I mean, does the sdram always runs at FSB(Front System Bus) speed? What happens if the sdram has slower or faster clock than the FSB clock?Does it have clock multiplier or divider internal circuit for these situations?
RAM modules have sets of timings and operational speeds for each of the common FSB speeds that DDR ram supports. These are defined in the JEDEC standards. In addition to JEDEC standards there are also Intel XMP specs that can allow overclocking, if the hardware supports them.
Modern RAM uses SPD information to match operation of the module and CPU, so that they can operate together. unless the parts are way off (and thus likely completely incompatible), SPD will allow your system to auto configure itself regarding RAM speed, Timing, and Memory bus speed.