Understanding Memory
April 9, 2002
By Stephen J. Bigelow
Random access memory (RAM) plays a critical role in your PC. It holds the operating system, the running applications, and the open data files for your applications. When you launch your favorite video editor, 3-D shooter, or other software, the program instructions and data load into RAM, and the CPU fetches those instructions and data from the RAM. Even your Windows desktop occupies a certain amount of RAM. A PC with more RAM will run more programs simultaneously, handle larger data files (such as those with complex graphic images), and generally exhibit better system performance. But how much memory do you need, and what type? We'll show you how to estimate your memory requirements, and we'll discuss the various types of RAM available today.
Adding It Up
Should you live with a minimum amount of RAM and save a few dollars, or should you spend some extra cash and load up the system? Just how much memory is enough? The answer really depends on what you're doing with the PC; RAM requirements can vary from system to system. Let's do the math.
Windows XP is rated to run with a minimum of 128MB, but it gets a real speed boost from 256MB. Windows 2000 puts its baseline at 64MB but will benefit from 128MB or more and as a server will see optimal performance with 512MB or more. Windows 98 and Me will run with 32MB, but you'll need at least 64MB for decent performance and 128MB or more to shine. Linux users will need at least 48MB to 64MB (depending on the version) but will certainly benefit from 128MB. For a Linux server, you could easily use 512MB or more.
Of course, the OS is useless without applications, so you'll also need enough memory to load all the programs you want to run at any one time. Applications like Microsoft Word or Excel generally use about 32MB each, but you can check the system requirements listed on each application's box to determine the recommended RAM. If you want to run three applications simultaneously (say, Word, Internet Explorer, and Media Player), you'll need to add the RAM requirements for all three to your total. Finally, you'll need RAM for data files—the open Word documents, Excel spreadsheets, MPEGs , and so on. More complex work demands more RAM. For example, a simple document may require less than 1MB, but a database file may fill several megabytes, and a high-resolution photo scan or graphic design may demand 50MB or more.
As a rule of thumb, if you base your overall memory requirements on the amount recommended for your operating system, you'll be in the ball park. As noted, you'll have to add more if you're processing large, complex files. Remember that Windows also uses virtual memory in the form of a swap file on your hard drive. If you don't want to buy the full amount of RAM that you expect to need, that's okay; your PC will make use of virtual memory to make up any difference. But be aware that this will result in a performance hit because of the hard drive access.
Time for More?
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So how do you know when it's time for more RAM? Windows 9x makes checking the available memory easy using System Monitor (Figure 1). This is a versatile reporting tool that can keep you informed of many different conditions in your PC, including unused physical memory. Start the system normally, then load any applications and data you use regularly to simulate normal use. Click on Start | Programs | Accessories | System Tools | System Monitor. Once System Monitor starts, click the Add button, select the Memory Manager category, then select the Unused Physical Memory item and click on OK. Memory use will be tracked as a graph over time. If System Monitor reports less than 10 percent unused memory, it's time to consider an upgrade. For example, if you have 128MB on the PC and System Monitor reports less than 12MB of RAM when the system is being used in a normal fashion, think about a memory purchase.
To check memory usage in the Windows 2000 and Windows XP families, press Ctrl-Alt-Del, choose Task Manager, and click the Performance tab. Here you see the current memory usage, as well as a graph showing memory usage history. Click the Processes tab and you can see the memory usage for each process. The Select Columns item in the View menu lets you choose to track other types of information—basically the same sort of things shown in the Windows 9x System Monitor.
In general, with regard to memory, more is better. Before you upgrade, though, you'll want to understand the various types of memory.
Ram Basics
Today, there are three varieties of memory in common use: SDRAM (synchronous dynamic RAM), Direct RDRAM (Rambus dynamic RAM), and DDR (double data rate) SDRAM. All three serve exactly the same role in a computer, but their internal designs are different, and memory module layouts for SDRAM and RDRAM are incompatible.
