What is RAM? - CodySnider.com

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What is RAM?

Your computer stores data in two places: long-term storage (including hard drives and CD-R/RWs) and short-term memory. (You'll also find memory chips on components such as video adapters, but we'll save that discussion for another day.) Storage devices, such as hard drives, maintain data even when they're turned off; memory, on the other hand, holds onto its contents only when the the computer is on and functioning. Turn off your computer, and the data in memory vanishes. Temporary memory is referred to as RAM, or random-access memory.

What is RAM?
Every time you start your operating system, launch a program, or open a file, the relevant program code and/or data is loaded into RAM. That's why you see RAM listed in the system requirements for the software you buy. If you have more RAM, you can open more files or programs at once and load bigger files or programs onto your system.

RAM capacity is rated in terms of megabytes (MB), and a typical computer will ship with 64MB to 128MB installed. However, you can increase that total to 256MB, 512MB, or even a whopping 1GB, if your system supports it.

Memory comes in a variety of packages, pin configurations, types, and formats. Do you need SIMMs, DIMMs, or RIMMs? Do you need parity or ECC? Confused yet? If terms such as PC100 SDRAM Non-ECC make your eyes glaze over, we're here to help.

There are two basic types of RAM available today: SDRAM (synchronous dynamic RAM) and RDRAM (Rambus dynamic RAM). And there are three incompatible forms of SDRAM memory modules currently in widespread use. The form you can use depends on the RAM sockets installed on your motherboard. Most systems available on the market today take SDRAM in 168-pin DIMMs (dual in-line memory modules). Older Pentiums and later 486 PCs use FPM and EDO RAM in 72-pin SIMMs (single in-line memory modules).

RAM modules are usually a little more than an inch high and four to six inches wide; look for a row of slots projecting from the motherboard with at least one module plugged in. The number of pins refers to how many gold (or tin) contact strips are on the edge connector side of the SIMM or DIMM.

You can buy DIMMs that hold up to 256MB or more, but note that not all motherboards support all module sizes. To determine the limitations of your system, check your motherboard or PC manual. On the expensive side are 30-pin SIMMs, which are rarely produced anymore. If you're thinking of getting 30-pin SIMMs to speed up an older machine, shop around first. In the long run, it may be cheaper to purchase a whole new system or to upgrade the motherboard.

Depending on the type of motherboard you have, memory modules can also be of a certain type, speed, or, in some cases, a metal lead type.

The types of RAM used in PCs over the past few years include (from fastest to slowest): SDRAM (used in DIMMs only), EDO (extended data-out), old-fashioned FPM (fast-page mode), DRAM (found in SIMMs only), and DRAM (used in both DIMMs and SIMMs). Most motherboards require that you use only one of these four types. Some motherboards can use more than one type but don't allow you to mix them. Consult your manual to find out which type of RAM you can use and whether you can mix them.

SDRAM speed is measured in megahertz (MHz). Your SDRAM must match the speed of your system bus, or it won't perform as well as it's rated. Most new computers use PC100 or PC133 SDRAM. In other words, the RAM communicates with the CPU via a 100MHz or 133MHz bus. PC66 RAM was once de rigueur, but Intel popularized the PC100 standard with its 400BX motherboard, which supports Pentium II and Pentium III CPUs.

Older DRAM (EDO and FPM) speeds are measured in nanoseconds (ns). The smaller the number, the faster the RAM. You can install DRAM with a faster speed rating than your system requires, but it won't improve performance. You cannot, however, install slower RAM or mix EDO and FPM DRAM in the same system and expect it to work.

SIMM socket contacts, or leads, are plated with tin or gold. Most manufacturers will insist that you buy SIMM modules that are made out of the same metal as your motherboard's memory connectors' or else run the risk of oxidizing the module contacts over time. Manufacturers of DIMMs addressed this issue by making all contacts out of gold.

Until a few years ago, virtually all PCs depended on parity RAM, a primitive form of error checking. Basically, for every eight bits of data stored, an extra parity bit was added and used in special calculations to ensure that the data was good. In the early days of the Pentium era, when memory was hard to come by and prices were high, many PC manufacturers switched to nonparity RAM. Most systems you'll encounter don't support parity at all; on others, it's an option at setup. If you're upgrading an older machine, make sure you don't mix parity and nonparity SIMMs.

Error correction code (ECC) RAM not only checks for errors but can even correct some of them along the way. ECC RAM is more expensive than other types of RAM, and it's typically found only in machines, such as file servers, that are used for mission-critical applications. ECC and non-ECC RAM cannot be mixed.

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