• Invalidate is usually better than update • Cache line state “DIRTY” bit records whether remote copies exist – If so, remote copies are invalidated by broadcasting message on bus – cache controllers snoop all traffic • Where to get the up-to-date data from? – Broadcast read miss request on the bus – If this CPUs copy is DIRTY, it responds – If no cache copies exist, main memory responds – If several copies exist, the CPU which holds it in “SHARED-DIRTY” state responds – If a SHARED-DIRTY cache line is displaced, … need a plan • How well does it work? – See extensive analysis in Hennessy and Patterson