Facilities specific to PowerPC that are not specific to a particular operating system are declared in sys/platform/ppc.h.
Read the current value of the Time Base Register.
The Time Base Register is a 64-bit register that stores a monotonically incremented value updated at a system-dependent frequency that may be different from the processor frequency. More information is available in Power ISA 2.06b - Book II - Section 5.2.
__ppc_get_timebaseuses the processor's time base facility directly without requiring assistance from the operating system, so it is very efficient.
Read the current frequency at which the Time Base Register is updated.
This frequency is not related to the processor clock or the bus clock. It is also possible that this frequency is not constant. More information is available in Power ISA 2.06b - Book II - Section 5.2.