HP Compaq d338 MT, Hardware Reference Manual

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If a memory frequency is paired with an unsupported processor bus
frequency, the system will run at the highest supported memory
speed. For example, if a 333 MHz DIMM is paired with a 400 MHz
processor bus, the system will run at 266 MHz, the highest supported
memory speed.
✎
The system will not start if you install unsupported DIMMs.
DIMM Sockets
The system will automatically operate in single channel mode or a
higher-performing dual channel mode, depending on how the DIMMs
are installed.
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In single channel mode, the maximum operational speed is
determined by the slowest DIMM in the system. For example, if
the system is populated with a DIMM that is 266 MHz and a
second DIMM that is 333 MHz, the system will run at the slower
of the two speeds.
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In dual channel mode, all DIMMs must be identically matched.
DIMMs in the XMM1 and XMM3 black sockets must be
identical; DIMMs in the XMM2 and XMM4 blue sockets must
also be identical. Therefore, if you have one preinstalled DIMM
in socket XMM1 and are adding a second DIMM, it is
recommended that you install an identical DIMM into the XMM3
socket. If you are populating all four DIMM sockets, use identical
DIMMs in each socket. Otherwise, the system will not operate in
dual channel mode.
Memory Frequency Required Processor Bus Frequency
266 MHz 400 MHz, 533 MHz, or 800 MHz
333 MHz 533 MHz or 800 MHz
400 MHz 800 MHz