The get_cyclecount function uses a register available in most modern CPUs to return a value that is monotonically increasing inside each CPU.
On SMP systems, there will be a number of separate monotonic sequences, one for each CPU running. The value in the SMP case is selected from one of these sequences, dependent on which CPU was scheduled to service the request.
The speed and the maximum value of each counter is CPU-dependent. Some CPUs (such as the Intel 80486) do not have such a register, so get_cyclecount on these platforms returns the number of nanoseconds represented by the structure returned by nanotime(9).
The Pentium processors all use the TSC register.
The Alpha processors use the PCC register.
The IA64 processors use the AR.ITC register.