Types of RAM

I recently added memory to a Pentium 3 system for my boss.  I purchased a 256 MB PC-133 RAM Chip and it worked just fine.  But then I tried to add the SAME type of RAM to another Pentium 3 system but it didn't work.  I found out that it took PC-100 RAM Chips.  Why is this?  I thought the type of memory you put in a system depended on the Processor you have; i.e.:

Old Petium = EDO
Petium MMX = PC -66
Pentium 2 / Celeron = PC-100
Pentium 3 = PC-133

I also found that my boss' computer also worked with PC-100 RAM chips, but this doesn't surprise me as much.  Can someone please give me a rule of thumb of the kinds of memory out there and what types of systems use them.  I also want to know the difference between ECC and non ECC memory and why it makes a difference?  And I also want to know about the RAMBUS hype of a few years ago.  Is that the FASTEST memory chip out there?

I will double the points if you come up with a thourough answer...
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Thats quite difficult to answer, there are many variations.  For instance, an early Pentium 3 Motherboard may only support upto pc-100 as pc-133 was not around yet.  A later board capable of supporting 133 will also support slower speeds like 100 and 66.  As a rule I would say P3/Athlon will use either PC100 or 133 depending on how old it is, P4/AthlonXP generally support DDR only but some boards can also take PC-100/133.

Take a look at these links to get you started:



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Also, if you put a 133 module in with a 100, it will usually work but at the reduced speed of 100.  It's best practice however not to mix modules.
Glen A.IT Project ManagerCommented:
Morpheus 30:  The issue here isn't usually just one of whether it's PC133, PC100, PC66 etc., (or DDR, or EDO, or what-have-you) but usually one of chip density and configuration.

It's true that PC133 is backward compatible to PC100 and PC66, and as such you can (sometimes) use PC133 in a celeron system spec'd for PC66 or even PC100.  That's the nature of RAM, they do try to make it backward compatible.

However....  It should be noted that many of the newer modules are made in a higher density than they used to be.  As such, you often run into compatibility issues in some motherboards.  Even buying PC100 instead of PC133 isn't a safeguard, because you can buy PC100 modules of higher density than some PC100 systems take.

There are also time when only gold-edged instead of tin-edged RAM will work.  As a rule of thumb, it is best to match up the edge contact material with the material in the slot.  If the slot has gold contacts, use gold-edged RAM.  If it has Tin contacts, use tin-edged modules.

One of the easiest ways to get RAM that will work is to use the memory 'configurators' available at either www.crucial.com or www.kingston.com .  On these sites you can enter system make/model, or motherboard make/model and get their RAM recommendations for your product.  You can then get more details on those specified parts, and from their learn the required density, configuration, edge, ECC (or not), speed, etc.

What you'll find is systems such as Compaq, HP, IBM, and other Tier1 vendors more often has very specific requirements on what will work in their system than the average custom-built system.  As such, when dealing with those types of PC's it is best to get the exact specifications required, and often it can simply be best to pay the premium and get your RAM from kingston or crucial.  They do charge a little more, but their RAM is guaranteed #1: to Work and #2: to work forever.  If you have to upgrade RAM on a budget, you can choose another vendor, and get a better chance of getting what you need by knowing the exact specifications listed on their sites.  But I never, ever recommend 'budget' memory for a system.

Also of note, as a rule of thumb:

Pentium I -  look at the slots.  If it has 72pin slots, it will most likely allow for both EDO or Non Parity 72pin SIMMS.  If it has 168pin DIMM slot, it could be for either an EDO DIMM or possibly for PC66 SDRAM.  Please never assume it's an SDRAM slot, as many systems did use 168pin EDO DIMMS rather than PC66 SDRAM.  Often crucial and kingston have these older boards/systems, but not always.

Pentium II 233, 266 and Celeron systems were generally using PC66 SDRAM.  Pentium II - 300 and up most often used PC100.  You can usually get more / better specifications on the type of RAM that could be used by getting the specifications of the motherboard controlling chipset.  for example, the Intel 440LX chipset used only a 66MHz system bus, and as such didn't benefit from anything faster than PC66.  The Intel 440BX could often use PC66, PC100 or PC133 depending on the motherboard.   Crucial and Kingston usually have specs listed for Pentium II and above motherboards.

Pentium III systems almost always used PC100 and PC133 systems.  Especially if the cpu is > 500MHz, it's most likely using PC133 SDRAM.  Again, the crucial and kingston sites are a great place to get the information.

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Glen A.IT Project ManagerCommented:
Regarding ECC:  ECC stands for Error-Checking & Correction.  It is most often used in server or high-end workstation environments.

In a nutshell, if you need to find out if you have/need ECC, parity, or non-parity memory, count the number of chips on the module. If you can evenly divide the number of chips by three or five, the module is ECC or parity, if not, then it is a non-parity module.  Then look at the part numbers on the chips of your module. If each chip has the same part number, you have ECC. If one chip is different, you have parity.

ECC stands for "Error Checking and Correction". It's a technology that allows the memory to not only detect memory errors but also correct them on the fly.

Some of us who have been around computers longer may remember the old parity memory that used to be on sale at a slight premium over regular memory. ECC and Parity memory share some likeness in that both can detect errors in the memory, however, ECC takes it one step further by correcting the error and preventing it from affecting your application.

