System Profiler says PC-133 RAM is PC-100 WTF ?

The short answer: It's a reporting bug in System Profiler which has
unfortunately survived throughout many different versions of the code.
(I don't think it was present in 10.2 because I don't think that
version reported any DIMM speed information at all.) You should
ignore the value reported by System Profiler and instead use the Apple
Hardware Test CD or look at the label on the parts to determine their
speed. The problem is known by Apple.

The long answer: System Profiler bases it's report on the content of
the dimm-speeds property in the IOKit device registry. The property
is written there by OpenFirmware. OpenFirmware decides what to write
there by looking at information encoded at the factory on the DIMM
itself in a 128 byte field known as the Serial Presence Detect (SPD).
The format of the SPD field is described at:
ftp://download.intel.com/technology/memory/pc133sdram/spec/
Spdsd12b.pdf. Unfortunately the PC133 spec doesn't provide a
definitive way to distinguish between PC-100 and PC-133 RAM using the
SPD info. You can make an informed guess about whether RAM is PC-100
or PC-133 by looking for plausible values for each speed of memory in
some other locations in the SPD - but that's not foolproof. Some
applications and some versions of OF may do a better job of this than
others.

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