.\" Copyright (c) 2000 Mark R V Murray
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.Dd November 20, 2000
.Dt GET_CYCLECOUNT 9
.Os
.Sh NAME
.Nm get_cyclecount
.Nd get the CPU's fast counter register contents
.Sh SYNOPSIS
.In sys/param.h
.In machine/cpufunc.h
.In machine/cpu.h
.Ft u_int64_t
.Fn get_cyclecount "void"
.Sh DESCRIPTION
The
.Fn get_cyclecount
function uses a register
available in most modern CPUs
to return a value
that is monotonically increasing
inside each CPU.
.Pp
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.
.Pp
The speed and the maximum value
of each counter
is CPU-dependent.
Some CPUs
(such as the
.Tn Intel
80386 and 80486)
do not have such a register,
so
.Fn get_cyclecount
on these platforms
returns the number of nanoseconds
represented by the
structure returned by
.Xr nanotime 9 .
.Pp
The
.Tn Pentium
processors all use the
.Li TSC
register.
.Pp
The
.Tn Alpha
processors use the
.Li PCC
register.
.Pp
The
.Tn IA64
processors use the
.Li AR.ITC
register.
.Sh SEE ALSO
.Xr nanotime 9
.Sh HISTORY
The
.Fn get_cyclecount
function first appeared in
.Fx 5.0 .
.Sh AUTHORS
This manual page was written by
.An Mark Murray Aq markm@FreeBSD.org .