Everything About [Computers :: Memory]

Performance: General Rules of Thumb

1.) Duplicate sticks of memory regardless of capacity will always yield ~5% better performance (with motherboards supporting Dual Channel) i.e. 2x512MB is ~5% faster than 1x1GB. Don't make a mistake by putting in an odd number of memory sticks, by doing so you're disabling the dual channel capabilities. Stick to a single pair of memory if at all possible, the 2nd pair adds addressing overheads.

2.) All motherboard chipsets are only capable of adressing a certain amount of memory before the point of diminishing returns is met with regard to degrading performance (ie. there's a technical limit on capacity, as well as recommended limit - before performance degrades).

3.) Differing ratings in sticks of memory will only operate at the lowest common denominator... or there WILL be stability concerns. Running DDR333 w/ another DDR500 @ DDR500 speeds is a substantial overclock for DDR333. The same is true w/ CAS (latency). Running CAS4 with CAS2 is more than a substantial overclock for CAS4.

4.) Synchronous bus is VERY nice, but not nearly as much these days since newer chipsets do seperate the 2x better (which is very good).

VERY Generally speaking, recommended limits for various generations are:

486D4/Pentium 1/Cyrix/AMDK6 = 64MB
Pentium 2/AMDK6-2/AMDK6-3 = 128MB
Pentium 3/AMDK8 Thunderbird's = 256MB
Pentium 4/AMDK8 XP's = 1GB
Pentium D/AMD64/X2 = 2GB
Core Duo/Opteron/Xeon = 4GB+
Itanium = XTB

There are obvious differences in chipsets as it pertains to quality & capability. Therefore, if you buy a brand-x value brand motherboard for ~$5 and have a ~PD/A64/X2... you can expect a good performance hit w/ 2GB, so stick w/ 1GB. Also important to note, many 1st-round Core Duo motherboards are only capable of 2GB with all recent worthwhile boards supporting 4GB.

Performance: Ratings

Memory is something that can confuse many uninformed people. Back in the day, there was EDO, then there was SDRAM... with it was a bold move to 66MHz and above FSB's. However, the move from SDRAM (ie PC100, PC133, etc.) to DDR required a little design change. DDR = Double Data Rate, and to grossly over-simplify things, it means twice the data is sent per clock or 1MHz. SOoooooooooooooo, that 200MHz stick of memory we say is DDR 400 ("acts" like PC400 aka 400MHz = 2x200MHz). DDR, DDR2, DDR3, etc. are NOT interchangable, just a heads up.

Memory is "rated" by an international board called JEDEC. They devised the following ratings:
PC8000 = 1000MHz or 1GHz DDR-2
PC6400 = 800
PC5400 = 667
PC4800 = 600MHz DDR (DDR-1 Max Speed Limit)
PC4400 = 550
PC4200 = 533 (Slowest DDR-2)
PC4000 = 500
PC3700 = 466
PC3200 = 400
PC2700 = 333
... and so on ...

There's alot of marketeering, jargon, buzzwords that the various companies like to throw at the consumer. Rather than detailing the differences between DDR, DDR-2, and DDR-3... let's just say for simplicity sake that DDR-2 is able to operate at the Higher Bandwidth of 1,000MHz or 1GHz because of SUBSTANTIALLY increased latency. Same example as CPU's: DDR-2 = 10 lanes of traffic at 45mph vs DDR-1 of 5 lanes of traffic at 90mph.

Performance: Latency

We measure this latency or responsiveness in various "timings" given as a CL X-X-X-X number. The lower the number, the lower latency... we want low numbers here. All 4x numbers "mean something" and define the latency as a whole, but generally it's the CAS (the first number) that holds most sway. Therefore, most will use latency and cas hand in hand... Ideally, you want high bandwidth memory that can operate at the lowest latency. So, in DDR-1... you should get the highest quality, highest bandwidth you can afford with a CAS of TWO (2)!!! You do NOT want CAS 2.5, and CAS 3 is the bottom of the barrel as most motherboards don't even support a cas of 3!!! Yes, there is a CAS lower than 2, but AMD CPU's aren't designed to handle such low latency memory... so rather than lowering it needlessly, we instead raise the bandwidth.

Currently, the best DDR-1 memory can operate @ 533MHz DDR w/ CAS 2... so that's ideally what we want. U can go higher than 533MHz w/ DDR-1 no problem, but you'd have to raise the timings (ie CAS 2.5, etc.) which MAY result in increased performance... it's a balancing act as it could just as easily lower performance. Just depends how well the motherboard manufacturer nailed down the bios and overall board quality.

Memory operates only at the least common denominator, otherwise you're overclocking... therefore, if you put CAS2 and CAS3 sticks in, 'you' should be setting the BIOS to CAS 3. Especially if you bought the cheap stuff, otherwise you're going to run into stability problems. If you were confident in the component quality, you *could* feasibly overclock the CAS3 memory to run at CAS 2 (more reason to buy quality). If you buy CAS 2 (like you should be this whole time), then you will be gauranteed to have no problems running it with your older memory (since the new stuff will be better/faster/more capable).

Manufacturers

Brand *technically* means nothing... it's all about the rating/specifications. (eg. DDR400MHz @ CAS2). What brand names get you are a history of success, warranty, and stability. You CAN mix brands no prob.

Pick Mushkin, Corsair, or OCZ... that's it. G.Skill and Patriot are both wanna be value brands that claim some performance at this point in the game. Not enough history to develop a track record and I don't like getting burned with crap components. So stick with what you know is quality, especially with price differences being neglible.

AMD vs Intel

AMD's AthlonXP/64/X2 architecture simply LOVES low latency memory... it's why early-on they kept with DDR instead of moving onto DDR2. Runing a system with CAS2 is a pure delight as everything definately appears more snappy. It's been argued in the days of Pentium 4, that even though AMD's chips were faster (could encode a DVD quicker), the Pentium 4 just has an enormous amount of bandwidth available to it. It's why the Pentium 4/D will forever seem more snappy than AMD and can multitask several magnitutes better than AMD. Moral of the Story: All AMD systems *should* be using the lowest latency memory possible (ie. CAS2 in terms of DDR)... you'll thank me!