While the "batch code" (stepping) is very important, the serial number is not as important. The stepping tells you the microcode version burnt on to the chip. If you get a mismatched set of processors they may work but I wouldn't bet the bank on how reliable they would be. The serial number is a tracking number and may tell you where and when the chip was made. Just because the serial number is within a few digits it doesn't mean the stepping is the same. Use the stepping to measure the compatibility. Sam. Chris Scheidecker wrote: >Each cpu should have a serial number on it. With xeons or other cpus, it's >always ideal to get two cpus with the same batch code and an incremental >serial number, i.e. cpu0=serial#1122445, cpu1=serial#1122446 (or whatever). > > > >