Computer memory has become incredibly cheap — the lowest cost it’s ever been. And it’s a good thing; Windows (all flavors: Me, 2000, soon to add Windows XP) loves all the memory you can supply. My Windows Me computer sped up noticeably when I increased its memory from 128 MB to 256 MB. Maybe this is the time to invest in a memory upgrade, if your computer is reasonably modern. 

What is memory?
Computer memory, also called RAM, or Random Access Memory (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 uses the hard drive to temporarily store overflow information. Information is maintained in RAM 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, or Me, 128 MB is probably the minimum acceptable memory, although the operating system will run with less. But if you launch several programs, your system will slow down as it writes information to the hard drive. For Windows NT or Windows 2000, 128 MB is not enough for serious use; 256 MB will see improvements. If you are doing memory-intensive operations, like serious graphics, video editing, games, voice recognition, or massive spreadsheets, you will benefit from even more memory.

How much memory does my computer have?
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 shows the screen for my computer, which currently has 256 MB of RAM.

Will a RAM upgrade solve your problems and make your old computer fast enough to run the latest software? 
Up to a point; however, 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 are SIMMs (Single Inline Memory Modules) and DIMMs (Double Inline Memory Modules). 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. The relatively new Pentium 4 computers use types of DIMMs called RIMM, but those are really DIMMs with Rambus memory chips aboard.

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.

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

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 (for SDRAM); PC1600 or PC2100 for DDR RAM. 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, SDRAM, or Synchronous Dynamic RAM, 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.

Rambus memory
The Pentium 4 chip requires a different type of RAM, called RAMBUS, or RDRAM. It is a very high speed type of RAM, befitting the high clock speeds of the Pentium 4. Rambus memory is able to achieve effective data transfer speeds of 800MHz and higher. All memory slots in a Rambus motherboard must be filled. Even if all the memory is contained in a single module, the “unused” sockets must be populated with a PCB, known as a continuity module, to complete the circuit. Rambus DRAM modules are known as RIMM modules (Rambus Inline Memory Modules). They are distinguished by a metal shield called a heat spreader cover plate, which acts as both a heat sink and physical protection for the chips on the board. 

DDR memory
The newest type of memory is called DDR, or Double Data Rate RAM. It’s currently used on some computers equipped with faster Athlon processors. DDR is cheaper than RDRAM, but slightly more expensive than SDRAM. It is nearly as fast as RDRAM, but much cheaper. But your motherboard must be designed to accommodate DDR memory. Since DDR RAM is nearly as fast as Rambus memory, it’s just a matter of time until we see motherboards that use DDR with Pentium 4 processors. Figure 4 shows a DDR DIMM.

DDR memory comes in two speed ratings: PC1600 DDR is designed for a motherboards and processors with a 200MHz front side bus, while PC2100 DDR is designed for motherboards and processors with a 266MHz front side bus.

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 Crucial Technology memory for an online memory installation guide. Local stores often feature free installation of RAM boards, so watch those ads closely. Even if you have to pay someone to install RAM, it should be fairly inexpensive, since it’s a 15-minute job.

How much should I pay?
There is no substitute for shopping. I use one of our advertisers, Amenity Computers as my benchmark. They are always very competitive and also very close to my home, so a visit there is a short trip for me. I would recommend a call or visit to our other advertisers who may be closer to your residence or office.