PC100 SDRAM: A Closer Look

What are the two most important things to look for when purchasing PC100 SDRAM? If you think that the Access Time rating on the chips themselves is one of them then you are in for a bit of a surprise.  According to the Intel PC100 Specification the chips used on a PC100 module must be PC100 compliant, meaning that they carry no specific access time rating, the only stipulation is that they must work at the 100MHz bus speed.  The theoretical limit for stable 100MHz operation is 10ns, and therefore, as long as your module is equipped with 10ns chips or faster than you'll be ok.

There have been a number of posts online about vendors offering 6ns PC100 SDRAM, the only PC100 Compliant Memory that can boast faster access time than the cream of the crop PC100 SDRAM is the high quality and high performance 133/166MHz ESDRAM which AnandTech should receive for testing within the month.  Most vendors that advertise 6ns SDRAM only do so on Price Search Engines and actually list 10ns chips on their site, or justify their7 claims by saying that the SDRAM brands "vary" from time to time.  Basically, as far as the public is concerned, < 8ns chips (manufactured by Samsung, no other PC100 chips are mentioned in this review and will be discussed later on, Hitachi does apparently manufacture PC100 SDRAMs rated at 6ns) are not available and you should be careful of ordering any products from a company that incorrectly advertises their memory. 

Now that we've settled the number issue, the two most important things when purchasing PC100 SDRAM are: the Quality of the Printed Circuit Board (PCB - the board the RAM Chips are placed on) and the type and rated latency of the SDRAMs on the DIMM.

How to Print [a circuit board]

When dealing with levels of importance, the lesser of the two when dealing with PC100 SDRAM is the Quality of the PCB.

The reason for this is because Intel has already implemented such strict regulations on the design and implementation of the PCB which must be used with PC100 Compliant SDRAM modules.  The regulations are so strict in fact, you won't notice much difference between a PC100 Complaint module that is manufactured on a 6-Layer PCB in comparison to one manufactured on an 8-Layer PCB (when dealing with circuitry, in most cases the more layers you have in PCB the better the PCB).   Basically, as long as a module's PCB is PC100 Compliant, you won't have any problems running that module at bus speeds up to 133MHz, on the part of the PCB that is.

Chips

Here's where the problems come in to play.   Take two PC100 SDRAM modules, a Corsair PC100 DIMM and an Advanced American Megatrends PC100 DIMM for example.  At the 100MHz Front Side Bus Speed (the frequency at which the System RAM operates at), they will each perform, and run as reliably as the other.  Bump the Bus Speed up to 112MHz and they both continue to run strong, neither even so much as flinching under the out-of-spec conditions they are placed under.   Give the 133MHz Bus Speed a try and one of your test systems will most likely fail to boot, which one?  The one holding the Corsair PC100.  Why?

At a glance both the Corsair PC100 and the AMM PC100 appear to be using the same chips, and as mentioned above the difference in the PCB's shouldn't matter since both PCB's (6-Layer Corsair vs 6-Layer AMM) are PC100 Compliant.  But why is it that the Corsair fails to boot at 133MHz?  Upon closer inspection you may notice a difference between the chips used on the Corsair module and those used on the AMM module.  While both chips are manufactured by Samsung, and carry the SEC markings on them, there is one subtle difference that can mean the world at 133MHz. 

The Corsair PC100 DIMM, like most other PC100 Modules out today, makes use of the slightly less expensive yet more available '-GL' (or in some cases '-G8') SDRAM chips, while the Advanced American Megatrends boasts the '-GH' Samsung chips.   The difference between these two?  The '-GL" and '-G8' chips are rated for a Column Access Strobe Latency Rating of 3 (RAM is accessed in Rows/Columns, the CAS rating determines how quickly a device in your system can return a value to your RAM - lower is better), while the '-GH' chips carry a CAS Latency Rating of 2.

Above - Samsung '-G8' Chip - CAS 3 Below - Samsung '-GH' Chip - CAS 2

At 100 & 112MHz Frequencies, the GL/G8/GH chips don't seem to show any visible differences other than the physical markings.   However taking the '-GL/G8' chips up to 133MHz is virtually impossible, simply because of the fact that the RAM cannot cope with such high frequencies, whereas the '-GH' chips, which are rated for a faster CAS Latency, can cope with higher bus speeds and will most likely work perfectly fine at the 133MHz FSB Speed.

With that said, let's compare the current SDRAM that is available and put an end to the debate as to which SDRAM is right for you.