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Troubleshooting September 2004 • Vol.15 Issue 9	Add To My Personal Library
Basic Troubleshooting Memory

When we need to remember information for a short period of time, we use our short-term memory. When we recall the location of our car keys or recently provided directions to a friend's house, we're using short-term memory. RAM is the PC equivalent of short-term memory; it's the location where a PC stores the data it's currently using. When you close an application, data needed in future sessions (such as changes made to a Word document) is saved to the hard drive for long-term storage and other information is simply discarded.

We couldn't function properly if our short-term memory didn't work reliably. In the same manner, RAM problems can cause any number of computer glitches. Diagnosing RAM problems, however, is not always easy. Many of the symptoms that may suggest a RAM problem have more common causes. A Page Fault or General Protection Fault error message, for instance, is usually a sign of a poorly written program but, in some instances, these types of faults can suggest a problem with a computer's RAM. In this article, we'll take a look at some common glitches bad RAM may cause.

Preliminaries.

RAM has its own set of specifications that you should know if you plan to replace or upgrade RAM. Incompatible RAM is the cause of most memory problems, so it's worth taking a moment to cover the different types of RAM and some RAM specifications you should look for when buying memory.

Problem: What's the difference between DIMMs, SIMMs, EDO, FPM, SDRAM, and DDR SDRAM? How do I find out what kind of memory my system uses?

Solution: There are many types of memory technology. In most cases, your system will only support a specific technology. You should be able to find detailed information in your system documentation on what type of memory your system uses. If you can't find any information in your documentation, open the system and remove a memory module

There are four types of memory packaging you can identify by counting the pins at the bottom of the module. SIMMs (single in-line memory modules) have 72-pins and are common in older systems while DIMMs (dual in-line memory modules) may have either 168-pins or 184-pins and are popular in newer systems. High-end systems may use 184-pin RIMMs (not an acronym; known as Rambus dynamic RAM). A RIMM has two notches cut out along the pins while a DIMM has a single notch.

A SIMM measures 4.25-inches long and consists of 72-pins along the bottom of the module (36 on each side). A SIMM can use either FPM (fast page mode) or EDO (extended data output) technology. Unfortunately, there's no easy way to tell by looking at the module whether it uses FPM or EDO. You'll need to consult your system documentation.

The FPM and EDO speeds are often presented in nanoseconds. The lower the number, the faster the RAM. Unfortunately, adding faster memory probably won't improve performance because your PC may not be able to take full advantage of faster memory.

SIMM sockets (the slots on the motherboard that hold a SIMM) are typically paired into a single memory bank. You must fill an entire memory bank when installing memory, so you typically have to install SIMMs in pairs. If you want to add 8MB of memory, you'll need to install two 4MB modules.

A DIMM consists of either 168 pins or 184 pins (84 pins per side and 92 pins per side, respectively) and measures 5.375- inches long. DIMMs with 168 pins use SDRAM (synchronous dynamic RAM) technology and come in speeds measured in megahertz. 66MHz SDRAM is known as PC66 SDRAM. You'll also find PC100 and PC133 modules available. The numbers typically correspond to the speed of the motherboard's frontside bus. Thus, you'd use PC66 SDRAM in a motherboard with a 66MHz frontside bus and PC133 SDRAM in a motherboard with a 133MHz frontside bus.

DIMMs with 184 pins use a memory technology known as DDR SDRAM (double-data-rate SDRAM). Rather than using a naming system that corresponds to the motherboard's frontside bus, DDR SDRAM uses a naming scheme based upon the theoretical maximum speed of the memory. Therefore, PC2100 DDR SDRAM has a maximum speed of 2.1GBps (gigabytes per second) and is compatible with a 266MHz frontside bus. Additional speeds include: PC2700 (333MHz), PC3200 (400MHz), PC3500 (433MHz), and PC3700 (466MHz). Each DIMM socket is its own memory bank. That means you do not need to install DIMM modules in pairs.

