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Mixing DRAM Speeds

Mixing memory speeds refers to the use of DRAM of more than one speed in the same computer. For example, you might have bought a machine in 1994 that came with 70 ns DRAM (the fastest speed generally available then) and later upgraded with more memory in 1997 that was 60 ns. While it's generally preferable to avoid doing this, it can work without problems as long as certain caveats are followed:

• Use Identical Memory Within a Bank: PCs read a certain bit width of memory at a time, typically 32 or 64, and the memory making up this width is called a bank. PCs always read data from more than one chip at a time within a single bank of memory. If different memory is used within a bank, some of the bits may arrive later than others and all sorts of system problems can result. For this reason you should only use the same type and speed of memory within a bank. This also means using the same technology--never mix EDO and FPM memory (for example) within a bank.
• Put The Slowest Memory in the First Bank: Some memory systems automatically detect the speed of the memory being used, and set the system timing accordingly. They usually only look at the speed of the memory in the first bank when setting the timing. If you have 60 ns RAM in the first bank and 70 ns in the second bank, the system may set the timing at a rate that works fine for the 60 ns memory, but causes problems for the 70 ns. You should put the 70 ns memory in the first bank instead. (If your system doesn't do autodetection this won't be an issue but it is still good practice). Note that the first bank of memory is often called "Bank 0".

Some systems just generally have a hard time working with dissimilar banks of memory. I once tried to upgrade a system that had a pair of 8 MB, fast page mode 60 ns SIMMs, with another pair of 8 MB, fast page mode 60 ns SIMMs of another brand. The two pairs just would not work together in any configuration, even though they worked fine separately. See this section of the Troubleshooting Expert for more on RAM problems.

Next: Memory Bus Speed and Required DRAM Speed

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