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+<table class="head">
+  <tr>
+    <td class="head-ltitle">TIMECOUNTERS(4)</td>
+    <td class="head-vol">Device Drivers Manual</td>
+    <td class="head-rtitle">TIMECOUNTERS(4)</td>
+  </tr>
+</table>
+<div class="manual-text">
+<section class="Sh">
+<h1 class="Sh" id="NAME"><a class="permalink" href="#NAME">NAME</a></h1>
+<p class="Pp"><code class="Nm">timecounters</code> &#x2014;
+    <span class="Nd">kernel time counters subsystem</span></p>
+</section>
+<section class="Sh">
+<h1 class="Sh" id="SYNOPSIS"><a class="permalink" href="#SYNOPSIS">SYNOPSIS</a></h1>
+<p class="Pp">The kernel uses several types of time-related devices, such as:
+    real time clocks, time counters and event timers. Real time clocks are
+    responsible for tracking real world time, mostly when the system is down.
+    Time counters are responsible for tracking purposes, when the system is
+    running. Event timers are responsible for generating interrupts at a
+    specified time or periodically, to run different time-based events. This
+    page is about the second.</p>
+</section>
+<section class="Sh">
+<h1 class="Sh" id="DESCRIPTION"><a class="permalink" href="#DESCRIPTION">DESCRIPTION</a></h1>
+<p class="Pp">Time counters are the lowest level of time tracking in the kernel.
+    They provide monotonically increasing timestamps with known width and update
+    frequency. They can overflow, drift, etc and so in raw form can be used only
+    in very limited performance-critical places like the process scheduler.</p>
+<p class="Pp" id="tc_windup">More usable time is created by scaling the values
+    read from the selected time counter and combining it with some offset,
+    regularly updated by
+    <a class="permalink" href="#tc_windup"><code class="Fn">tc_windup</code></a>()
+    on
+    <a class="permalink" href="#hardclock"><code class="Fn" id="hardclock">hardclock</code></a>()
+    invocation.</p>
+<p class="Pp">Different platforms provide different kinds of timer hardware. The
+    goal of the time counters subsystem is to provide a unified way to access
+    that hardware.</p>
+<p class="Pp">Each driver implementing time counters registers them with the
+    subsystem. It is possible to see the list of present time counters, via the
+    <var class="Va">kern.timecounter</var> <a class="Xr">sysctl(8)</a>
+  variable:</p>
+<div class="Bd Pp Li">
+<pre>kern.timecounter.choice: TSC-low(-100) HPET(950) i8254(0) ACPI-fast(900) dummy(-1000000)
+kern.timecounter.tc.ACPI-fast.mask: 16777215
+kern.timecounter.tc.ACPI-fast.counter: 13467909
+kern.timecounter.tc.ACPI-fast.frequency: 3579545
+kern.timecounter.tc.ACPI-fast.quality: 900
+kern.timecounter.tc.i8254.mask: 65535
+kern.timecounter.tc.i8254.counter: 62692
+kern.timecounter.tc.i8254.frequency: 1193182
+kern.timecounter.tc.i8254.quality: 0
+kern.timecounter.tc.HPET.mask: 4294967295
+kern.timecounter.tc.HPET.counter: 3013495652
+kern.timecounter.tc.HPET.frequency: 14318180
+kern.timecounter.tc.HPET.quality: 950
+kern.timecounter.tc.TSC-low.mask: 4294967295
+kern.timecounter.tc.TSC-low.counter: 4067509463
+kern.timecounter.tc.TSC-low.frequency: 11458556
+kern.timecounter.tc.TSC-low.quality: -100</pre>
+</div>
+<p class="Pp">The output nodes are defined as follows:</p>
+<dl class="Bl-inset">
+  <dt id="kern.timecounter.tc."><var class="Va">kern.timecounter.tc.</var><var class="Ar">X</var><var class="Va">.mask</var></dt>
+  <dd>is a bitmask, defining valid counter bits,</dd>
+  <dt id="kern.timecounter.tc.~2"><var class="Va">kern.timecounter.tc.</var><var class="Ar">X</var><var class="Va">.counter</var></dt>
+  <dd>is a present counter value,</dd>
+  <dt id="kern.timecounter.tc.~3"><var class="Va">kern.timecounter.tc.</var><var class="Ar">X</var><var class="Va">.frequency</var></dt>
+  <dd>is a counter update frequency,</dd>
+  <dt id="kern.timecounter.tc.~4"><var class="Va">kern.timecounter.tc.</var><var class="Ar">X</var><var class="Va">.quality</var></dt>
+  <dd>is an integral value, defining the quality of this time counter compared
+      to others. A negative value means this time counter is broken and should
+      not be used.</dd>
+</dl>
+<p class="Pp">The time management code of the kernel automatically switches to a
+    higher-quality time counter when it registers, unless the
+    <var class="Va">kern.timecounter.hardware</var> sysctl has been used to
+    choose a specific device.</p>
+<p class="Pp">There is no way to unregister a time counter once it has
+    registered with the kernel. If a dynamically loaded module contains a time
+    counter you will not be able to unload that module, even if the time counter
+    it contains is not the one currently in use.</p>
+</section>
+<section class="Sh">
+<h1 class="Sh" id="SEE_ALSO"><a class="permalink" href="#SEE_ALSO">SEE
+  ALSO</a></h1>
+<p class="Pp"><a class="Xr">attimer(4)</a>, <a class="Xr">eventtimers(4)</a>,
+    <a class="Xr">ffclock(4)</a>, <a class="Xr">hpet(4)</a></p>
+</section>
+</div>
+<table class="foot">
+  <tr>
+    <td class="foot-date">August 12, 2015</td>
+    <td class="foot-os">FreeBSD 15.0</td>
+  </tr>
+</table>