Original Link: http://www.anandtech.com/show/108

PC100 SDRAM: A Closer Look

by Anand Lal Shimpi on June 15, 1998 11:44 AM EST


PC100 SDRAM: A Closer Look
A few months back the topic of PC100 SDRAM became a hot topic for discussion on the Internet, unfortunately what we lacked was a proper platform to test the slowly emerging PC100 DIMMs.  Now, after the release of the Intel BX Chipset, memory manufacturers have finally started to release their PC100 SDRAM DIMMs, however what we're finding out through a process of trial and error is that not all PC100 SDRAM is created equal. 

While most modules (more or less) work similarly at the 100MHz bus speed, pushing the limits to 112 and the speed terror 133MHz bus speed separate the average PC100 SDRAM modules from the Best of the Best.  Taking   a closer look at today's PC100 SDRAM AnandTech compared 4 different brands and 6 different types of PC100 SDRAM.  Which manufacturer came out on top?  With performance eliminated from the list of features to look for in SDRAM we must turn to reliability, which is where the number one modules shine and the rest are given a long "wait-state".  Keep your Bus ready as AnandTech explores the world of PC100 SDRAM.



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.



CAS Latency 3 PC100 SDRAM Modules ('-G8/GL')

Advanced American Megatrends PC66 SDRAM

While the AMM PC66 SDRAM isn't PC100 Compliant (10ns 100MHz non-PC100 Chips), it will work on most BX motherboards at the 100MHz bus speed.  The 112MHz bus speed is a bit flaky on the AMM PC66 modules, but is still attainable.  If you have already invested your money in 64MB or more of AMM PC66 SDRAM, try using it on your new BX motherboard before tossing it out in favor of a more expensive PC100 module.  You may be in for a surprise. 

Corsair PC100 SDRAM

Probably the most reliable and best overall CAS-3 rated modules out on the market, Corsair PC100 SDRAM boasts an extremely high success/compatibility rate with the newly released BX motherboards.  Corsair PC100 was the only brand of its class to run flawlessly in every single BX Motherboard AnandTech Reviewed in the BX Motherboard Comparison at bus speeds up to 112MHz.  Corsair is known for manufacturing high quality PC100 SDRAM, and if you happen to find some at a good price, and don't plan on using the 133MHz bus speed then there is no point in passing these excellent modules up.  

Memory Man PC100 SDRAM

The Memory Man also produces high quality CAS-3 rated PC100 SDRAM, with a price about $10  higher than the Corsair modules there is no real benefit the Memory Man PC100 SDRAM offers you over the competition except for the lower profile design.  The Corsair PC100 has a clearance of about 5mm greater than that of the Memory Man PC100, there have been cases in which a matter of millimeters was the differentiating factor between getting a motherboard to fit into a case and buying a new case.  Especially with BX boards like the ABIT BX6, which place the Memory Expansion Slots directly underneath the 3.5" HDD Bays in most cases, every millimeter of space saved counts.  Whether or not it is worth the extra money is your call, it depends on where you need the DIMMs to fit.  As far as reliability is concerned, the Corsair PC100 is difficult to tell apart from the Memory Man PC100.

CAS Latency 2 PC100 SDRAM Modules ('-GH') 

Advanced American Megatrends PC100 SDRAM

It is amazing how long the old AMM PC66 SDRAM has lasted, considering it made its debut just under a year ago.  The key to its success was a manufacturing process much more advanced than the competition, while many buyers were frightened by the seemingly outrageous prices of the AMM PC66 SDRAM those that purchased the modules aren't regretting their decision one bit.  The PC66 SDRAM made use of an 8-Layer PCB, well beyond the unwritten specification for standard SDRAM out at the time of its release, however, migrating into the next generation of PC100 SDRAM, the AMM PC100 modules are built on a different 6-Layer PCB.  The only difference between the older PC66 and the newer PC100 AMM SDRAM is the presence of the '-GH' SDRAM chips on the newly released PC100 SDRAM, allowing it to run at the 133MHz bus speed.  Stability at this bus speed is a tad on the shaky side, however it is still quite reliable. 

Azzo PC100 SDRAM

The Azzo PC100 SDRAM, otherwise known as The Ram Warehouse PC100 SDRAM, also makes use of the same '-GH' SDRAM chips found on the AMM PC100 DIMMs, the only real difference seems to be that the Azzo DIMMs carry a price tag about $1 higher than their AMM counterparts.

During AnandTech's tests with the ABIT BX6 the Azzo 64MB Test System never crashed at 466MHz (133 x 3.5) while the AMM 64MB Test System crashed once out of a total of 3 test runs.  The decision is yours, if you happen to be ordering parts from AMM, then go with the AMM SDRAM, if you happen to be ordering the rest of your parts from Azzo (The Ram Warehouse) then you might as well pick up one of their PC100 DIMMs.  Remember that there is really no point to going with the '-GH' modules unless you plan on making use of the 133MHz bus speed, or unless you want to squeeze a few more percent of stability out of your system. 

Memory Man PC100 SDRAM

The Memory Man also offers their SDRAM using '-GH' chips like the above two contenders, the major difference here is that The Memory Man has undercut the price of AMM and Azzo on their 64MB PC100 modules by about $20, which is enough to cover for Overnight Shipping on the SDRAM.  The Memory Man's PC100 SDRAM is virtually identical to the Azzo modules.  If all you happen to be purchasing at the time is memory, then there is no place better to get your modules from than the Memory Man, otherwise it is better to purchase as much as possible from the same retailer.    



Basically the final decision comes down to what you plan on doing with your system.  If you already have some Advanced American Megatrends SDRAM laying around (PC66) and you don't plan on breaking the 112MHz Front Side Bus Speed barrier, then you might as well stick with your current SDRAM and spend the money you saved on something more worth while.  If you're purchasing new SDRAM for your BX Motherboard, and either don't plan on overclocking at all, or don't plan on going past the 112MHz FSB Speed then you might as well pick up your choice of '-G8/GL' CAS-3 PC100 SDRAM Modules, depending on who you feel more comfortable ordering from.  The only two reasons for going with the more expensive '-GH' CAS-2 PC100 Modules are either that you want to run at the 133MHz bus speed, or that you have some money to waste and feel like getting the best out there.  There have been cases in which older PC100 modules or even PC66 modules rated at 100MHz have miraculously worked at the 133MHz bus speed, however situations such as those are very rare.  In conclusion, we can pretty much say that all PC100 DIMMs are created equal, some are just more equal than others.


PC100 SDRAM Provided By: Advanced American Megatrends Technologies, Azzo Computers (The Ram Warehouse), The Memory Man, and New Frontiers Computers (Corsair SDRAM).

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