C'est en anglais, mais google peut vous aider à traduire... ou une âme charitable... moi j'ai pbcp trop de choses à faire...
Repris sur le Site de Crucial :
- What does "CL" stand for?
CL stands for "CAS latency," which is the number of clock cycles it takes before data starts to flow once a command is received. Low CAS latency is faster than high CAS latency. However, faster memory will NOT necessarily make your system faster. Your computer will only run as fast as the "slowest link" in your system, so you can't speed up your computer by adding memory that is faster than your current memory. The way to make your system faster is to add more megabytes (MB) of memory.
If you're building your own system, we recommend using parts with low CAS latency.
- Do I need error checking?
When choosing among ECC, parity, and non-parity memory, you need to match what is already in your system. You can tell which kind you have by looking at one of the modules currently in your system. Count the number of black chips on one module. If the number of chips can be evenly divided by three or five, you should buy ECC or parity (whichever is offered for your system). If not, you should buy non-parity.
For example, if one of your modules has nine chips, you should buy ECC or parity. If one of your modules has eight chips, you should buy non-parity.
If you're building your own system, we recommend using non-parity parts unless you will be using your system as a server or have a need for error checking. ECC and parity parts are slower than non-parity parts.
- Which speed and memory technology should I buy?
In general, you should match the speed and type of memory technology that is already in your system. If you don't know exactly what type of memory you currently have, don't worry; any of the modules listed in the Memory Selector™ for your system should work fine.
Common types of memory technology include fast page mode (FPM), extended data out (EDO), synchronous dynamic random access memory (SDRAM), and double data rate SDRAM (DDR or DDR SDRAM). Most systems will accept only one type of memory technology. (The very few motherboards that can accept more than one type of memory will only allow you to use one type at a time.)
Memory speeds are written in different formats depending on the type of memory technology your computer uses. FPM and EDO speeds are written in nanoseconds (ns). SDRAM and DDR speeds are written in megahertz (MHz) or with a "PC" rating.
It may seem confusing, but faster memory will NOT necessarily make your system faster. Your computer will only run as fast as the "slowest link" in your system, so you can't speed up your computer by adding faster memory. The way to make your system faster is to add more megabytes (MB) of memory.
- What's the Truth about Memory Manufacturers?
A lot of memory companies claim to be memory manufacturers even though they're just memory module assemblers. By putting two pre-manufactured memory parts together to build a memory module, they feel they've manufactured memory.
Truth is, there is only one company in America that actually manufactures the DRAM chips, the memory printed circuit board, and then assembles them into memory modules — Micron and its memory upgrade division, Crucial Technology.
Memory modules are made of two pieces — the DRAM semiconductor chips that store data and the printed circuit board (PCB) connecting the chips to the rest of the computer. There are only a handful of semiconductor manufacturers with the engineering expertise to make DRAM chips. They include Micron, Samsung, Hitachi, and Hyundai.
Micron has over 2,000 engineers and a 1.8 million-square-foot manufacturing facility with state-of-the-art clean rooms, memory testers, and exacting quality control. Crucial memory is recognized worldwide for its power and performance. Our memory is used by Apple, Gateway, HP, IBM, Micronpc and more — for good reason. Crucial offers over 110,000 upgrades for more than 20,000 different computers, notebooks, servers and printers. And we guarantee that the high-quality memory you buy is 100% compatible with your system or your money back.
- The difference between top tier and generic memory.
So your computer is dragging and you've decided you need more memory (don't we all?). You start searching the Web for a place to buy it. Suddenly, you realize you've found 50 suppliers and a very wide range of pricing.
How do you pick the right place to buy memory?
You want to make an informed decision so you read about SIMMs, DIMMs, and SODIMMs; parity and ECC; registered and buffered; DDR and SDRAM; and then you scream "Stop! I just want to upgrade!" Yes, memory upgrades involve a lot of technical information. But for most people (other than the ones building their own systems) most of it doesn't matter. All you have to do is remember the next sentence.
The most important factor in selecting memory is who makes it.
It's pretty simple actually. Once you have a company that manufactures high-quality, reliable parts and is going to stand behind its product with technical support and customer service, finding the right product for your system is relatively easy.
- Comparing DRAM...why not buy the cheapest?
For good reason. Not all memory is created equal.
There are many fast food chains that sell cheeseburgers. Even though it's just a cheeseburger everyone makes it a little differently.
