Like many of you, we have seen the guide written by eBay user aussieram. While it is very informative and helpful on some issues, it is absolutely wrong on some points… 8 of them to be exact. We would like to set the record straight with our own RAM tutorial. This guide will be split into 4 sections:

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Section 1 – Explains SDR, DDR, DDR2, their speeds, and density markings
Section 2 – Busts a few RAM myths and confirms some RAM facts
Section 3 – A short section on how RAM is manufactured
Section 4 – Cross-reference table of Kingston & Micron part numbers

Section 1

The most common types of RAM are SDR, DDR, and DDR2.

SDR RAM (or SDRAM) stands for Synchronous Dynamic Random Access Memory, it comes in 168pin DIMMs (Dual Inline Memory Modules) for desktop PCs or 144pin SODIMMs (Small Outline Dual Inline Memory Modules) for laptops. The speeds for this type of RAM are PC66, PC100, and PC133.

DDR (also called DDR1) stands for Double Data Rate. Like EDO RAM (an older type of RAM technology), if your computer needs DDR you need to make sure the auction specifically states the RAM is DDR. Some common speeds for DDR modules are PC2100, PC2700, and PC3200.

There is also DDR2, do not mistake them with regular DDR as they are not the same. This RAM also comes in 184pin DIMMs for desktop PCs or 200pin SODIMMs for laptops, but the more common speeds are PC2-3200, PC2-4200, and PC2-5300. Note the overlap at PC3200 - make sure you know what your PC needs, then make sure you're buying the right thing.

 

Now we need to talk about how the density plays into the picture.

First let’s explain what Low Density and High Density refers to. Using 256MB modules as an example… you would think the 16 chip module would be called the High Density module and the 8 chip version would be the Low Density module… but it’s backwards, isn’t it? The problem is – the density isn’t referring to the number of chips on the module, it refers to the density of the memory units inside the individual chips on the module.

When people talk about a Low Density module, they’re incorrectly abbreviating their speech, and it’s misleading. What they really mean is – it’s a module with Low Density chips. Think of two pepperoni pizzas. On one pizza each slice has 2 pieces of pepperoni. On the other each slice has 4 pieces of pepperoni. The one with 2 pieces per slice is the low density pizza, and the one with 4 pieces per slice is the high density pizza.

This is what it’s like on the RAM modules. The High Density module only needs half as many chips per side because there are more memory slots inside those chips. If you want Low Density chips, you need twice as many, and so modules with Low Density chips cost twice as much. (The labor to build the module from the assembled parts is about the same, regardless.)

 

So how do you make sense of the RAM's description?

Here's a chart for quick reference, with an explanation below it:

(If you need this often, we suggest you bookmark this page)

128MB module: total Module layout is 16Mx64
High density - each Chip is 16Mx16
Low density - each Chip is 8Mx16

256MB module: total Module layout is 32Mx64
High density - each Chip is 16Mx16
Low density - each Chip is 16Mx8
 
512MB module: total Module layout is 64Mx64
High density - each Chip is 32Mx16
Low density - each Chip is 32Mx8

1GB module: total Module layout is 128Mx64
High density - each Chip is 64Mx16
Low density - each Chip is 64Mx8

2GB module: total Module layout is 256Mx64
High density - each Chip is 256Mx16
Low density - each Chip is 256x8

4GB module: total Module layout is 512Mx64
High density - each Chip is 512Mx16
Low density - each Chip is 512Mx8

For some reason (just to confuse everyone) the memory quantities in these numbers are not written as MB for Megabytes... they just use M. In any case, you'll notice that the total module layout simply doubles at each step: 128MB is 16Mx64, 256MB is 32Mx64, and so on. What changes at each step is how the memory is laid out on each chip - remember our pizza analogy? Let's use a 256MB module as an example:

In the 256MB category you have 2 types of chips you can use. The first one has 16Mx16 worth of memory - this is the high density chip. The low density chip only has half that much memory on it - 16Mx8. This is why you only need 8 chips on a high density stick of 256MB and you need 16 chips on a low density stick of 256MB.

So how do you know what you're looking at when you're reading a description of some RAM? Look at the second number in the descriptor. If the last number is x64 then that descriptor is talking about the overall layout of the module, not the memory on each individual chip. If the second number in the descriptor is x8 or x16, then that's telling you about the memory on the chips themselves.

 

Section 2

Ok, now let’s bust some myths and confirm some facts!

 

Statement: High Density RAM is only 10% compatible. FALSE!

Explanation: This statistic varies greatly depending on what kind of RAM you need and who the manufacturer of the RAM is. Some companies make RAM that is much more widely compatible. We have been selling sticks of 256MB PC100 RAM for over a year and find that they are about 75% compatible. We also carry Low Density RAM and state clearly in our listings certain models that we know require Low Density RAM, enabling our customers to save a lot of money if their model can use the High Density RAM.

 

Statement: Manufacturers of High Density RAM don’t put their name on the sticks because “JEDEC doesn't want the modules to be built that way”. FALSE!

