It's important to understand the difference between RAM memory, which is the computer's workspace while it is powered on, and system's storage memory.  When the computer is shut off, even for an instant, the RAM memory is completely erased.  Anything that needs to be saved while the computer is turned off (programs, data, etc.) must be SAVED TO DISK to be stored while the computer is without power.  This is the function of the hard drive.  Hard drives will be discussed further in the next section.

The amount of RAM memory in a computer dramatically affects its performance.  Modern Windows programs require a large amount of memory to perform quickly and effectively.  A computer with a fast CPU and little RAM memory will perform sluggishly, especially when dealing with large files and multiple open programs.

RAM memory is measured in the basic unit of the "Byte".  A byte is 1 character of information (a letter, number, symbol, etc.).  A "Kilobyte" is approximately 1000 bytes (1024 bytes), and a "Megabyte" is approximately 1 million bytes (1024 X 1024, or 1048576 bytes). 

The original IBM XT computers were typically installed with 64k-640k byte of memory (64,000 - 640,000 bytes - that's LESS than a single megabyte!).  286's typically had 1-2M installed (1-2 Megs or megabytes = 1,000,000 to 2,000,000 bytes),  386's usually had 2-8M, 486's had 2-64M (8-16 Megs was standard).  Most "Pentium Class" (i.e., "modern") machines have 16-512Megs installed (32-128M is a standard configuration at the time of this writing). 

The most common ram chips are called "Dimms" or "SDRAM".  SDRAM chips are approximately 5.25" long, they have two notches placed at asymmetrical locations on the chip (so that they can't  be put into the motherboard backwards).  They are lined by 168 metal connectors along the edge which connect to the memory socket on the motherboard.  SDRAM chips are placed straight down into the MB's memory socket, and are snapped into place by two clips at each end of the chip.

PHOTOS:  Dimm, SDRAMs, SDRAM, memory chips, Memory - Simms and Dimms

VIDEOS:  memory chip comparison, SDRAM install

Adding SDRAM is very easy:  Turn the computer off, find an empty memory slot on the motherboard, and push the chip straight down into the socket until it clicks into place.  Make sure you align the chip correctly - it can only go in one way, don't force it in.  A firm press will lock it into place.  Adding memory to a computer is one of the simplest, quickest, and least expensive ways to improve performance.

SDRAM currently comes in 3 speed varieties:  66Mhz, 100Mhz, and 133Mhz.  When adding memory to a machine, you will need to know the "bus frequency" of the computer's CPU/motherboard.  The CPU's bus frequency will match one of the SDRAM speed ratings. (66, 100, 133)  The fastest new CPUs require the fastest new memory chips (i.e., 133Mhz is used in new machines).  Older CPUs may be fitted with 66 or 100 MHz memory chips. 

New memory is typically "backwards compatible" with old systems, which means that the new, fast SDRAM will usually work in an old system, but the older, slower memory will NOT work with a new CPU (the machine will freeze, act inconsistently, or not work at all).  If you are not sure of a computer's bus frequency, simply make note of the CPU speed and type, which is typically shown on the screen just as the computer starts up.  Your vendor will be able to tell you which type of memory you need to install.

PHOTO:  CPU indentification at startup

VIDEO:  Computer startup, and entering the BIOS (watch the screen BEFORE the BIOS is entered - MORE ON THE BIOS IN LATER SECTIONS).

Older types of memory chips include "SIMMs" and individual memory modules. The original 8086's and some 286 machines used single memory chips that held only 16-256k of memory and plugged directly into pin pads on the motherboard (NOT a convenient way to add memory - these fragile memory "bugs" usually ended up with broken pins when added or removed by novice users). 

PHOTO:  memory bug (NOTE: this type of memory is no longer used in modern computers)

VIDEO:  memory bug

SIMMs combined several smaller memory modules onto a chip which looked similar to a modern SDRAM chip.  The original (now obsolete) SIMM chips used in 386 and 486 machines had 30 pin connectors, usually holding between 256k and 4 Megs of memory (tiny by today's standards).  A newer type of SIMM with 72 pins can be found in many of the Pentium class machines still in use today.  SIMMs snap diagonally into place on a motherboard.

PHOTO:  memory chips, Memory - Simms and Dimms (older obsolete 30 pin SIMMs, 72 pin SIMMS, along with some SDRAM for comparison)

VIDEOS:  Simm install (inserting 72 pin SIMMs), 4 memory chips installed in 486, removing 4 SIMMs from 486 MB

There are typically between 2-4 72 pin sockets on the motherboards which take that type of memory.  Some SIMMs need to be installed in pairs of two, called "parity".  Others can be installed individually, called "non-parity".  The most common types of 72 pin SIMMs are "EDO" and "fast page".  In general, different types of SIMMs should not be mixed within a machine.  SIMM memory within a machine should be matched not only for type, but also for speed:  When upgrading a machine with SIMM memory, you should find the speed marking of the currently installed chips, typically denoted by a "-6" or "-7", and buy chips which are the same speed.  (These numbers refer to the access speed of the chips, measured in nanoseconds:  usually either 60 or 70 nanoseconds).

VIDEO:  memory nanosecond cut (speed rating on 72pin SIMMs)

The newest types of memory chips and sockets are called "DDR" and "RAMBUS".  They offer very fast bus speeds (266 and 800 MHz respectively, compared to the older 66, 100, and 133) which can be used by only the newest processors to dramatically improve the performance of a machine.  The newest AMD Athlon CPU's for example, use a 266 MHz bus speed motherboard, and will perform significantly faster when fitted with 266 MHz DDR RAM.

The process of figuring out what type of memory to use when upgrading or repairing a machine can be daunting to the beginner.  If you are not sure what type to get for your machine, simply remove and bring in the chips that you are currently using - your hardware vendor will be able to match them for you.  If you are building a new machine, tell your vendor the type of CPU and motherboard you will use, and that will provide the information needed for choosing appropriate RAM chips.

Memory prices:  You will notice that different types of RAM vary widely in price.  Standard SDRAM chips used in most modern machines should cost between $.25 and $.50 per Meg (ie. $49 for a 128 Meg chip is a reasonable price) at the time of this writing.  All other types are more expensive.  SIMMS and older varieties are typically more expensive because they are no longer manufactured in great quantities, and newer varieties such as DDR are more expensive because they offer performance improvements, and like all new technology, they cost more to manufacture.  Be careful not to spend too much money on old memory when upgrading an old machine.  Adding memory will only produce a limited improvement in system performance, and the price of older memory may make it more cost effective to buy a new motherboard and CPU which supports newer, faster, and cheaper memory.  Performing such an upgrade will much more dramatically improve the overall system speed and performance.