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[ The PC Guide | The PC Buyer's Guide | Designing and Specifying PC Systems and Components | Detailed Considerations and Tips for Specifying Particular Components ]

System Memory (RAM)

Description: The PC's system memory, also sometimes called random access memory or RAM (for historical reasons I won't go into here), is a high-speed temporary storage area used by the system to hold programs while they are running and data that those programs are using. A defining characteristic of the system memory is its temporary nature: it is erased whenever the PC is turned off. It is thus (roughly) homologous to short-term memory in the human brain, while the hard disk drive and other permanent storage are more comparable to human long-term memory and "personality".

Tip: For lots of additional information on system memory, including more discussion of many of the technical details, criteria and features mentioned below, see the Reference Guide section on system memory.

Role and Subsystems: The system memory is a component in the system processing core. It facilitates the running of programs and processing of data by providing them with a place to reside so the CPU and other hardware can access them.

Related Components: The system memory is most closely related to the system processor and the motherboard. The choice of motherboard essentially defines the choice of system memory technology and also theoretical maximum memory size, since both of these are functions of motherboard (and chipset) characteristics.

Key Compatibility Selection Criteria: The system memory must be matched to the needs of the motherboard, and the requirements for the type of memory needed are usually spelled out in the motherboard manual. There are a number of different issues for compatibility; these are the most important ones:

  • Memory Technology: The general type of memory must be matched to the requirements of the motherboard. Current (and upcoming) systems use SDRAM, RDRAM or DDR SDRAM; older systems used EDO DRAM, FPM DRAM and even older technologies. Whatever the technology type, you must use whatever the motherboard supports. Some motherboards support more than one type of memory.
  • Physical Packaging and Module Types: System memory is comprised of silicon chips that are placed into small black packages (themselves also sometimes called chips, oops). These packages are in turn placed onto standard-sized modules, to make installing and removing memory easier; these modules are really very small circuit boards. The standard module types today are 168-pin dual inline memory modules (DIMMs) for SDRAM, 184-pin DIMMs for DDR SDRAM, and Rambus inline memory modules (RIMMs) for RDRAM. Older systems used 30-pin and 72-pin single inline memory modules or SIMMs. You must be sure to get the right packaging for your motherboard.
  • Bus Speed: The system bus speed of the memory must be certified to be at least as fast as that the motherboard requires. It is usually indicated by a "PC" specification, for example, "PC133 SDRAM". Most motherboards will work with memory that is faster than the system bus speed required, though not all will If you use memory that is too slow you will have a problem (or you will have to scale back the operating speed of the motherboard to suit the memory you are using, if that is possible.)
  • Specific Module Requirements: Certain memory types have other technical details you may need to pay attention to. For example, some memory modules are buffered and others unbuffered. You have to use whatever your system manual calls for, or check the manufacturer's web site for details. (Problems with mixing up incompatible module types are normally avoided through the use of modules that are slightly modified, for example, through use of a notch or slightly different sizing.)
  • Error Detection And Correction: If you want the added security and integrity that comes from using memory with error protection (parity or ECC memory), then you will need either parity or ECC memory (of the appropriate type) and a motherboard that supports its use.
  • Standard And Proprietary Modules: Some systems (typically large retail PCs) use proprietary modules. If you are upgrading such a machine be sure to use the correct memory.

Performance and Capacity Selection Criteria: In selecting the technology type and bus speed of the memory to match that required by the system, you are defining most of the memory's performance characteristics. Many specific memory types have available different variants that run slightly faster or slightly slower than others. These are based on the specific chips used on those modules and may vary in terms of latency or how much "headroom" they have in meeting their respective performance specifications. Differences are small and not really worth worrying about unless you are an overclocker or a real performance wonk.

The most important issue related to performance when it comes to memory is not what kind you have, but rather how much of it. The amount of memory is more important to overall performance than the specific technology or module type. I recommend a minimum of 128 MiB for new systems, and with memory so inexpensive 256 MiB is not a bad idea, especially if you plan to run Windows 2000 or do a lot of work with multimedia or large files. Don't skimp on memory. (A 128 MiB DIMM of SDRAM memory costs less today than 4 MiB of older FPM DRAM did just a few years ago.)

When you are selecting modules, try to use the largest modules that your motherboard can reliably handle; this saves room for future expansion. Watch out for systems that have two DIMM sockets and two 64 MiB DIMMs filling them; one 128 MiB DIMM in one socket may cost slightly more but leaves you room to add more memory later if you need to. Check the motherboard manual carefully to see what size modules are supported.

Quality Selection Criteria: Memory modules have two main quality issues: the quality of the chips themselves and the quality of the modules. The chips on almost all modules are made by one of a handful of large chip makers, and almost all are of good to excellent quality. The modules, however, are assembled by companies ranging from the same ones that make the chips, to small shops that you have likely never heard of. For more on this, see the "Importance of Manufacturer" section below.

Important Features: None really; it's either the right type of memory, or it isn't.

"Magic Numbers" To Watch For: The two numbers usually associated with system memory are bus speed and size, and I consider both important.

Performance Impact: As described above, the size of the system memory is more important than the exact type being used, to a point. The right amount of memory depends on how you are using the system.

Retail, OEM and Gray Market Issues: Traditionally, memory has been one of those components that a lot of buyers tried to get "as cheap as possible", which meant a lot of OEM and gray market memory was being sold. Today, the trend is back towards retail-branded memory, or at least, OEM memory sold by a vendor willing to back it up with a proper warranty. As with CPUs, most memory fails quickly or not at all, but I still recommend buying from a reputable firm and not taking your chances with cheap stuff.

Importance of Manufacturer: Very high, if you care about quality. As the speed of memory increases, its stability becomes increasingly a function of not just the raw memory chips but also the module and how the module is assembled. Look at not just the manufacturer of the chips but the maker of the module. Avoid generics unless they are backed by a long warranty from a vendor you trust (some people do have good luck with generics when it comes to memory, I must admit, but memory today is just not that expensive--buy it properly and save yourself a lot of potential hassle.)

Typical Component Lifetime: System memory is one of the more "long lived" of components. If it lasts a week, it will most likely last for over a decade easily without failing. Technologies change faster than that of course, but not much: memory gets faster but usually support for slower versions is maintained for many years. There's a reasonable chance of being able to move memory from one system to another if you get a new system within a few years.

Warranty Issues: Warranty is provided almost exclusively by the vendor of the memory. Warranty periods vary from as little as 30 days to "lifetime". In fact, many vendors have these "lifetime" warranties, which are as much a testament to the relative scarceness of memory failures over time as anything else.

Driver Support Issues: Not applicable.

Special Specification Considerations: If you are not sure of the exact type of memory you need for a motherboard or your system, get help from the maker of the motherboard or system, or from a reputable dealer. Crucial Technology, the retail arm of Micron Technologies (one of the biggest memory makers in the world) has a memory selector on their web site that will help you determine exactly what memory you need for your system.

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