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+.\" Copyright (c) 2000 Mark R V Murray
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.\" $FreeBSD$
+.\"
+.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 sys/systm.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
+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 .