Computer Memory Upgrade Install Guide

Frequently Asked Questions (FAQ's)

Can I mix memory modules from different manufacturers? Yes.

What is the difference between SIMM and DIMM? They are different form factors (physical construction) for the memory module. SIMM stands for Single In-Line Module. DIMM stands for Dual In-Line Module. Most computers use either a SIMM or DIMM form factor. SIMM is an older technology. DIMM's have been the prevalent technology since around 1997. Physically they look similar. One difference is that SIMM's are installed in pairs, whereas DIMM's are installed singly. Another is that they are "keyed" differently - their notch positions along the connector edge are in different locations. They also install differently. SIMM's are first inserted into the memory slot at a slight angle and then rotated into a straight-up position. DIMM's are inserted straight-down into the memory slot.

Do I need to use all of the memory slots? Not with SDRAM or DDR, although the slots must be used in order. With Rambus, all of the slots must be used, but you can get continuity modules that fill Rambus slots without adding memory.

Can I mix memory modules of different speeds, such as:
   PC100 and PC133 memory
   PC1600 and PC2100 memory
   CAS 2 and CAS 3 memory
Yes, in all these cases. Sometimes it's cheaper to get the higher speed memory even if your computer does not support it, such as purchasing PC133 memory for a computer with a Front Side Bus speed of 100MHz. The memory just won't run at the higher speed.

Can I mix memory modules of different size? Yes, you can mix and match memory modules of different megabyte capacity any way you like.

Can I use newer memory modules with older memory modules? In terms of compatibility, yes. Most of the time there will be at least one empty memory slot you can use to just add new memory. However, it may be necessary to "swap out" an older memory module to reach the amount of memory you desire. While all SDRAM memory modules (or DDR memory modules) look pretty much alike, they can provide different amounts of memory. So let's say you have a computer with three DIMM's and all three memory slots are filled with 64MB SDRAM for a total system memory of 192MB. One way to get to 256MB total system memory is to remove one of the 64MB SDRAM memory modules and replace it with a 128MB SDRAM memory module. The swapped out memory module is ready for eBay!

How many memory slots do I have in my computer? You'll have to look and see or check your documentation to be sure. Many computers have three. But it's not unlikely it could have two or four.

What is PC100, PC133, PC1600, PC2100, PC2700, and PC3200 memory? PC100 and PC133 are types of SDRAM. PC1600, PC2100, PC2700 and PC3200 are types of DDR. They are all DIMM's in physical construction. The type of memory, SDRAM or DDR, that can be used in a computer is a characteristic of the motherboard. Most motherboards support either SDRAM or DDR, but not both. However, there are some motherboards that allow either, but not both at the same time. SDRAM and DDR use different types of memory slots for installation. The numbers identify the FSB speed of the RAM.

• PC100 memory runs at a FSB speed of 100MHz
• PC133 memory runs at a FSB speed of 133MHz
  
  For DDR,
• PC1600 memory runs at a FSB speed of 200MHz
• PC2100 memory runs at a FSB speed of 266MHz
• PC2700 memory runs at a FSB speed of 333MHz
• PC3200 memory runs at a FSB speed of 400MHz
  
  What is the front side bus? The front side bus (FSB) is the connection between the processor and memory. It's speed is expressed in MHz. The higher the MHz, the faster the FSB. The maximum speed of the FSB is a characteristic of the motherboard.
  
  For SDRAM
• 100 FSB processor should be used with PC100
• 133 FSB processor should be used with PC133
  
  For DDR,
• 200 FSB processor should be used with PC1600 RAM
• 266 FSB processor should be used with PC2100 RAM
• 333 FSB processor should be used with PC2700 RAM
• 400 FSB processor should be used with PC3200 RAM
  
  The newest DDR memory is PC3200. You should be aware that the JDEC standard for PC3200 RAM has not been finalized, so not all PC3200 memory will work with all motherboards and processors that support 400 FSB. Using the Memory Selector I provide below will take you to Crucial memory guaranteed to work with a particular motherboard or computer.
  
