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1.\" Copyright (c) 2000 Mark R V Murray
2.\" All rights reserved.
3.\"
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25.\" $FreeBSD: head/share/man/man9/get_cyclecount.9 89193 2002-01-10 10:48:39Z ru $
25.\" $FreeBSD: head/share/man/man9/get_cyclecount.9 91537 2002-03-01 18:43:15Z markm $
26.\"
27.Dd November 20, 2000
28.Dt GET_CYCLECOUNT 9
29.Os
30.Sh NAME
31.Nm get_cyclecount
32.Nd get the CPU's fast counter register contents
33.Sh SYNOPSIS
34.In sys/param.h
35.In machine/cpufunc.h
35.In sys/systm.h
36.In machine/cpu.h
37.Ft u_int64_t
38.Fn get_cyclecount "void"
39.Sh DESCRIPTION
40The
41.Fn get_cyclecount
42function uses a register
43available in most modern CPUs
44to return a value
45that is monotonically increasing
46inside each CPU.
47.Pp
48On SMP systems,
49there will be a number of
50separate monotonic sequences,
51one for each CPU running.
52The value in the SMP case is
53selected from one of these sequences,
54dependent on which CPU
55was scheduled to service the request.
56.Pp
57The speed and the maximum value
58of each counter
59is CPU-dependent.
60Some CPUs
61(such as the
62.Tn Intel
6380386 and 80486)
64do not have such a register,
65so
66.Fn get_cyclecount
67on these platforms
68returns the number of nanoseconds
69represented by the
70structure returned by
71.Xr nanotime 9 .
72.Pp
73The
74.Tn Pentium
75processors all use the
76.Li TSC
77register.
78.Pp
79The
80.Tn Alpha
81processors use the
82.Li PCC
83register.
84.Pp
85The
86.Tn IA64
87processors use the
88.Li AR.ITC
89register.
90.Sh SEE ALSO
91.Xr nanotime 9
92.Sh HISTORY
93The
94.Fn get_cyclecount
95function first appeared in
96.Fx 5.0 .
97.Sh AUTHORS
98This manual page was written by
99.An Mark Murray Aq markm@FreeBSD.org .