5.6 Memory Selection Guidelines

The following sections provide guidelines for selecting memory for a new system or to upgrade an existing system. Follow these guidelines as closely as possible to ensure the memory you select functions optimally in your system.

5.6.1 Upgrading Older Systems

When upgrading an older motherboard, it is sometimes impossible to match the installed memory. Some motherboards have 30-pin and 72-pin SIMM sockets, and many have both SIMM and DIMM sockets. If the installed memory occupies all available sockets of one type, you may have to add memory of a different type.

If so, consult the manual to determine supported configurations. For example, many 30/72-pin motherboards provide four 30-pin and two 72-pin sockets, but allow you to populate both 72-pin sockets only if no memory is installed in the 30-pin sockets. Similarly, motherboards with both SIMM and DIMM sockets may allow using a DIMM only if one or more SIMM sockets are vacant or populated only with single-sided SIMMs.

Mixing memory types may degrade performance. Some chipsets run all memory at the speed of the slowest module. Others, including the 430HX and 430VX, run each bank at optimum speed. Mixed-memory configurations are nonstandard and best avoided. Memory is cheap enough that it's often better to use all new memory.

Be very conscious of costs when upgrading older systems. Although older memory types are still available, the cost per MB may be very high. For the same cost as a large amount of old-style memory, you may literally be able to purchase a new motherboard, processor, and the same amount of modern memory. If for some reason we need to upgrade memory in a SIMM-based system, we try to scrounge compatible SIMMs from hangar queens rather than buying new modules.

Observe the following general guidelines when selecting memory:

  • Verify your motherboard can cache the full amount of RAM you install. For example, the Intel 430TX chipset supports 256 MB of RAM, but caches only 64 MB. Adding RAM beyond 64 MB actually decreases system performance. Some motherboards have insufficient cache installed to cache the full amount of cacheable RAM supported by the chipset. Some of these have sockets that you can fill with additional cache. The cacheable RAM area of Slot 1 systems is determined by the processor itself. The L2 cache present on Pentium II CPUs can cache a maximum of 512 MB of system memory. Modern socketed processors from Intel and AMD can cache more main memory than can physically be installed.

  • Purchase only name-brand memory, especially if you overclock your system. Motherboards vary in their tolerance for different brands of memory. Some motherboards accept and use nearly any compatible memory, but others are reliable only with some brands of memory. A name-brand memory module functions properly on nearly any motherboard designed to accept that module. We use only Crucial memory (http://www.crucial.com) and Kingston memory (http://www.kingston.com) in our systems, and have never had a problem with them.

  • Don't attempt to save money by mixing newer memory with older memory. For example, if you buy two 32 MB PC133 SDRAM DIMMs for a system that currently has one 16 MB EDO DIMM installed, you may be tempted to use all three DIMMs for a system total of 80 MB rather than 64 MB. That's usually a mistake. Some systems accept mixed EDO and SDRAM memory, but most systems will run the newer, faster memory at the same speed as the older, slower memory. The same holds true for upgrading newer systems. For example, if you buy two 256 MB PC2700 DIMMs to upgrade a system that currently has one 128 MB PC1600 DIMM, settle for 512 MB of pure PC2700 memory rather than going for 640 MB of mixed-speed memory that will operate at only PC1600.

  • Buy one larger-capacity module in preference to two smaller ones of the same total capacity. This is particularly important with DIMM modules. Most motherboards have only two to four DIMM sockets, so buying smaller-capacity DIMMs may later force you to replace existing memory when you want to add more memory to the system.

  • Memory modules and sockets may use tin or gold contacts. Manufacturers often recommend using modules with the same contact metal as the sockets. In theory, mixing gold and tin can cause corrosion and other problems. In practice, we've never seen this happen.

5.6.2 Upgrading Newer Systems

Newer systems use DIMM or RIMM modules exclusively. DIMM and RIMM memory are available in many more variants than SIMM memory, so be careful to buy DIMMs or RIMMs that match your requirements. Use these guidelines when choosing DIMM or RIMM memory modules:

  • For SDRAM DIMMs, get the proper voltage. All DDR-SDRAM DIMMs use 2.5V. Most SDR-SDRAM DIMMs use 3.3V. Some DIMMs, often called Macintosh DIMMs, use 5V. Some early DIMM-based PC motherboards use 5V DIMMs.

  • DIMMs are available in EDO, SDR-SDRAM, and DDR-SDRAM. Some transition motherboards support two types?EDO and SDR-SDRAM, or SDR-SDRAM and DDR-SDRAM. If you have the choice of EDO or SDR-SDRAM, buy SDR-SDRAM DIMMs for future flexibility. If your motherboard supports both SDR-SDRAM and DDR-SDRAM, buy DDR-SDRAM.

