What is memory?
Computer memory, also called RAM (Random Access Memory), or (or sometimes DRAM, which means Dynamic RAM), are the chips in your computer that the CPU (Central Processor Unit) uses to store information and programs while it’s working. You might think of RAM as a scratchpad where the computer stores information while it’s thinking. Memory is not your hard drive, although when Windows runs out of RAM, it stores overflow information on the hard drive temporarily. The main thing to remember about RAM is that information is maintained there only as long as your computer is turned on. When you turn off the computer, the information in RAM is gone. 

How much memory do I need? 
If you are using Windows 95 or 98, 64 MB is probably enough for most programs. For Windows NT or Windows 2000, 128 MB will usually suffice. If you are doing memory-intensive operations, like serious graphics, Video editing, games, voice recognition, or massive spreadsheets, you may benefit from even more memory. 

How much memory does my PC have?
Assuming you are running Windows 95, 98, or NT, right-click on the My Computer icon. Then click Properties. You should see the System Properties window, with a number at the bottom of the screen that shows how much RAM is installed in your computer. Figure 1 show the screen for a computer which has 16 MB of RAM. 

Will a RAM upgrade solve your problems?
Yes, up to a point; if you have a 486 computer, there really isn’t much reason to upgrade the memory — start saving for a faster machine instead. But if you have an older Pentium, Pentium II, or Pentium Pro, adding memory will probably make your computer noticeably faster. Will it be fast enough? That depends on your expectations. Additional RAM won’t work miracles, but may give you up to 20% overall speed increase. But 20 % of 100 MHz is just 20 MHZ, and while you would notice the speed increase, it would not be enough to matter on most applications. 

What does RAM look like?
RAM is actually small integrated circuit chips. These look like small black rectangular insects with two rows of small legs. The "legs" are really the contacts that go into a circuit board to connect the chip to the board. Fortunately, designers have found a way to spare us from using RAM chips directly, which requires a very steady hand, excellent eyesight, bright lighting, and the patience of Job. For computers, RAM is now packaged on small, longish circuit boards called inline memory modules, and sometimes referred to as "sticks." Memory modules come in several forms; the most common, and at present, the only two forms you can find at your local computer store, are single inline memory modules (SIMMs) and double inline memory modules (DIMMs). SIMMS usually have a single notch on the connector edge of the board, while DIMMs usually have two notches. DIMMs also have 168 connector contacts on the board, while SIMMs have either 30 or 72 (30-pin SIMMs are obsolete and difficult to find). DIMMs are about an inch longer than SIMMs. Figure 2 shows a DIMM above a 72-pin SIMM. Although this SIMM is taller than the DIMM, that’s not necessarily the norm; it just depends on how the manufacturer decided to build the DIMM. For comparison, the SIMM has 8 MB of RAM, while the DIMM has 64 MB of RAM. 

Early Pentium systems and 486 computers required matched pairs of SIMMs, so you must add two SIMMs to increase the memory. Later Pentiums and all current computers use (mostly) SDRAM packaged on DIMMs. Since DIMMs are really two pairs of SIMMs on a single board, they are already matched pairs; thus you only have to install a single DIMM for a memory increase. 

When adding RAM, be sure to add the same type of module already in your computer. 

Notebook computers usually use proprietary memory modules to conserve space. That’s one of the many reasons notebooks tend to be more expensive than desktop computers. 

Why is RAM speed-rated?
CPU chips come in a wide range of speeds. The fastest CPUs demand RAM that can keep up with them. Therefore, RAM has speed ratings. Until recently, those ratings were expressed in nanoseconds (ns), which is the shortest length of time in which a chip can respond to a request for information. Typical speed ratings were 60 or 70 ns. When the Pentium III appeared, and later the Celeron, memory speed was specified according to the speed of the memory bus, which, in computer terminology, means the traces on the motherboard that connect the memory to the CPU. A motherboard’s memory bus usually operates at 66 MHz, 100 MHz, or 133 MHz, with the higher numbers being faster. The 66 MHz motherboards are used on slower processors, like Celerons (but newer, faster Celerons will soon get 100 MHz buses).  Pentium IIIs running at up to around 600 MHz benefit from a faster bus, running at 100 MHz. So do the Athlon chips. Finally, the latest Pentium III chips can use an even faster bus, running at 133 MHz. 

RAM may be speed-rated to indicate what bus speed it can support. These speed ratings are expressed as PC66, PC100, and PC 133, corresponding to the bus speed they will work in. You can use a faster speed DIMM than the bus supports without problem, but using a slower speed DIMM than the bus supports will cause slowdowns and erratic behavior. For example, you can use a PC100 DIMM in a 66 MHz motherboard, but not in a 133 MHz motherboard. Using a DIMM faster than the motherboard’s rated bus speed will not make the computer faster; the motherboard controls the speed, not the RAM. 

What kinds of RAM are available? 
EDO (Extended Data Out) RAM was the most popular memory until a few years ago. It is packaged on SIMMs, which must be upgraded in pairs. EDO RAM supports bus speeds up to 66 MHz. But when faster bus speeds became necessary, a new type of RAM, Synchronous Dynamic RAM, or SDRAM, became the standard, and still is the most popular type of RAM. It is available in PC66, PC100, and PC133 versions. Most RAM sold today is SDRAM, packaged on DIMMs. 

Several newer types of RAM are beginning to appear. Rambus DRAM, or RDRAM, comes in versions with speed ratings up to 800 MHz; however, it is very expensive, and tests show that is may not really be that much faster than PC133 SDRAM. If you want to delve into the details, I recommend a website called Tom’s Hardware Guide. Most motherboards won’t support RDRAM, and it probably isn’t cost-effective for most of us. Still, if your motherboard comes with RDRAM installed, you will need to add RDRAM to expand the memory. RDRAM comes in unique DIMM packages, encased in metal shielding. 

What’s coming in the future?
A new type of RAM called DDR (Double Data Rate) SDRAM is really an advanced form of SDRAM which transfers data twice as fast as standard SDRAM. I have not seen it used as system RAM, but it’s probably just a matter of time. High-speed graphics cards are beginning to use DDR SDRAM. 

Is it hard to add RAM?
Probably not, but that depends somewhat on the design of your computer. You must remove the case, find the empty slots for RAM expansion, and then insert the appropriate type of memory (SIMMs or DIMM). SIMMs and DIMMs have different types of sockets, so you can’t inadvertently use the wrong type. Unfortunately, actual insertion of memory modules is beyond the  scope of this article. I recommend you visit an online memory installation guide, courtesy of Crucial Technology memory.