MCH (memory controler hub ) or northbridge is the chipset, eg intel's 965p chipset. It serves as the link between memory and cpu, the interface is the FSB. It also links to the PCI-ex16 (or x8) port for graphics. It links the southbridge to itself through an interconnect (FSB) and thus to the CPU. In AMD cpu's since the athlon 64 debuted AMD has done away with a discrete memory controler hub and instead located it inside the CPU die, meaning better performance (still less than core 2 uarch). IT generally gets hotter than the southbridge and thus requires a passive heatsink in modern motherboards, if you do 'huge' overclocks (eg over 4.5ghz) active cooling (eg fan) is required.
the southbridge is reffered to as ICHx with intel motherboards eg ICH7, ICH8 and so on. The southbridge controls the stroage and drives and PCI, PCI-E. Also HD or AC '97 audio, onboard LAN etc
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