Explanation: Our manufacturer builds their RAM and sells it at retail prices with their name on it on their own website. They also sell it wholesale without their name on it – at our request – because we spent MONTHS tracking down a good manufacturer and negotiating a reasonable price. We don’t just sell retail – we also sell wholesale, and we want to keep our large customers – if they knew who our supplier was they could go over our heads and buy direct! After all the work we’ve done, we don’t want it to have been for nothing! So we ask our manufacturer not to put their name on the labels of the ones we buy. Our manufacturer is not ashamed of their product, and neither are we!

 

Statement: “High Density module is also by far slower than Low Density.” FALSE!

Explanation: The Density and Speed ratings are independent. If the RAM is PC100, PC133, or PC3200 then that’s the speed it runs at when you put it in your PC or a RAM tester. It cannot be marked as a higher speed based on the density. To use an analogy: If you’re driving 100mph it doesn’t matter if your engine has 4 cylinders or 8. 100mps is 100mph, period.

 

Statement: “Low Density modules have 100% compatibility”. FALSE!

Explanation: We’ll use the car analogy again. Ford and IBM are manufacturers, Mustang and Thinkpad are models, the Mustang GT and the Thinkpad A21m are sub-models. You might think that every Mustang GT (or Thinkpad A21m) are exactly the same under the hood – but they’re not. You might have 10,000 of them built exactly the same and then later in production some minor piece is changed. Maybe a fixture that a light bulb plugs into... does it change how the car works? No. Does it change the fact that it can use a light bulb? No. But you may need a differently shaped bulb. We use this example from our own experience. We know for a fact that the A21m requires Low Density RAM, and we’ve sold many modules to our customers. But for some reason 1 time out of every 1000 the customer reports that their A21m won’t accept the RAM. This is why we have a return policy and our listings will never state 100% compatibility. We will only claim 99.9% because we know there is always that one PC out there that’s going to be picky and we don’t want our customers to be upset with us when that happens – so we tell you the truth up front and we offer a money-back guarantee.

 

Statement: "Almost all Branded-Name systems such as Apple/MAC, Compaq/HP, Dell and IBM only uses LOW DENSITY modules." FALSE!

Explanation: We sell High Density RAM for all those name brand systems and more – check our listings and feedback. We have looked but cannot find any website that will tell us which models need Low Density RAM and which can use High Density RAM. Our best guess is either they want to sell the more expensive Low Density RAM and boost their sales figures, or they don’t want to have to deal with as many returns/exchanges. The second is more likely. Our company (SBC Memory) is still small, we sell about 75 items each week and deal with 3-5 returns each week (usually from people who did not read the auction listing correctly and bought High Density when they needed Low Density). That works out to a 4%-6% return rate. When you’re a large manufacturer selling 5,000 items a week, 4% would be 200 returns a week and 800 returns a month – way too many to handle. Hopefully our business will grow and at some point we’ll start to sell only Low Density RAM too. But for now we’re small and can take the time to do what we do, enabling our customers to buy the cheaper High Density RAM if their PC can use it. We’ve started to compile our own list of PCs that require Low Density and will continue to add to this list as we learn. Our customers are our partners in this, and you can help us just as much as we help you.

 

Statement: "Many has [sic] been MISLEAD by the myth that PC133 modules would not work on older spec PC100 systems ... higher speed modules are ALWAYS backward compatible in speed with slower system, and the problem source is the DENSITY". FALSE!

Explanation: There is always a cutoff point where a manufacturer changes their support for technology. You wouldn’t expect a steering wheel from a 2007 Chevy Camaro to bolt on to a 1987 Chevy Camaro. PC133 refers to the speed, and it can always run slower (at PC100 or PC66 speeds), but that doesn’t mean the PC will be able to recognize it. When you boot up your computer the motherboard does a Power On Self Test (POST), and it checks the components attached to it. Say your PC was made in a certain year and knows about PC66 and PC100 (but not PC133) and you put in a PC133 module. When the PC does its POST it sees the RAM and says, “Hi! What are you?” If the module replies, “Hi! I’m a RAM module, PC133!” … the PC thinks “PC133?? It’s probably a PC100 module with an error… I should report this error so it can be replaced.” At this point your PC gives an error and stops booting. But if your PC was made after the cutoff point the manufacturer may have built your PC to run at PC100 but recognize the code for PC133. So some PCs will get the module’s response and reply, “Ok but we run at PC100, so you’ll have to slow down to our speed.” Whether or not your PC can do this will depend on your make and model and when it was built. Again, because of differences even within the same make/model and product line, we cannot know for sure. All we can do is tell you what speed RAM the manufacturer says your PC was built to use. If you want to try putting in faster RAM and hope the PC can see it and slow it down, you can. But it’s not guaranteed to work… and it has NOTHING to do with the density of the RAM.

 

Statement: You can tell the density by counting the chips on the module. FALSE!

