Use the following guidelines when choosing SIMM memory modules:

• Unless you are upgrading an older Socket 7 system that accepts only 72-pin SIMMs, buy only 168-pin DIMM memory. Many Socket 7 motherboards have both SIMM and DIMM sockets. Buying DIMMs for such a board makes more sense than buying obsolete SIMMs. However, note that some early boards support DIMMs only with very slow memory timings.

• SIMMs are available in FPM and EDO. If your motherboard supports either type, buy EDO (assuming you can't scrounge it somewhere). Some motherboards support both FPM and EDO, but do not allow mixing types, either per bank or globally. Because FPM is obsolescent and demand for it is small, FPM often costs more than EDO. If FPM is installed and the motherboard does not allow mixing FPM with EDO, consider removing FPM SIMMs and installing all EDO SIMMs.

• Don't assume the motherboard supports large SIMMs. Check the manufacturer's web site to determine the supported SIMM capacities and memory configurations. Older motherboards often support nothing larger than 16 MB SIMMs. Conversely, don't assume the motherboard cannot support larger SIMMs than those currently installed just because the manual says so. For example, one of our technical reviewers from an earlier edition has a Compaq system whose manual lists 16 MB SIMMs as the largest supported. That was true when the manual was printed because 32 MB SIMMs were not yet available. But that system uses 32 MB SIMMs without problems.

• Some motherboards, particularly those with four or more SIMM sockets, restrict the number of double-sided SIMMs that can be used, or restrict the total number of SIMM sides that can be used. If your motherboard has such a limitation, verify that the combination of currently installed SIMMs and the SIMMs you plan to install does not exceed the limit.