Rambus provides some of the best performance, but it's often too expensive for consumer PCs. As with DDR SDRAM, Rambus comes in a 184-pin package. RIMM modules, however, have two notches along the pins at the bottom. DDR SDRAM modules only have one notch. RIMM modules are also slightly shorter than DDR SDRAM modules and measure 5.25 inches.

Problem: What kind of RAM does my notebook use?

Solution: Notebook PC components are often smaller than desktop components, so it should come as no surprise that notebook RAM is smaller than desktop RAM. Notebook memory modules are known as SO-DIMMs (small outline DIMMs) and come in two packages, 144-pin and 200-pin.

144-pin SODIMMs use SDRAM technology and measure only 2.625 inches. SO-DIMMs typically come in speeds of PC66, PC100, and PC133. 200-pin SO-DIMMs use DDR SDRAM technology. Despite the higher pin count, 200-pin SO-DIMMs still measure just 2.625 inches. You'll find the 200-pin SODIMMs available in PC2100 (266MHz) and PC2700 (333MHz) speeds.

Problem: How can you find memory compatible with your PC?

Solution: Many memory manufacturers have an online database where you can look for memory that's compatible with your system. Crucial Technology (http://www.crucial.com) has an excellent database that can provide you with all of the specifications you need to find compatible memory. The interactive guide asks you for your PC manufacturer and model and supplies a list of compatible Crucial memory modules. Although Crucial no doubt hopes you'll buy its memory, you can use the specifications next to each module to locate compatible modules elsewhere.

Unrecognized Memory.

It can be frustrating when you install additional memory and your PC refuses to recognize it. Getting everything to work depends on your hardware and OS (operating system).

Problem: When it first boots up, the computer lists installed RAM as less than the amount you've installed.

Solution: When your computer boots, it runs a variety of hardware tests and will often list the amount of RAM it detects. This screen may go by quickly on some systems or be completely masked on others. Often, you can press a key (such as ESC) to display the testing information. The key will vary among systems, so consult your system documentation for more information.

You may be able to add more RAM to your motherboard than the system can support. A motherboard with three DDR SDRAM sockets, for instance, may be able to support only 1024MB of DDR SDRAM. If you install three 512MB modules, you'll have 512MB more than the system can support. In some instances, a BIOS (Basic Input/ Output System) update may help (consult your manufacturer's Web site), but in most cases you'll have to keep your system's memory limits in mind when adding memory.

In some instances your PC may not be properly recognizing one or more memory modules. If you've recently installed new memory, check to be sure it's properly seated in its socket and the module is compatible with your system.

If the module is properly secured and appears to be compatible with your system, try cleaning the pins using rubbing alcohol and a cotton swab. Clean each socket on the motherboard, as well.

If problems persist, try changing the order of your memory modules. DIMM sockets on the motherboard are labeled (DIMM0, DIMM1, etc). You should always start installing memory in the lowest numbered socket available. In many cases, the BIOS will probe the memory module in DIMM0 and address all memory according to the specifications of the module in DIMM0. If you've installed faster memory in DIMM0, the system may not be able to communicate with slower modules in other slots. Placing a slower module in DIMM0 may fix your problems; however, the faster modules will then, of course, run at a slower speed).

Problem: The computer reports a Memory Mismatch error.

Solution: Older PCs require you to tell them how much memory you have installed. When you add more RAM, the PC may detect more memory than you've specified causing a mismatch error. You'll need to enter the BIOS Setup utility by pressing the correct key right after turning on your system. The correct key depends on your motherboard and BIOS, so consult your documentation to learn how to access the BIOS Setup utility. Specify the correct amount of memory, save the settings, and exit the BIOS Setup utility.

Problem: You have Windows 95/98/ 98SE/Me installed and Windows doesn't appear to recognize all installed RAM.

Solution: You can find out how much memory Windows recognizes by right-clicking My Computer and selecting Properties. If the amount of memory Windows reports doesn't match the amount of memory you know you have installed, you may need to make some adjustments to Windows. Older versions of Windows have an option that limits the amount of memory Windows recognizes. From the Start menu, click Run and type msconfig. Click the Advanced button and remove the check from the Limit Memory To checkbox. Reboot the system.