Manufacturing DRAM is similar. An 8Mb x 8 chip from Micron or any other DRAM manufacturer will give you the same amount of memory, but each manufacturer's parts will have slight differences in some critical parameters. In other words, there are differences between "identical" DRAMs.
Still not convinced? Why do you think major PC manufacturers qualify DRAM suppliers rather than purchase generic memory modules? It's because memory plays a critical role in the overall performance of the systems they build.
Let's face it. It doesn't matter if you're using a computer to surf at home or to work in the Space Shuttle. You're not going to tolerate failures. Shouldn't you expect the same level of performance from your memory supplier?
So which memory supplier is right for you? To answer that question, there are two things to consider:
Who made the chips?
Who assembled the module?
- Tracing quality to the source.
Very few companies in the world actually make memory chips, but literally hundreds of companies sell memory modules.
Take a look at the markings on any chip in your computer. Each chip is covered with numbers. Believe it or not, all those little numbers mean something. They are used for traceability.
If you have a problem with a module and want to return it, one of the first things you will be asked is to read this information. By decoding these numbers, any of the more reputable chip manufacturers (not just DRAM manufacturers) can determine when the part was built, in which Fab, and they can often trace it back to the actual wafer the part came from. By building databases with this information, chip manufacturers are better able to pinpoint the cause of a problem, correct it, and make the next lot that much better.
In addition to the state-of-the-art wafer processing equipment used in DRAM manufacturing, part of what you pay for when purchasing top-tier memory is testing.
The most quality-conscious chip manufacturers, like Micron, will put every single chip through an extensive series of tests rather than just checking a sample of parts. They will test chips under normal operating conditions as well as under varying voltages, temperatures, and other "stressful" conditions.
Chips also go through a burn-in process at elevated temperatures to help identify any borderline parts. This process accelerates failure normally seen as "infant mortality." Chips that would fail early during actual usage will fail during burn-in. Chips that pass have a life expectancy much greater than that required for normal usage. Burn-in further helps to prevent failures from reaching you, the end user.
- How testing influences which name appears on your module.
Sure, testing is important. But does it really matter what will happen to your memory at 200° F when it's a comfortable 70° F in your house? Actually?it does.
Even though it's 70° F in your house, the chips inside your system get a lot hotter. One common cause is a spike in power that reaches your PC (a good reason to use a surge protector). Also, any normal computer use will cause the parts inside your PC to heat up. Basically, your computer may see conditions that are more extreme than what you see in front of the monitor. For this reason, major DRAM manufacturers use what's called guardbands in their testing.
It's a pretty simple concept really, kind of like insurance. A guardband helps to ensure that a chip will work properly, even under many abnormal operating conditions. For example, let's say a chip needs to meet a particular parameter between 10 and 20 to be acceptable. Rather than set the test to fail anything less than 10 or greater than 20, the test is set to fail anything less than 11 or greater than 19. Yes, more failures occur this way, but these additional failed parts are close enough to the limits to imply some sort of process variation. It's not worth the risk to the chip manufacturer to ship these parts to customers under their brand name.
Top Tier Memory
After going through this extensive series of tests, you can be confident that the passing parts will work for a long time.
Generic Memory
So what happens to the parts that failed testing? Some get thrown out, but many are re-tested and graded to lesser speeds and/or conditions (i.e. without guardbands). Others aren't even re-tested (even though they might not have gone through burn-in). Yes, these lower-grade modules will probably work today, but they're more prone to marginal performance and failures, especially over time. DRAM manufacturers often sell this memory to third party vendors who then re-label them with their own brand name. Third party vendors are often required to remove the original DRAM manufacturer's name from the chips. This memory is typically referred to as "generic memory."
- Who assembled your memory module?
Micron is one of the very few companies in the world that actually makes memory modules from start to finish — that is, manufactures the memory chips then mounts them to printed circuit boards (PCBs). Other companies claim to be memory manufacturers, but the truth is, they just buy the parts from a manufacturer like Micron then assemble them into modules. Either way, whatever company assembles your memory module has control over three very important factors contributing to quality.
The PCB
The module assembly
The module test processes
The Printed Circuit Board
The PCB enables your computer to access its memory. It contains layers of circuitry used to connect the components to the outside. As you would expect, it also plays a critical role in quality. This is especially true at higher densities and speeds.