  Does the processor FSB rating need to match the speed of the RAM? No. It's quite common to use a processor and memory with different FSB ratings depending on the cost of the components. For example, PC2700 memory works just fine with a 266 FSB processor. It will not be getting the most out of the PC2700 memory by running it with processor that has a slower FSB, but it will be at least as fast as the PC2100 memory. And if the cost of the PC2700 is about the same as the PC2100 memory then you may as well get the faster memory. That way if you upgrade the processor someday to a 333 FSB processor then you'll get an extra performance boost by getting more out of the memory.
  
  Is CAS2 (or CAS3) memory the same thing as CL2 (or CL3) memory? Yes. For example, saying "CAS2" is the same as saying "CL2". It's just two ways of saying the same thing.
  
  What is CAS Latency? CAS is a mnemonic for Column Access Strobe, but it's not much help to know that, is it? The number, such as CAS2, refers to the number of clock cycles required by the memory. The more cycles required, the slower the memory. So CAS2 memory is faster than CAS3 memory. The performance difference shows up more with the faster bus speeds and faster processors.
  
  What's the cost-performance consideration? It's a safe generalization to say you should get the fastest memory your computer supports. For example, if your computer supports PC133 SDRAM then purchase PC133 CAS2 SDRAM. Memory tends to be one of the more troublesome performance bottlenecks in the computer, so the small extra cost for the faster memory is worthwhile. The cost difference is so narrow that it can't be automatically assumed that the slower memory is cheaper!
  
  How much faster is one memory over another? In raw terms, PC133 SDRAM memory is 33% faster than PC100 SDRAM memory. A similar statement can be made for PC2100 DDR memory and PC1600 DDR memory. However, the overall system performance you actually observe will more likely be in the range of 5% to 10%. That's because the overall system performance is affected by other bottlenecks at the hard drive, video card and other peripherals. And computer operations that occur entirely within the processor are unaffected by the speed of the FSB. CAS2 yields a similar improvement range over CAS3. In theory, PC2100 DDR memory is twice as fast as PC133 SDRAM memory, but in practice the SDRAM motherboards make double-use of the 133MHz FSB speed so the actual overall system performance improvement given by PC2100 DDR over PC133 SDRAM is again in the single-digit percentages. Nevertheless, with computer memory being such a troublesome bottleneck on system performance and with the cost difference as low as it is, it's best to go with the faster memory.
  
  What are the differences between unbuffered, buffered and registered memory? Home computers should use unbuffered memory modules that do not have a register. That's good because this type of memory is faster and less expensive. Buffered modules contain a buffer to help the chipset deal with the type of electrical load required when the system has a large amount of memory. Registered modules are unbuffered modules that contain a register that helps ensure data is handled properly. Buffered and registered modules are typically used only in mission-critical systems, such as servers.
  
  What do the memory specifications such as 16x64 or 16x72 mean? This describes the memory format. The important thing to note is the second number. If it is divisible by 9 evenly, such as 72, then it is an ECC memory module. Otherwise it is a non-parity memory module.
  
  So what do I need to know about non-parity memory modules, parity memory modules and ECC memory modules? Not much, really. ECC stands for "error correcting code". ECC can actually correct memory errors - if they are not too severe! Parity memory modules can at least detect that there is a memory error. But such rigorous "self checking" capabilities are not needed for home computers and most do not support it. Non-parity memory is the norm for home computer use. Non-parity memory modules, parity memory modules and ECC memory modules cannot be mixed in the same computer.
  
  Is there a limit to how memory I can add? There are limits. The number of memory slots is one constraint. Windows 95, Windows 98 and Windows 98SE users should stick to no more than 512MB. Windows ME and Windows XP support up to 4GB. There may be other limiting factors imposed by your computer or motherboard. If you use the Crucial Memory Selector, however, it will inform you of your system limit on the same page that lists the compatible memory modules.
  
  How much memory should I have? It's safe to say you will see a performance improvement in at least one way or another by having at least a total of 256MB, if your computer supports that much. For running today's computer software and operating systems, it's entirely reasonable to equip your computer with up to 512MB of memory. I use my own computers for a wide variety of tasks and I equip them with at least 512MB as a standard practice. Few home computers make effective use of more than 512MB of memory, but many people go with more anyway for the satisfaction of "maxing out" the computer or the desire to utilize all the memory slots.
  
  What's the maximum amount of memory my computer supports? There's no standard amount so different models of computers have different maximums.
• Bring me back to memory purchase page.