Boot Problems.

There's nothing worse than when your PC greets you with a series of beeps and a black screen. In some cases, your memory may prevent your system from booting.

DIMM

Problem: You turn on your PC and hear a series of beeps. The computer refuses to boot.

Solution: This particular symptom may reflect a problem with any number of components. A bad CPU or video card, for instance, may prevent the computer from booting properly. Often the number of beeps will clue you in to the offending hardware. Check your system or your motherboard manuals' troubleshooting sections for a list of beep codes.

If the beep codes indicate a memory problem, make sure all memory is properly seated and secure. If you've recently added new memory, make sure it's compatible with your system. You may also want to try cleaning the pins on each module and each socket on the motherboard.

If you have multiple memory modules, try to isolate any bad modules by removing all but one module and booting the system. If the system boots, turn it off, replace the module with another and boot the system again. When the computer refuses to boot, you've found your problem module.

Again, you may want to reorder your memory modules. Faster modules in the DIMM0 slot can cause a problem on some systems.

Problem: When booting the system, the computer displays an error message such as "Memory parity interrupt at . . ."

Solution: Memory-related error messages that appear when your computer first begins booting usually point to a bad memory module. The computer performs basic tests on all hardware when it's first switched on. If the information it writes to memory is not the same as the information it reads from memory, the computer stops booting and displays an error message.

In most cases, you'll need to replace the bad module. If you have recently installed new memory, however, the problem could be a compatibility issue. Try removing the new memory and see if it solves the problem.

If you have multiple memory modules, try booting your PC with just one module installed at a time. This will help you isolate the bad memory module.

Performance.

Lack of memory or bad/ incompatible memory can limit a computer's effectiveness. Just as forgetting where your car keys are in the morning can cause you to be late for an important meeting, ineffective computer memory can cause system slowdowns, error messages, and even crashes.

Problem: When you have a lot of applications open, your system tends to slow to a crawl. You may even be able to see the PC redrawing the screen line by line when you switch between applications.

Solution: More memory. When a PC uses all the available space in RAM, it begins to offload information to the computer's hard drive. Accessing information from a hard drive is much slower than accessing data from RAM, resulting in a performance loss. Doubling the amount of memory, in most cases, should solve the problem. Alternatively, you can try running fewer applications.

Problem: The computer gives General Protection Faults, Page Faults, and/or Exception errors.

Solution: The messages listed above are memory errors, but the source of the error is usually unruly software. A Page Fault, for instance, can occur when software tries to write data to a portion of memory that's reserved for another application. When the PC reads back information from that area, it doesn't get the information it expects and generates an error message. In most cases, such error messages do not mean you have a real memory problem. Rebooting the system is usually enough to resolve the problem.

In some instances, however, it may be reasonable to believe that physical memory is the root of your problems. If you've recently installed or upgraded your memory, for instance, and you suddenly receive numerous error messages, your memory module may be suspect. Remove the new memory and return any older modules to the system and see if the problem disappears. If so, you may have memory that's incompatible with your system or a bad memory module.

Problem: The computer reports frequent Registry errors or the system often crashes or reboots at unpredictable times.

Solution: As with most potential memory problems, these errors may have causes not directly related to faulty memory. Poorly written software, for instance, may cause such problems. However, there is a chance that your memory may be to blame.

If you've recently installed new memory, remove it and see if the problems disappear. If so, make sure the new memory is compatible with your system. You can also try installing different modules in different sockets. If you have more than one module, try to isolate any problem modules by running with as little RAM as possible and seeing if the errors subside.

You can avoid most memory problems by simply making sure any additional RAM you install is compatible with your system. In most instances, memory problems are the result of incompatible memory and not a memory module gone bad. Many of the symptoms produced by bad or incompatible RAM have more common roots elsewhere in your system (such as software or other hardware problems). In most instances, you should suspect a memory issue only if you've recently installed new RAM or have eliminated all other possibilities.

by Chad Denton

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