  • SDR-SDRAM and DDR-SDRAM DIMMs are available in buffered (also called registered) and unbuffered versions. Some motherboards accept only buffered DIMMs, others accept only unbuffered DIMMs, and still others will accept either buffered or unbuffered DIMMs, but do not allow mixing types. Check the motherboard or system maker's documentation or web site to verify which type(s) of module your system accepts. If you are upgrading an existing system that accepts both types, open the lid to determine which type is currently installed. Even if the motherboard accepts mixed buffered and unbuffered DIMMs, avoid mixing types if possible.

  • Buy the fastest-speed memory available, which usually costs little or no more than slower versions. For SDR-SDRAM, buy nothing slower than PC133 modules, which are backward-compatible with systems that use PC100 or PC66 memory. For DDR-SDRAM, buy PC2700 or (better) PC3200 modules in preference to PC2100 modules. Like SDR-SDRAM modules, faster DDR-SDRAM modules are generally backward-compatible with motherboards designed to use slower modules. Verify backward compatibility using the Crucial Memory Configurator (http://www.crucial.com).

    No-name and off-brand memory may be fraudulently labeled. The manufacturer's reputation is the only guarantee that a module labeled "PC2700" or "PC3200" in fact meets the required specifications. Using substandard memory may at best cause frequent lockups and at worst corrupt data or cause boot failures.

  • Don't mistake modules labeled "100 MHz" for true PC100 memory. PC100 memory supports 125 MHz (8 ns) operation, but 100 MHz (10 ns) memory is usable at FSB speeds no higher than 83 MHz. Some vendors label modules "LX" to indicate they work with the 66 MHz FSB 440LX chipset, and "BX" to indicate they work with the 100 MHz FSB 440BX chipset. "BX" modules usually run at 100 MHz FSB, but are not PC100 modules unless so labeled. Similarly, some vendors sell "133 MHz" DIMMs, hoping buyers will mistake them for PC133.

    Access time for memory modules is rated for the entire module rather than for the individual chips that comprise it, which is why a module that uses 60 ns chips can have a 10 ns access time.

  • Make sure DIMMs support Serial Presence Detect (SPD), a serial EEPROM that stores DIMM parameters?including memory type, size, speed, voltage, number of row and column addresses, and so forth?which can be read by the system BIOS at boot time and used to correctly configure memory. Some motherboards won't boot with non-SPD memory. Others boot, but with a warning message. Some vendors label SPD modules "EEPROM," but a module so labeled isn't necessarily SPD-compliant. Ask.

  • Purchase modules with lower CAS latency whenever possible. PC100 and PC133 DIMMs are readily available in CAS2 or CAS3 (sometimes shown as CL2 or CL3). CAS2 modules provide a small performance boost relative to CAS3 modules. More important, CAS2 modules have a bit more in reserve that allows them to function more reliably in overclocked or heavily loaded systems. CAS2 modules typically cost 2% or so more than CAS3. You may find CAS1 modules, which ordinarily sell at a high enough premium over CAS2 modules to make them a poor choice.

  • When upgrading an older system, determine whether it requires two-clock or four-clock modules. Recent PCs use four-clock memory, which is now ubiquitous. If your system requires two-clock memory, you'll have to ask for it. It may be a special-order item, for which you will be charged a premium.

All 168-pin DIMMs, 184-pin DIMMs, and 184-pin RIMMs use similar sockets, but both modules and sockets are keyed to prevent using the wrong type of module. If there is such a thing as a "standard" SDR-SDRAM DIMM, it would be 3.3 volt, nonparity (x64), unbuffered, PC100, CAS3. Similarly, a "standard" DDR-SDRAM DIMM would be 2.5 volt, nonparity (x64), PC2700 or PC3200, CAS2.5.

5.6.3 SIMM Guidelines

Use the following guidelines when choosing SIMM memory modules:

  • Unless you are upgrading an older Socket 7 system that accepts only 72-pin SIMMs, buy only 168-pin DIMM memory. Many Socket 7 motherboards have both SIMM and DIMM sockets. Buying DIMMs for such a board makes more sense than buying obsolete SIMMs. However, note that some early boards support DIMMs only with very slow memory timings.

  • SIMMs are available in FPM and EDO. If your motherboard supports either type, buy EDO (assuming you can't scrounge it somewhere). Some motherboards support both FPM and EDO, but do not allow mixing types, either per bank or globally. Because FPM is obsolescent and demand for it is small, FPM often costs more than EDO. If FPM is installed and the motherboard does not allow mixing FPM with EDO, consider removing FPM SIMMs and installing all EDO SIMMs.

