Adding memory to a system is one of the most useful upgrades you can perform and also one of the least expensiveespecially when you consider the increased capabilities of Windows 9x/Me, Windows NT/2000/XP, and Linux when you give them access to more memory. In some cases, doubling the memory can practically double the speed of a computer.

The following sections discuss adding memory, including selecting memory chips, installing memory chips, and testing the installation.

Upgrade Options and Strategies

Adding memory can be an inexpensive solution; at this writing, the cost of memory has fallen to about 12 cents per megabyte or less. A small dose can give your computer's performance a big boost.

How do you add memory to your PC? You have two options, listed in order of convenience and cost:

• Adding memory in vacant slots on your motherboard

• Replacing your current motherboard's memory with higher-capacity memory

If you decide to upgrade to a more powerful computer system or motherboard, you usually can't salvage the memory from your previous system. Most of the time it is best to plan on equipping a new board with the optimum type of memory that it supports.

Be sure to carefully weigh your future needs for computing speed and a multitasking operating system (Windows 9x/Me, Windows NT, Windows 2000, Windows XP, or Linux, for example) against the amount of money you spend to upgrade current equipment.

To determine at what point you should add memory, you can use the Performance Monitor (Perfmon.msc) built in to Windows 2000 and Windows XP. You can launch it remotely or from the server's own console. To check memory usage, select Memory as the Performance object and enable the following counters:

• Pages/Sec. This counter measures the number of times per second that the system uses virtual (swapfile) memory rather than physical memory. A value above 20 indicates a potential problem. Check the virtual memory settings; if the counter remains above 20, install more memory.

• Committed Bytes and Available Bytes. Committed Bytes tracks virtual memory in use; Available Bytes tracks physical memory available. Add more memory if you run short of Available Bytes.

• Cache Bytes. Measures the amount of RAM used for file system cache. Add more RAM if this amount exceeds 4MB.

Before you add RAM to a system (or replace defective RAM chips), you must determine the memory modules required for your system. Your system documentation has this information.

If you need to replace a defective memory module or add more memory to your system, there are several ways to determine the correct module for your system. You can

• Inspect the modules installed in your system. Each module has markings that indicate its capacity and speed. You can write down the markings on the memory module and use them to determine the type of memory you need. Check with a local store or an online memory vendor for help.

• Look up your system using the online memory configuration utility provided by your preferred memory vendor. Originally, these configuration utilities were primarily for users of name-brand systems. However, most vendors have now added major motherboard brands and models to their databases. Thus, if you know your system or motherboard brand and model, you can find the memory that is recommended.

• Download and run analysis software provided by the memory module maker or from a third party. SiSoftware Sandra and similar programs use the SPD chip on each module to determine this information.

• Consult your system documentation. I list this option last for a reason. If you have installed BIOS upgrades, you might be able to use larger and faster memory than your documentation lists as supported by your system. You should check the latest tech notes and documentation available online for your system and check the BIOS version installed in your system to determine which memory-related features it has.

Adding the wrong modules to a system can make it as unreliable as leaving a defective module installed and trying to use the system in that condition.

Selecting and Installing Memory

Installing extra memory on your motherboard is an easy way to add memory to your computer. Most systems have at least one vacant memory socket where you can install extra memory at a later time and speed up your computer.

If your system requires dual-channel memory, as some high-performance systems do, you must use two identical memory modules (same size, speed, and type).

Purchasing Memory

When purchasing memory, there are some issues you need to consider. Some are related to the manufacturing and distribution of memory, whereas others depend on the type of memory you are purchasing. This section covers some of the issues you should consider when purchasing memory.

Considerations in Purchasing or Reusing SIMMs

When purchasing SIMMs, the main things to consider are as follows:

• Do you need FPM (Fast Page Mode) or EDO (extended data out) versions?

• Do you need ECC or non-ECC?

• What speed grade do you need?

• Can you use memory from other systems or from parts stores as an alternative to purchasing (very expensive) new memory?

Most Pentium systems after 1995 used EDO SIMMs that were non-ECC and rated for 60ns access time. If your system is older than that, you might need regular FPM versions. The FPM and EDO types are interchangeable in many systems, but some older systems do not accept the EDO type. If your system is designed for high-reliability using ECC, you might need (or want) ECC versions; otherwise, standard non-ECC types are typically used. You can mix the two, but in that case the system defaults to non-ECC mode.

Unfortunately, FPM and EDO SIMMs are obsolete by today's standards, so they are much more expensive than newer, better, and faster types of memory. This can make adding memory to older systems cost prohibitive.

Considerations in Purchasing DIMMs

When purchasing DIMMs, the main things to consider are as follows:

• Do you need SDR or DDR versions?

• Do you need ECC or non-ECC?

• Do you need registered or standard (unbuffered) versions?

• What speed grade do you need?

• Do you need a specific column address strobe (CAS) latency?