Just like DRAM, the PCB must meet strict requirements to be called PC3200 or PC2700. (See Is your PC100 up to Spec?) Most large computer manufacturers require every PCB design they use to be qualified.
- How poor assembly procedures affect you.
Module assembly can have a considerable effect on how the end product works. Is it automated? Or are people soldering on parts by hand in a warehouse? You probably won't be able to tell if the parts went through proper material storage, screen print, and reflow processes during assembly just by looking at them. However, these are some of the practices that play key roles in the short and long term reliability of your memory module.
During the reflow process, (where the DRAM components are permanently attached to the PCBs) the boards go through a heat cycle of a few hundred degrees. If they are heated too quickly or remain at an elevated temperature too long, they can be significantly damaged. However, this damage may not be visible. If the parts weren't properly stored, any large amount of moisture that was trapped inside tends to expand and cause a failure almost immediately. Additionally, the reflow process can sometimes affect the module just enough so that it performs properly initially but degrades over time. This can result in memory that doesn't work by the time the end user gets it, or memory that fails intermittently.
Another item that drives process improvements and widens the gap between upper and lower echelon assemblers is component packaging. More and more DRAM components are being packaged in ball grid arrays (BGAs). Instead of having leads that come out from the side of the chip, the chip attaches to the PCB through solder spheres, or balls, that are on the bottom of the component. BGAs can be placed using the same equipment that is used to place leaded components. However, the tricky part is verifying that the solder joints are good.
Since the solder joint is under the body of the device, you cannot visually look at it and determine if it is acceptable. The only way to inspect these packages is with an x-ray machine. And the only way to replace or repair the part is with specialized equipment.
Even the best memory chips in the world can be damaged by poor assembly techniques.
- You would think all memory assemblers would test every module they build.
Some assemblers test only a sample of parts and call the entire lot good based on an "acceptable defect level." These companies argue that it's cheaper to get a certain amount of returns than to detect and/or prevent them. This may be true for their bottom lines, but if you're in the small percentage of end users that ends up with a failed part, you probably won't think it was a good idea. At Micron, we test each and every memory module we manufacture.
There are several different methods used to assess memory modules. Many of them take modules through a series of tests, including checking for opens and shorts, leakage, verifying refresh rates and running pattern tests. All of these tests are typically performed at high speed to ensure functionality.
As an additional safeguard, many assemblers have invested in automatic handlers to manage their modules. This prevents an operator from accidentally placing a failing module into a good tray. The yield at this step should be greater than 99%.
On top of these production tests, reputable assemblers will also pull additional samples for outgoing quality checks and to qualify the modules on different motherboards. As processors and memory speeds become faster and faster, it becomes increasingly critical for memory modules to do more than just meet standard specifications. They must also be proven to work in specific systems.
Unfortunately, many assemblers consider comprehensive testing an unwarranted expense. Some of the equipment is extremely expensive and requires a great deal of engineering support. On the other hand, it can be very easy for an assembler to buy an inexpensive hand held tester, just to check for opens and shorts and perform no additional testing.
- Symptoms of Bad Memory
How the quality of your module affects you.
So now you know the shortcuts some memory manufacturers will take to save money. Still, you're wondering, "How does this affect me?"
Unfortunately, substandard memory can affect you in more ways than you may realize. It can cause problems ranging from annoying error messages to a complete system crash. When these failures occur, critical data that's stored in memory is typically lost or overwritten. And it's not always obvious that memory is the cause of your problem.
Let's say you're building a system, you assemble it completely then turn on the power and nothing happens. You may not suspect the memory at first. Most people will double check all connections, maybe swap a power supply with a different machine and spend hours trying to solve the problem, when in fact it was a failure due to bad memory.
Or, your system may seem to work fine at first then mysteriously crashes later. Most of us have experienced a "blue screen" at one point in our computer lives and we all know how unreliable PCs can be. However, it's not always easy to tell what caused the blue screen. Was it a bad cell in the memory? Was it an intermittent failure in the memory? Was it something else?
Low-grade memory is also more prone to compatibility problems. For example, your computer might only recognize a portion of the memory, or even worse, it won't see the memory at all. Or maybe you upgrade your PC only to discover that the new generic module won't work with the existing memory, so you're stuck using just one module or the other.
Marginal memory is certainly not the only cause of computing problems. But it can contribute to them. Why do you think major memory manufacturers like Micron refuse to ship these lower grade parts? It's because they don't want their name on it.