Intel x86-64 PMU (Nehalem)
The x86-64 PMUs are specific to each underlying architecture. The Netburst architecture of the Pentium-4 has given way to the Core architecture, in use in Nehalem, i7 processors.
The recent PMUs seem to be somewhat structured similarly across Core and Atom CPU models.
The diagram of PMU registers shows the registers available depending on which processor family you are using. You are offered at least 4 counters.
The Nehalem PMU supports a hardware bufering scheme to collect multiple samples from the PMU. When a counter overflows, the CPU interrupts into firmware, which in turn copies the counters into a PEBS buffer record. The following graph shows the indirection levels and the content of each record.