Computer memory

What is it?

"Memory" is the shorthand term commonly used to refer to the high-speed microchip-based storage of a computing device.  Memory can be distinguished from the much larger volume, but much slower speed, storage provided by magnetic hard disks and floppies, or from optical storage like CDs and DVDs.

Normally any part of memory can be read from or written to directly, hence the name random-access memory or, in abbreviation, RAM.  

Data is stored in memory when it is being directly acted on by the computer's processor (CPU) -- and for this reason is sometimes referred to as "primary" storage.  Data is read into or written out from the RAM to the computer's "secondary" storage -- most commonly to and from the hard drive.

Computer RAM is typically "volatile," meaning it retains data only as long as your computer is on.  By contrast, the micro-chip memories used in flash drives and removable card memories (CF, MMC, SD, et al) are non-volatile, requiring no power source to retain data.

How much is enough?

A computer's performance is strongly affected by the power (speed) of its processor, the amount of RAM, the speed of its hard drive or other secondary storage, and the communications channels among those elements.  As in other areas of life, a computer is only as strong (or as fast) as its weakest link.

In general, the amount of RAM in a PC determines how many applications (programs) can be active simultaneously and how much data can be readily available to each.  The faster you want each individual application to perform, and the more applications you want to toggle among at one time, the more RAM you need.

Each new generation of computer operating system and applications software tends to require more memory.  Fortunately, the price of memory has fallen dramatically with each new generation of microchip.

We currently recommend at least 512 megabytes (MB) of RAM for a PC running Windows XP.  Additional memory -- 1024 MB or more -- is recommended for applications like graphics, statistics, databases, or if you tend to have many applications running at the same time.

We currently recommend at least 1024 MB for Apple/Mac PCs.

What kind should I select?

All memory is not the same.  In fact, there are many varieties, labelled with bewildering abbreviations like SDRAM, DDR and DDR2, ECC and non-ECC.    Fortunately, your choices for a particular computer are much more limited, because memory must be matched to the other hardware, notably the CPU.

When you purchase a new computer, the manufacturer will limit the memory choices to those compatible with the CPU.  If you add memory to your computer after purchase, be sure to correctly specify the make and model so that you get compatible RAM.

You'll notice, for example, that memory descriptions usually have a "speed" in MHz (millions of cycles per second).  A computer's CPU is made to handle a specific memory speed.  Even if you were to add a faster speed memory, it would only run at the speed allowed by the CPU.

One choice you will often have is in the number of "DIMMs" for a given amount of memory.  DIMM is an abbreviation for "dual in-line memory module" -- the term for the small circuit board assembly to which the actual memory chips are attached.  

Picking the smaller number -- one instead of two, two instead of four -- means that more memory capacity is packed onto a single assembly.   Why is that good?  Computers have a limited number of "slots" into which DIMMs can be plugged, so it leaves open more memory slots for future expansion.  Whether that benefit is worth the extra price depends on how likely you are to expand the memory on that computer in the future.

Most PCs use what's called "non-parity" memory and do not need error correction code (ECC) or "parity" memory.  ECC memory can check for errors in data and are most often found in servers and other mission-critical applications used by large networks and businesses.

ROM, PROM, EPROM, RAM, DRAM, SDRAM, etc.

You can usually get through a computer purchase without learning the meaning of all these abbreviations.  But if you're curious, here are some of the more common ones:

RAM is distinguished from read-only memory, or ROM.  ROM can be read randomly too, just not written to.  ROM memory is non-volatile; it may or may not be changeable once initially configured.  ROM that is erasable and (re)programmable carries abbreviations like PROM, EPROM and EEPROM.

Each "cell" of memory is actually a microscopic capacitor, capable of storing an electric charge.  If the capacitor is charged, the memory's value is "1", and if un-charged it is "0".   As noted, RAM memory is volatile and so must be continuously refreshed in order for the "1" cells to retain their charge.   This is called "dynamic" refresh, and so the memory is referred to as "dynamic RAM" or DRAM.

Memory reading and writing must be synchronized with the CPU, hence "synchronous DRAM" or SDRAM.   Common types of memory technology today include DDR2 (double data rate, second generation) SDRAM and DDR (double data rate, first generation) SDRAM.  Many older systems use SD (single data rate) SDRAM, fast page mode (FM), or extended data out (REDO).

For more details, consult one of the references below.

Learn more

How computer memory works (How Stuff Works)
Good basic information, with links to information on memory types.

Random access memory (Wickipedia)
How the varieties work, with links to technical detail on the various branches of the memory family