Currently, DIMMs come in SDR (SDRAM), DDR, and DDR2 versions. They are not interchangeable because they use completely different signaling and have different notches to prevent a mismatch. High-reliability systems such as servers can use ECC versions, although most desktop systems use the less-expensive non-ECC types. Most systems use standard unbuffered DIMMs, but file server or workstation motherboards designed to support very large amounts of memory might require registered DIMMs (which also include ECC support). Registered DIMMs contain their own memory registers, enabling the module to hold more memory than a standard DIMM. DIMMs come in a variety of speeds, with the rule that you can always substitute a faster one for a slower one, but not vice versa. As an example, if your system requires PC2700 DDR DIMMs, you can install faster PC3200 DDR DIMMs but not slower PC2100 versions.

Another speed-related issue is the column address strobe (CAS) latency. Sometimes this specification is abbreviated CAS or CL and is expressed in a number of cycles, with lower numbers indicating higher speeds (fewer cycles). The lower CAS latency shaves a cycle off a burst mode read, which marginally improves memory performance. Single data rate DIMMs are available in CL3 or CL2 versions, with the CL2 being faster. DDR DIMMs are available in CL2.5 or CL2 versions, with CL2 being the faster and better version in that case. You can mix DIMMs with different CAS latency ratings, but the system usually defaults to cycling at the slower speeds of the lowest common denominator.

Considerations in Purchasing RIMMs

When purchasing RIMMs, the main things to consider are as follows:

• Do you need 184-pin (16/18-bit) or 232-pin (32/36-bit) versions?

• Do you need ECC or non-ECC?

• What speed grade do you need?

RIMMs are available in 184-pin and 232-pin versions, and although they appear to be the same size, they are not interchangeable. Differences exist in the notches that prevent a mismatch. High-reliability systems might want or need ECC versions, which have extra ECC bits. As with other memory types, you can mix ECC and non-ECC types, but systems can't use the ECC capability.

Replacing Modules with Higher-Capacity Versions

If all the memory module slots on your motherboard are occupied, your best option is to remove an existing bank of memory and replace it with higher-capacity modules. For example, if you have a motherboard that supports two DIMM modules (each representing one bank on a processor with a 64-bit data bus) and runs in single-channel mode, you could remove one of them and replace it with a higher-capacity version. For example, if you have two 256MB modules giving a total of 512MB, you could remove one of the 256MB modules and replace it with a 512MB unit, in which case you'd then have a total of 768MB of RAM.

However, just because higher-capacity modules are available that are the correct pin count to plug into your motherboard, don't automatically assume the higher-capacity memory will work. Your system's chipset and BIOS set limits on the capacity of the memory you can use. Check your system or motherboard documentation to see which size modules work with it before purchasing the new RAM. You should make sure you have the latest BIOS for your motherboard when installing new memory.

If your system supports dual-channel memory, you must use matched pairs of DDR or DDR2 modules (depending on which type your system supports) and install them in the correct location on the motherboard to achieve the superior memory performance that dual-channel access offers. You should consult your motherboard manual for details.

Installing DIMM or RIMM Modules

This section discusses installing memoryspecifically, SIMM or DIMM modules. It also covers the problems you are most likely to encounter and how to avoid them. You also get information on configuring your system to use new memory.

When you install or remove memory, you are most likely to encounter the following problems:

• Electrostatic discharge
• Improperly seated modules
• Incorrect memory configuration settings in the BIOS Setup

To prevent electrostatic discharge (ESD) when you install sensitive memory chips or boards, you shouldn't wear synthetic-fiber clothing or leather-soled shoes because these promote the generation of static charges. Remove any static charge you are carrying by touching the system chassis before you begin, or better yet, wear a good commercial grounding strap on your wrist. You can order one from any electronics parts store. A grounding strap consists of a conductive wristband grounded at the other end through a 1-meg ohm resistor by a wire clipped to the system chassis. Be sure the system you are working on is unplugged.

Follow this procedure to upgrade DIMM or RIMM memory on a typical desktop PC:

After adding the memory and putting the system back together, you might have to run the BIOS Setup and resave with the new amount of memory being reported. Most newer systems automatically detect the new amount of memory and reconfigure the BIOS Setup settings for you. Most newer systems also don't require setting any jumpers or switches on the motherboard to configure them for your new memory.

After configuring your system to work properly with the additional memory, you might want to run a memory-diagnostics program to ensure that the new memory works properly. Some are run automatically for you. At least two and sometimes three memory-diagnostic programs are available for all systems. In order of accuracy, these programs are as follows:

• POST (power on self test)

• Disk-based advanced diagnostics software

The POST is used every time you power up the system.

Many additional diagnostics programs are available from aftermarket utility software companies.

Installing SIMM Modules

SIMM memory is oriented by a notch on one side of the module that is not present on the other side. The socket has a protrusion that must fit into this notched area on one side of the module. This protrusion makes installing a SIMM backward impossible unless you break the connector or the module.