The question is, does it configure your SDRAMs correctly? I assume it's
on auto config, then the BIOS has to figure out what timings to use for
the RAMs by reading their SPD ROM.
Problem 1: you run PC133 memory in PC100 configuration. Some PC133
DIMMs contain no configuration data for PC100, so they theoretically
couldn't legally be run as PC100. They could be run using the PC133
config - not optimal, but should work. The BIOS has to figure that out,
however. This problem is more likely to occur if you're using no-name
RAMs instead of branded RAMs by a reputable vendor (you might not even
have fully valid config data for PC133 in that case).
Problem 2: your BIOS is broken and configures all DIMMs to the
configuration values of the first, causing at least one to run with the
wrong values, if they have different timing requirements. Given your
problem (either DIMM works alone, two don't), this sounds like a more
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