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apm - APM BIOS interface
apm is an interface to the Intel / Microsoft APM (Advanced Power
Management) BIOS on laptop PCs.
apm provides the following power management functions.
1. When the system wakes up from suspended mode, apm adjusts the
system clock to RTC.
2. When the system wakes up from suspended mode, apm passes a
message to syslogd(8) comprising of system wakeup time and
elapsed time during suspended mode.
3. apm slows CPU clock when there are no system activities
(runnable processes, interrupts, etc.). This function is
available only on systems whose APM supports CPU idling.
4. apm exports an application interface as a character device.
Applications can control APM, or retrieve APM status
information via this interface. apm exports the following
interfaces. These symbols are defined in
Get power management information.
Enable / Disable power management.
Control execution of HLT in the kernel context
Get per battery information.
Some APM implementations execute the HLT (Halt CPU
until an interrupt occurs) instruction in the
“Idle CPU” call, while others do not. Thus
enabling this may result in redundant HLT
executions because “Idle CPU” is called from the
kernel context switch routine that inherently
executes HLT. This may reduce peak system
Also the system hangs up if HLT instruction is
disabled in the kernel context switch routine, and
if the APM implementation of the machine does not
execute HLT in “Idle CPU”. On some
implementations that do not support CPU clock
slowdown, APM might not execute HLT. apm disables
APMIO_NOTHALTCPU operation on such machines.
The current version of apm does not call “Idle
CPU” from the kernel context switch routine if
clock slowdown is not supported, and it executes
HLT instruction by default. Therefore, there is
no need to use these two operations in most cases.
These interfaces are used by apm(8).
5. apm polls APM events and handles the following events.
Name Action Description
PMEV_STANDBYREQ suspend system standby request
PMEV_SUSPENDREQ suspend system suspend request
PMEV_USERSUSPENDREQ suspend system user suspend request
PMEV_CRITSUSPEND suspend system critical suspend
PMEV_NORMRESUME resume system normal resume
PMEV_CRITRESUME resume system critical resume
PMEV_STANDBYRESUME resume system standby resume
PMEV_BATTERYLOW notify message battery low
PMEV_UPDATETIME adjust clock update time
Tatsumi Hosokawa <hosokawa@jp.FreeBSD.org>
WARNING! Many, if not most, of the implementations of APM-bios in
laptops today are buggy. You may be putting your LCD-display and
batteries at a risk by using this interface. (The reason this is not a
problem for MS-Windows is that they use the real-mode interface.) If you
see any weird behavior from your system with this code in use, unplug the
power and batteries ASAP, if not immediately, and disable this code.
We are very interested in getting this code working, so please send your
observations of any anomalous behavior to us.
When apm is active, calling the BIOS setup routine by using hot-keys, may
cause serious trouble when resuming the system. BIOS setup programs
should be called during bootstrap, or from DOS.
Some APM implementations cannot handle events such as pushing the power
button or closing the cover. On such implementations, the system must be
suspended only by using apm(8) or zzz(8).
Disk spin-down, LCD backlight control, and power on demand have not been
supported on the current version.