SDRAM was first used in 1996 with support for a PC processor bus (also called front-side bus or FSB) speed of 66 MHz, but by 1998 it had advanced to support the 100-MHz FSB. SDRAM certified to run at 100 MHz was dubbed PC100 SDRAM, and it is available in 168-pin dual in-line memory modules (DIMMs). Since each DIMM offers a 64-bit data bus, the peak bandwidth for SDRAM is 800 MBps (8 bytes x 100 MHz). This means a PC100 SDRAM DIMM can ideally pass up to 800 MBps between the DIMM and motherboard (though speeds rarely get this high in actual practice). By 1999, SDRAM was available to support a 133-MHz FSB speed, and this was termed PC133 SDRAM. With the same 64-bit data bus on a DIMM, PC133 SDRAM DIMM can theoretically handle up to 1.1 GBps (8 bytes x 133 MHz).
Rambus introduced its memory technology in 1995. Rather than using the existing processor bus, as other memory technologies do, Direct RDRAM employs a small 16-bit data bus with a dedicated high-speed (300-MHz) clock. Since Rambus handles two operations per clock cycle, this effectively doubles the clock speed to 600 MHz. Rambus also supports dual channels, effectively doubling the data bus to 32 bits and providing a peak bandwidth of 2.4 GBps (300 MHz x 2 operations per clock x 16 bits x 2 channels). Rambus modules rated for 300-MHz operation are termed PC600 RIMMs (the 300-MHz clock x 2). By 1998, Rambus had advanced to dual 400-MHz channels, so the effective clock rate became 800 MHz (400 MHz x 2) with a 16-bit data bus, yielding a peak bandwidth of 1.6 GBps. This doubles to 3.2 GBps when you add a second channel. 184-pin Rambus modules rated for 800-MHz operation are called PC800 RIMMs. Today, Rambus is advancing to 1,066 MHz (PC1066) and 1,200 MHz (PC1200) for even higher performance.
DDR SDRAM. The issue with SDRAM is that each data line passes only one bit per clock cycle (resulting, for a 64-bit memory device, in 64 bits per clock). To compete more closely with RDRAM, SDRAM creators developed memory that would perform two operations per clock cycle. This memory is called double data rate or DDR SDRAM. For a 100-MHz FSB, DDR SDRAM provides twice the bandwidth (8 bytes x 100 MHz x 2) or 1.6 GBps. For a 133-MHz FSB, DDR SDRAM can reach a peak bandwidth of 2.1 GBps (8 bytes x 133 MHz x 2). With a 166-MHz FSB, DDR SDRAM can offer a peak bandwidth of 2.7 GBps (8 bytes x 166 MHz x 2). These are called DDR200, DDR266, and DDR333 for 100-MHz, 133-MHz, and 166-MHz speeds, respectively. But memory makers often name the modules based on their bandwidth, such as PC1600 (1.6 GBps), PC2100 (2.1 GBps), and PC2700 (2.7 GBps). A DDR SDRAM module uses 184 pins (like a RIMM). Since DDR SDRAM builds on well-established SDRAM technology, it's often cheaper than Direct Rambus modules.
The important thing to remember is that you cannot mix SDRAM, Rambus, and DDR SDRAM on the same motherboard. When you're adding memory, select the type specifically intended for your motherboard. SDRAM modules each have 168 pins. Rambus and DDR SDRAM modules have 184 pins. Although they are all similar in size, the difference in pin count and keying prevents you from using them in the wrong slots. You'll also need to check whether the memory in your system uses error-checking techniques such as parity and ECC (error correction code) and whether it's buffered or unbuffered.
Memory is one of the most popular PC upgrades. It's relatively inexpensive and easy to do, yet it can improve performance and allow the system to support more (and more sophisticated) applications. When planning a memory upgrade, you need to evaluate the amount of RAM you need and understand the memory types and characteristics that are appropriate for your system. For tips that will ease the installation process, see "Adding More Memory".
Stephen J. Bigelow is the author of Troubleshooting, Maintaining, and Repairing Personal Computers (Fifth Edition). |
