Dave wrote:

what is the system managment bus? for the p4c800 deluxe?


More properly known as the I2C bus.

I2C is an acronym for Inter-IC bus. The name literally explains it's
purpose - to provide a communication link between Integrated Circuits.
On mainboards, it's used for jumperless configuration - e.g. the BIOS
uses I2C to program the PLL with the configured FSB clock speed during
boot. Programs like SoftFSB use it to change configuration on the fly.

http://perso.club-internet.fr/mbouget/i2c-faq.html