Computer Basic Info Memory RAM DDR Rambus SDRAM DIMM Troubleshooting

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Memory

Your computer's memory is necessary for its operation. It is fully tied in to the processor, chipset, motherboard, and cache.

Ok, so we've all been wondering where the heck its been. DDR-SDRAM, the supposed next step for the memory industry, has proven itself...by actually showing up in the mainstream memory market. Although it has had a lot of press, anyone who has been involved with computers knows that this doesn't really mean anything, as vaporware has always been around. In any case, DDR-SDRAM is here, and its here to stay.

DDR-SDRAM

DDR-SDRAM has actually been on the market for some time, as it has been used in memory speed intensive things like graphics accelerators and whatnot. But now we actually get to see it used as the primary RAM in computers, so it is our duty as consumers to know the beef about this. Lets start from the top.

Double Data Rate Synchronous Dynamic Random Access Memory
Not only is Double Data Rate Synchronous Dynamic Random Access Memory a heck of a long name for something, its annoying to type. Now that we've established that, we can move on.

The Beef Traders
DDR-SDRAM and Rambus DRAM have been the two competitors for the memory of the future. Rambus DRAM was seen by most, myself included, as the future. This was caused by the continuing trend that when Intel speaks, everybody listens. Intel said that RDRAM was the future, and as such it was accepted as inevitable. However, RDRAM was DAMN expensive ($340 at the time of writing for a 64MB stick). It was to the point of people needing an RDRAM slush fund, right next to their Roth IRA, in case one needed to upgrade. A number of RDRAM chipsets were produced, primarily for high end servers and whatnot, due to the effectiveness of Rambus. Then we forgot about it. It currently only holds about 2.5% of the market for memory technologies this year, expected to only jump marginally in the next couple years.

DDR-SDRAM, on the other hand, is currently holding a 9% market share this year, expected to jump all the way to 20% next year, and estimated to be holding perhaps as high as 50% of all the memory shipped in 2002. This is caused by a number of fiscally interesting factors. Primarily, unlike RDRAM, this stuff doesn't force you to sell your grandmother's organs to buy some. Its price is comparable to that of the run-o-the-mill SDRAM available at your neighborhood tech kid's garage computer chop shop. Current prices show a difference of less than $10 on the same size modules, which is impressive for new technology.

DDR-SDRAM won't just fit into your old motherboard, though. Newer motherboards need to have both the slots for it as well as BIOS support. However, here as well the cost of making the changes is marginal. At only $5 - $10 to make the upgrades, there is no real worries about getting a cheap motherboard that supports this (we aren't even going to compare to RDRAM motherboards).

Direct Rambus RAM

On the outside, this looks to be a very good solution for memory due to its really fast speed at up to 800MHz. The truth is that the design that Rambus is using is only about twice as fast as current SDRAM because the bus width of 16 bits vs. 64 bits.

Aside from the claims from both Intel and Rambus, Inc., there are some major issues that to be brought into the light. The higher speeds at which DRDRAM will be running at require short wire lengths and additional shielding to prevent problems with electro-magnetic interference. In addition, latency times are actually worse than current PC-100 and PC-133 SDRAM. Since most of today’s applications do not actually utilize the full bandwidth of the memory bus even today, simply increasing the bandwidth while ignoring latency issues will likely not provide any performance improvements. Processors operating with 800MHz bus speeds will certainly require more than double the current memory bandwidth.

While all of these other issues are very serious, probably the biggest drawback to manufactures is that those wishing to use DRDRAM will have to pay royalties to Intel and Rambus, Inc., and they will also have no control over this technology. This doesn’t make DRDRAM look to appealing to any outside manufacturer. Fortunately for the consumer Intel is definitely considering the use of PC-133 due to low cost, availability, and complete backwards compatibility with current systems.

Installing Memory

First there are some things you need to watch out for when choosing memory.

Buy memory that fits into the memory banks on your motherboard. You'll know what kind it is because of the length of the slot.
Get memory that is the right speed for your computer so that it can use the memory without tripping. Look on the memory chip. Somewhere, it should say how fast it is. Look for a number after a dash, like "-100" or "-70". This is the speed in nanoseconds. Consult your motherboard's manual to see what kinds of memory it can handle. In general, you don't want to get memory that is any slower than the memory currently on your computer.
Make sure you don't exceed the amount of memory your motherboard can handle. Different boards have different limits. Again...consult your board's manual for this info. Also pay attention to chipset limitations, such as the TX's 64MB cacheable memory limit.

Now I'll give you instructions to put it in. The actual installation is as easy as could be. The problems usually come from finding compatible memory, but even this is not a huge problem.

1) Turn off the computer, unplug it, and take off the case cover.

2) Get your memory. Here is where you make sure the above guidelines are met.