Explanation: This is false for two reasons. First, you have to keep in mind the capacity of the module you have. On a 128MB module 4 chips on each side is Low Density, but on a 256MB module 4 chips on each side is High Density. So the number of chips required to classify it as High or Low Density will depend on what capacity module you’re looking at Secondly, there is some new technology (we haven’t found out what it’s called yet) that blows this right out of the water. On a 256MB stick Low Density is supposed to have 16 chips. But we’ve come across a few sticks that are 256MB Low Density and not only do they not have 16 chips, they don’t even have 8. They have 2 strange looking bumps on each side (4 total). You can see a picture of one in the listing we have up for 256MB Low Density. It’s the strangest thing we’ve ever seen, but it works!

 

Statement: “Consider only buying modules that were made with the above Top 5 memory chips company for lifetime trouble free operation.” FALSE!

Explanation: You do not have to buy from ONLY those manufacturers to get quality RAM modules. This goes back to the returns issue we talked about further up in this guide… those top manufacturers sell mostly Low Density RAM because they don’t have the time to deal with thousands of returns. If you buy one of their sticks, you’re probably getting Low Density RAM that’s more likely to be compatible. But as long as long as you check what you’re buying there’s no reason to automatically assume that a stick without a major-label name is going to be incompatible. Bottom line: Bottom line: be an informed shopper… which you are now after having read this guide! Just read the eBay listing carefully and if in doubt – just ask before you buy.

Section 3

Now that we’ve explained how to understand the speed and density and cleared up some misconceptions, we’re going to talk about how RAM is manufactured. While not often covered in guides, this is important because we get at least one email every 2-3 months asking for RAM that is “specifically PC100 RAM, not reprogrammed PC133 RAM.”

To explain, we’re going to describe something else for a moment: When Intel makes processors, they make ALL of them to be the best processors they can be, and there is a round of quality assurance (QA) tests at the end of the process that checks each processor. Years ago, if the processor didn’t pass all the QA tests they were scrapped. Then one day someone looked at all the processors to be scrapped and said “You know, so what if they only work at 90% of the speed they’re supposed to? I’m sure there’s a whole market of people out there who don’t need their PC to run at the fastest speed on the market. Instead of scrapping these we could market them as a slightly cheaper alternative and still sell them.” And that’s how the Celeron name brand was born. For those of you who were into sports when you were in school – Pentium is the varsity team, Celeron is the Junior Varsity team. Celerons are just processors who didn’t make the varsity team.

Likewise, ALL PC133, PC100, and PC66 modules are manufactured to the same standards.  They all have a tiny chip on them that is programmed to tell the module what speed it is. (We discussed this earlier in this guide.) RAM manufacturers will build a million of the same module, but some aren’t able to meet the PC133 speed rating. These get programmed to be PC100 or PC66. Also, they’ll look at what they have in stock. Even though all the rest of the modules are capable of PC133 speeds, maybe they don’t need any more PC133 in stock. So they’ll program some of them to think they’re PC100 or PC66 and put them into stock that way. Now say the manufacturer has 10,000 of each speed in stock and all of a sudden someone comes and places an order for 15,000 modules of PC100. Rather than try to quickly build 5,000 new modules, all they have to do is take 5,000 of the modules they’ve already built and reprogram them to think they’re PC100. There really is no such thing as a pure PC100 module (or any other speed). This is also true for the newer DDR modules. They’re built to be the best they can be, and can be reprogrammed to tell the motherboard that they’re slower if needed. (For an explanation of this, read some of the topics higher up in this guide if you haven’t already.)

Section 4

Here's a small list of Kingston and Micron part numbers. Hopefully this will help you determine what kind of RAM you have, or what you need. If you need RAM equivalent to any listed here check our auctions, if you don't see what you need just email us and we'll help you out.  Again, feel free to bookmark this page if you deal with this regularly.

128MB PC100 SODIMM Low Density
Kingston KVR100X64SC2/128 KVR100X64SC2L/128
Micron MT8LSDT1664HG-10EB1 PC100-222-620

128MB PC133 SODIMM Low Density
Kingston KVR133X64SC3/128 KVR133X64SC3L/128
Micron MT8LSDT1664HG-133 MT8LSDT1664HY-133 MT8LSDT1664HY-13E

256MB PC100 SODIMM High Density
Kingston (unknown)
Micron (unknown)

256MB PC100 SODIMM Low Density
Kingston KVR100X64SC2/256 KVR100X64SC2L/256 KTM-TP390X/256
Micron MT16LSDF3264HG-10EB1 PC100-222-620

256MB PC133 SODIMM High Density
Kingston (unknown)
Micron MT8LSDT3264HG-133 MT8LSDT3264HG-13E MT8LSDT3264HIY-133 MT8LSDT3264HY-133 MT8LSDT3264HY-13E

256MB PC133 SODIMM Low Density
Kingston KVR133X64SC3/256 KVR133X64SC3L/256
Micron MT16LSDF3264HG-133 MT16LSDF3264HY-133 MT16LSDF3264HY-13E

512MB PC133 SODIMM Low Density
Kingston KVR133X64SC3/512 KVR133X64SC3L/512
Micron MT16LSDF6464HG-133 MT16LSDF6464HY-133 MT16LSDF6464HY-13E

Ok – hopefully this has corrected some of the misleading information out there. As always, please check our auctions and if you have any questions feel free to ask. Thanks for reading!