Issues to consider before buying RAM:

How many memory sockets are open? If there are none open, you will have to replace your current RAM with higher capacity ones.

What type of memory sockets are open? SIMMs? DIMMs? The form factor is important for choosing the right RAM. Also, putting SIMMs into a board that is currently using DIMMs may result in speed compromises.

What kind of memory are you currently using? SDRAM does not mix with EDO DRAM, and few other types don't mix at all.

How much money are you willing to spend? SIMMs/DIMMs come in several speed grades, each at different price levels. It may be more cost effective to buy one large chip instead of several small ones, or vice versa.

Determine whether you have Tin or Gold sockets.

Determine if you need to buy your RAM in pairs.

What maximum amounts of RAM/Cache does your motherboard support?

Memory Pins: Tin versus Gold

Some people talk about the danger of tin and gold combining. What happens is that when the two metals (found on the connecting pins on both the motherboard and RAM) come into pressure contact with each other, the tin begins to pass particles over to the gold, where it oxidizes (rust). In several years, enough tin will transfer to affect the memory slot, and subsequently the rust will damage the memory (or vice versa for gold slots!)

The problem is only solved by using SIMMs or DIMMs with the same metal as their motherboard uses. For most users though, by the time the problem may have full effect, the computer is already upgraded or replaced.

Buying RAM in pairs - An issue of bus width

It was generally well publicized in stores after the Socket 7 Pentium was introduced that 72-pin SIMMs would need to be installed in pairs. But why is this? 486 users had no such restrictions. The answer is found by looking at bus width sizes.

The Intel 486 uses a single 32-bit wide memory path to communicate with RAM. Because 72-pin SIMMs are 32-bit, the fit was perfect. In order to boost memory bandwidth to accommodate a new level of CPU, Intel decided on twin 32-bit pathways starting with the Pentium, hence requiring twin SIMMs.

If one pathway is unconnected, then the system will fail to boot up, as data will attempt to pass to an empty memory socket. With Slot 1/Socket 7 solutions, this is not a problem for the DIMM, which in fact has a 64-bit wide path, and can handle the twin pathways. They face the same problem, however, in systems using the DEC Alpha CPU, which uses a 128-bit wide path.

Experimentation has shown that the paired SIMMs or DIMMs need not be of the exact same name brand (most of the time) as long as the speeds and type are the same.

Cache RAM Issues

Motherboard chipsets not only control the maximum amount of RAM you can insert, but the cache area as well. The Cache area is the first X megs of RAM that the L2 cache can retrieve information from for relaying to the CPU. If there is more RAM on board than the maximum cache area can handle, all memory after the limit must be accessed directly by the CPU, which can involve performance hits depending on application.

Generally, you do NOT want to exceed the limit, but in some cases it is warranted because the performance loss is much less than the performance loss due to overuse of the hard disk swap file. Also, the Pentium 2 Processor is not cache area limited - the maximum amount of RAM is also the maximum cacheable amount, as Intel uses the cartridge to decide limits.

* The 430HX only supports 512 MB with a specific Tag RAM. Otherwise, the chipset only supports 64 MB.

**The Aladdin IV can support higher amounts of memory only with certain Tag RAMs. For most boards there is a 64 MB limit.

*** The 440LX supports 512 MB of SDRAM, but can support up to 1024 MB of EDO DRAM.

Additional Informationg Sources:

Memory Store PC100 Website
An incredible site for finding out the differences between types of PC100 SDRAM. They also have a comprehensive letters section to answer specific questions and misconceptions.

Jeff's PC Hardware Review
His memory FAQ is still bare bones, but he has a good deal of information on misconceptions related to NS timing, and is all around knowledgeable (which you might expect from a Micron employee).

Tom's Hardware Memory Guide
A dated, but valuable reference for discovering how exactly memory works. Tom's guide clears up some misconceptions on SRAM along with an overview of major DRAM technologies.

AnandTech's September 98 RAM Comparison
Anand provides a slick, easy to read comparison of today's top PC100 SDRAMs and shows you which are the fastest.

Kingston Technologies Ultimate Memory Guide
A well laid out technical reference guide for common questions and ECC concerns.

TweakIt's Test of 430TX with 96 MB RAM
This page shows the benchmark results from exceeding the maximum cache area limit on a Pentium motherboard.

PCGuide's Video Memory Reference Guide
A quick guide for deciphering memory abbreviations on video RAM types.

Intel's DRAM Technology Evolution Document
A quick description of Intel's observations on the move to Rambus DRAMs.

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