

If Jon is correct it may not work at full speed but it brought a nice add to the speed.
For example overclocking my Pentium2 350MHz to 392MHz by setting the FSB (and the ram speed) from 100 to 112MHz brought +10% score in 3D Mark 2000 and 2001!

jam,

I see no reason good 100Mhz memory can't run at a higher bus speed. I'm sure memory marketed before the 133 stuff was sold to consumers also came in good, average, and crappy flavors; just like it does now.

I said "The pc100 can not handle the 133 speed. If it could it would be sold as 133."
I should have said it "MIGHT NOT instead of "can not".
What I am pointing at is similar to cpu's that sell for one speed but it may run at a higher speed. This is because they were tested for the speed they were sold for and not graranteed to run reliable at a higher speed.

Quick and easy:

PC100-->Runs at 100. Can be overclocked to higher, but make sure you have high quality memory for overclocking.
PC133-->Runs at 133. I would reccommend just getting two of these, or how many ever you need, because they aren't much more money than the pc100, and there's not a chance of the memory chip lighting up and destroying/damaging the motherboard.

When you put in a new cpu the mobo bios immediately reads a few bytes off of it at boot up in order to know the proper multiplier, fsb, vcore, cache size, etc. Those settings may later be modified by the user, if indeed the cpu can handle different settings.

When you put in a memory stick the mobo bios does the same thing, by reading the SPD chip

http://www.pc100.com/dimmspd.jpg

which is on all memory. There it finds the cas latency, clock speed, and other settings that the stick is capable of. That is why the default settings in bios for memory is usually called "auto" or "spd detect"

When you put in sticks of different speeds, the mobo bios will use the easier setting, which means the default speed of both sticks becomes the default speed of the weaker stick.

But just like with a cpu, the user can then change whatever settings he wants, to see if both sticks can run at a better speed. In your initial post you asked if you were right or wrong. You were right.

johnoh,

The initial question isn't really about whether I'm right or wrong, but more about what is the correct answer. I've seen the question on here many times & the answer usually is, "the system will run at the speed of the slowest stick", which I feel isn't necessarily true. I understand what you're saying about the mobo autoconfiguring the memory to the easiest setting...but if you have quality memory, you should be able to tweak the settings to make PC100 perform as PC133. Wouldn't the same go for DDR? PC2100 as PC2700? I haven't jumped on the DDR wagon yet so I have no 1st hand experience with it...

Thanks to Johnoh for making it perfectly clear.

'I've seen the question on here many times & the answer usually is, "the system will run at the speed of the slowest stick", which I feel isn't necessarily true.'

Maybe the best way to phrase it is "if the mobo is set to autoconfigure, it will use the settings located on the spd chip of the weaker stick. If not set to auto, the system will use the setting you put in manually."

When a memory speed is brand new, like pc4200 is right now, it tends to not be overclockable. A maker will make a bunch of sticks and test them all, and the few that can live up to a pc4200 rating will be rated that way. The others are labelled lower according to more testing, but also very much due to product demand. So the result is that the speeds that are being commonly purchased have large lots of sticks that are capable of higher speeds but are not rated higher because the demand is not there for them. So the result is that the pc2100 sticks may be just as good as the pc2700 sticks.

My method of buying memory is to buy the fastest rated cheap stuff that is still below the knee of the price curve, such as if the generic pricing was

fast - $x
faster - $1.1x
even faster - $1.2x
fastest - $1.6x

I would buy the "even faster" stick, which in today's terms would be a toss up between pc3500 and pc3700 for $75-$90. The objective being to maximize the economic efficiency of the purchase. An $80 generic pc3500 stick is going to run at a higher mhz than an $80 corsair pc2700 stick.

Some will say yeah but what about timings? Well timings are just a trade off for mhz. You can tighten timings by lowering mhz (usually). And if person A pays $70 for a stick that runs at 220mhz (meaning 440mhz since its ddr) at 7-3-3-3 and person B paid $100 so he can run at 220mhz at 5-2-2-2, person B paid 40% more to get maybe 1% of real world benefit.

Gotta hand it to ya johnoh, you sure have a way with words ;)

Thanks