As I mentioned, ECC is something only a server or mission critical system would need (sort of like Registered Memory) because memory errors are very rare and because the number of errors likely to be experienced by a system is proportional with the amount of RAM on that system - since servers tend to have a few Gigabytes of memory, and because they are on 24hours a day and also deal with huge amounts of pretty important data, it's not hard to figure out why ECC is considered a must for any serious server application.

ECC Slows down Boot time too.  Another reason why you wouldn't want ECC in a regular system is because once ECC checking is enabled you better be prepared for the longest RAM check you've ever seen in your life. It is literally several times slower than regular memory checking during POST.  

Also of note, because of the way ECC works, you can expect it to perform roughly 2% slower than non-ECC memory.

Glen A.IT Project ManagerCommented:
Heres some information I provided in another PAQ:  ( http://www.experts-exchange.com/Hardware/Microchips/Q_20799798.html )

A big difference that made RDRAM desirable (once, but not now) was one of bandwidth.

When DDR first came on the market, it was running at a maximum of 2.1GB/s of bandwidth.  RDRAM at the time was doing 3.2GB/s.

Now, however, if using Dual-DDR PC3200 you can get a maximum of 6.4GB/s (PC3200 @400MHz, Dual-Channel) which is superior to that of RDRAM's PC1200-Dual 4.8GB/s.

Also of note, DDR RAM is much less expensive than RDRAM.

How The Two Systems Differ and General Summary

This is taken from overclockers.com:  

"RDRAM is a narrow, high speed serial connection.
SDRAM/DDR is a wider, lower speed parallel connection. (DDR is simply SDRAM that squeezes out two actions per clock cycle; something RDRAM already does. I'll use the term "SDRAM" when what I'm saying applies to both current SDRAM and DDR, and "DDR" when it applies only to that).

Just about all the major advantages and disadvantages between RDRAM and SDRAM stem from this difference.
These are two much different ways of handling data flow. RDRAM does a little very quickly. SDRAM does much more but more slowly.

The wiring on RDRAM modules is technically more challenging, but there is much less of it. It operates best with simple configurations; performance degrades as you add more devices. This is typical of serial devices. "
Glen A.IT Project ManagerCommented:
Now, to be specific on the 256Mb module that didn't work in your bosses PIII system:  It is likely that the generic 256Mb module you purchased was high-density, and that your bosses sytem required single density pc100 of a certain spec.  As I mentioned, a good place to find exactly what was required is on www.crucial.com

Did you need any more information?  If you have more questions, please let me know and I'll be happy to help out.

Glen A.IT Project ManagerCommented:
MrSkinny:  Re: your comments:

>>Thats quite difficult to answer, there are many variations.  For instance, an early Pentium 3 Motherboard may only
>>support upto pc-100 as pc-133 was not around yet.  A later board capable of supporting 133 will also support slower
>>speeds like 100 and 66.  As a rule I would say P3/Athlon will use either PC100 or 133 depending on how old it is,
>>P4/AthlonXP generally support DDR only but some boards can also take PC-100/133.

As mentioned, it's not necessarily the jedec specification (that's what the PC66/100/133 is, a jedec spec.), but often there is density, speed and chip configuration requirements.  And although some boards that supported 133 could support 100 and 66MHz ram, there were times that they wouldn't.  

Also of note, if the cpu is 100MHz, you can't use 66Mhz ram.  The memory bus speed must match the cpu bus speed.

Initial P4's used RDRAM from rambus.  The first P4's to use non-rambus memory actually used SDRAM (performance was pitiful, though) and later P4's started being offered with DDR.  It wasn't the cpu though, but the chipset and design of the motherboard that determines what kind of RAM it takes.  The cpu has nothing to do with it, really.

Athlon XP's were at release able to use SDRAM and DDR, depending on the board, and many boards offered support for both (but not at the same time).

Now, you'd be hard-pressed to find a new system being sold with SDRAM, or RDRAM.  Almost everything whether intel or AMD comes with DDR.  PC3200 is currently the fastest ram in the jedec specification (at 400MHz), but currently many manufacturers also offer ram in 433MHz (PC3500), 466MHz (PC3700), 500MHz (PC4000) although jedec has yet to make any PC3500, PC3700 or PC4000 specs, so it should not officially be called such.


Whilst I am aware of all that, I did not have the time to write three or four pages on it! :-)


Glen A.IT Project ManagerCommented:
lol.  I type faster than most.  I've played piano for 30 years and credit that to my typing speed of >120wpm.

Once at work while working on a manual for software I developed, I realized a bunch of people were standing behind me watching . . . so I turned around to ask what the wanted and one of them said "We just want to see how long it takes before smoke comes out of the keyboard..."

Lol.  So it's not that I really have that much time...  It's mostly that I type really fast.  
Impressive! :-)
Glen A.IT Project ManagerCommented:
Morpheus30 -  Did you need more information?  If so, please ask in detail.  If not, please accept the answer that provided the information you required, or split points among experts if appropriate.

Thank you,

EE Page Editor, Microchips/Desktops TA's
morpheus30Author Commented:
Sorry it took me so long to get back, but my Internet connection at home was down until Sunday.  I'm increasing the points and splitting them...
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