  • Don't assume the motherboard supports large SIMMs. Check the manufacturer's web site to determine the supported SIMM capacities and memory configurations. Older motherboards often support nothing larger than 16 MB SIMMs. Conversely, don't assume the motherboard cannot support larger SIMMs than those currently installed just because the manual says so. For example, one of our technical reviewers from an earlier edition has a Compaq system whose manual lists 16 MB SIMMs as the largest supported. That was true when the manual was printed because 32 MB SIMMs were not yet available. But that system uses 32 MB SIMMs without problems.

  • Some motherboards, particularly those with four or more SIMM sockets, restrict the number of double-sided SIMMs that can be used, or restrict the total number of SIMM sides that can be used. If your motherboard has such a limitation, verify that the combination of currently installed SIMMs and the SIMMs you plan to install does not exceed the limit.

5.6.4 Upgrading Memory in Pentium Pro and Pentium-Class Systems

Pentium Pro and P54C/P55C Pentium-class systems, including those with an AMD K6 or higher, are too old to be good upgrade candidates, but many such systems are still used as firewalls and fax servers, and for other appliance duties, where it may make sense to extend their useful lives for another year or two by installing a memory upgrade. When upgrading memory in a Pentium or Pentium Pro, note the following issues:

  • Try to install the memory identical to that already installed. Early Pentiums and most Pentium Pros use 72-pin SIMMs, but either type of system may use DIMMs in addition to or instead of SIMMs. Existing memory may be FPM, EDO, or SDRAM. Many systems can use different memory types?e.g., FPM or EDO?but do not allow mixed types. If you have a choice, install SDR-SDRAM DIMMs. Even PC66 DIMMs work fine in systems this old, and most systems of this type yield better memory performance with SDRAM than with older memory styles.

  • Most such systems have nonparity memory installed. Some do not support parity memory. Pentium Pro systems often have parity memory installed, and may use ECC. Some systems support parity or nonparity memory, but do not allow mixed types. Others accept parity or nonparity RAM interchangeably, but disable parity if any nonparity modules are installed. For SIMM-based systems, count chips to determine memory type. For DIMM-based systems, locate the identification number on a module and check the manufacturer's documentation to determine its type.

  • These systems use a 64-bit memory bus, and therefore require adding 72-pin SIMMs in pairs. DIMMs may be added individually.

  • Most SIMM-based systems use 60 or 50 ns memory. Make sure new memory is at least as fast as that already installed. Early DIMM-based systems use FPM or EDO DIMMs, which you should match as closely as possible. Later DIMM-based systems may use JEDEC (PC66) SDRAM. You can use PC66 SDRAM, but it's better to buy PC133 SDRAM, which can be recycled later into systems that will themselves be approaching their design lifetimes.

5.6.5 Upgrading Memory in Pentium II/III/4, Celeron, and Athlon Systems

These systems are excellent candidates for memory upgrades. Early Pentium II systems often have only 16 MB of RAM. In the price-sensitive consumer Celeron market, many early systems shipped with only 16 MB, and some Celeron systems have been sold with only 8 MB. Expanding memory to 128 MB or more is the most cost-effective upgrade you can make. When upgrading memory in one of these systems, note the following issues:

  • Some early Pentium II and Celeron systems use EDO SIMMs or DIMMs, but most use 3.3 volt 168-pin unbuffered JEDEC SDR-SDRAM DIMMs.

  • Conserve DIMM sockets. A few motherboards have four DIMM sockets, most have three, and some low-end systems have only two. If you have the choice, always install one larger DIMM rather than two smaller ones that total the same amount of memory. Note, however, that older systems may not recognize large-capacity DIMMs or those that use 128-megabit or larger memory chips. In that situation, a BIOS upgrade may help, but the limitation is often hardcoded into the chipset.

  • Most of these systems have nonparity memory installed, but can use either parity or nonparity DIMMs interchangeably. Unless you plan to install 512 MB or more, install nonparity DIMMs. We have been told that when using very large amounts of memory?more than 512 MB?memory errors introduced by cosmic rays make it worthwhile to pay the additional cost for parity/ECC memory and to accept the small performance hit that using ECC produces.

  • Memory must always be added in full banks. These systems use a 64-bit memory bus, and therefore require adding 72-pin SIMMs in pairs. DIMMs may be added individually.

  • Most SIMM-based systems use 60 ns or 50 ns memory. Make sure memory you add is at least as fast as the memory that is already installed. Early DIMM-based systems use FPM or EDO DIMMs, which you should match as closely as possible. Later DIMM-based systems may use JEDEC (PC66) SDRAM. You can install PC66 SDRAM, but it usually makes more sense to buy PC133 SDRAM for these systems because it can be recycled later if you upgrade to a faster system.