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800mhz fsb cpu not same as 400mhz mainboard socket
(most likely LGA775 CPU vs Socket478 MB)
Socket 478 will suppoprt up to P4 CPU's with 400mhz FSB.
CPU's with 533+ MHZ will not fit Socket 478 (this covers all dual-core CPU's)
However, if, when using a CPU with a higher FSB value (IE, e8xxx-series CPU w/1333 FSB on a 1066 MHZ FSB mainboard), the system should work without damage to the CPU, though at reduced performance (therefor is not recommended.)
to hythos : so,if i put pentium 4 socket 478 with 800mhz fsb to mb socket 478 max support 400mhz fsb,it cannot be worked??
or it still worked,but in reduced performance?
Hmm sort of. Knowing the MB spec would help most, though CPU voltage is different from 1.5 (800mhz) to ~1.7v(400mhz) ; Verify per CPU, then adjust on the MB. MB may have auto-sense with soft-set through CMOS, or jumper.
** Note - CPU will run at 1/2 speed rating - IE, P4@ 2.8ghz/800 = 1.4ghz/400