3) Install the RAM. First some info, though. Your computer, if it uses 30-pin or 72-pin SIMMs, organizes its SIMM sockets into groups called banks. Some boards say that two sockets make a bank. Some say that one is a bank. Nevertheless, a bank must be full. A half full bank will drive your computer nuts. Also, you can't mix two different kinds of memory in a single bank. For example, you can't put a 4MB SIMM and an 8MB SIMM in one bank and expect to get 12 MB of RAM. Also, many systems require you to put the memory in in pairs. Therefore, if you want 32 megs of RAM, you have to stick 2 16's in instead of one 32.

Here's a shortcut which is almost always true. An older computer with a 386 or an early 486 chip usually has a 4 socket bank of 30-pin SIMM modules. A later model 486 requires only one socket of 72-pin modules. Pentium machines have two socket banks of 72 pin modules, meaning you must install RAM in pairs. In all of these systems, the bank must be full for your system to operate. Following these guidelines, lets say you want to add 16 meg of RAM to your Pentium machine. You could buy one 16MB SIMM, but this won't work because you will have a partially filled bank. You must buy two 8MB SIMMS instead, and install them in a pair.

Now, we can all thank God that SDRAM came out, or we'd all have to decipher that Greek to install memory. SDRAM is much simpler. Most newer motherboards have the 168-pin slots for SDRAM. Each SDRAM slot is a bank, so one SDRAM chip will work fine. Other motherboards have both SIMM and DIMM slots. Usually, each DIMM slot is a bank, just like normal. The SIMM slots right next to them are usually paired in a bank, just like the normal Pentium bank setup.

Now that that is out of the way, lets install the RAM. First, ground yourself. Then, in the case of a SIMM module, pick up the memory module and look for the notched end. That notch will only let the thing go in one way, so if it goes in, its right. Push the module down into the socket. Tilt the module slowly until the little spring snaps into place and holds it in.

With DIMM chips, the setup is slightly different. There are little levers that hold the DIMM in place. When installing, you open the levers. They flip to the side. You then push the DIMM in place. Choose the correct memory slot. The slots are numbered, such as DIMM 0, DIMM 1, and DIMM 2. Choose the lesser, unused number. If this is the only module in the system, use DIMM 0. The notch is off-center, so it will only allow the chip in the right way. Once it is in all the way, close the levers. If you push the DIMM snuggly into the slot, the levers will actually close themselves. Very easy.

4) Consult the manual for any jumpers or DIP switches you need to mess with. Flip the ones you need to flip. Most boards do not have such jumpers, but this step is included for those of you that do have such hardware.

5) Turn the system on. Your computer should either greet you with an error message or just boot up normally. If it boots normally, just watch it count up the memory to see if it counts the new memory. If you get an error message, your computer has found the memory and wants you to confirm that you did indeed put more in it. This all happens in the CMOS. Every setup program is a little different. If your machine doesn't seem to be working at all, go over the checklist and make sure you put the right kind of memory in.

6) You're done! Is this really a step?

Memory Problems

There are a lot of problems out there to be had with memory. That's why it isn't too fun. Most of the time, these errors can be traced simply to faulty modules. Nevertheless, let's look at some common memory errors, just so you know and you can impress your friends.

Two very common memory errors are :
"NMI error at [address]" or "Memory parity interrupt at [address]".
If you have Phoenix BIOS, it goes on: "Type (S)hut off NMI, (R)Reboot, (I)gnore".
An NMI is a nonmaskable interrupt. This means that your computer isn't allowed to mask this one while finishing another task. When an NMI occurs, you'll know. The whole computer will lock up and will stay dead until you fix the problem.

Most PC's reserve a little chunk of memory so that it can test the rest of the memory. Almost all modern computers use non-parity memory. These computers will not generate an NMI if it finds only one bit or so bad during the test. With these non-parity computers, then, you may get a few random errors in your work due to these bad bits that the memory test hides. Its no big deal, though.

So, what causes this parity error? Most likely you have a bad RAM chip. Either that, or the voltage to the memory dropped and your computer forgot everything. These two possibilities are most likely. Otherwise, (1) you have bad address logic to the memory chips, making the computer confuse one chip for another, or (2) a bad chip that reports a problem but is just joking with you because there is none. These last two usually result in getting another motherboard, because the chips that are complaining are irreplaceable alone.

To find error sources, though, usually requires a software memory test. There are several out there. Hopefully these programs can point out the bad chip so you can just replace it. If you're lucky, the module just slipped out of the socket somehow. All you have to do is snap the case off and reseat the chip.

If a testing program doesn't find anything, you'll need to have your memory tested at a repair shop.

If you suspect the voltage drop to the memory, as stated above, check to make sure you have sufficient power from the outlet on your wall. Most computers expect around 110 volts from the wall. You can test the outlet with a voltmeter and see if you are indeed getting that much. Another possibility is a temporary brown out. This is when the power in your entire house reduces temporarily. This could be because of overloads at the power company in your area, or possibly a major electrical appliance in your house, such as an A/C, powering up.

Besides this, there could be a myriad of problems. You may need to replace your power supply. The most drastic measure: replace the motherboard or all of the memory.

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