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NAME
apm — APM BIOS interface
SYNOPSIS
device apm
DEPRECATION NOTICE
This driver is scheduled for removal prior to the release of FreeBSD 13.0.
DESCRIPTION
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 <machine/apm_bios.h>.
APMIO_SUSPEND
Suspend system.
APMIO_GET
Get power management information.
APMIO_ENABLE
APMIO_DISABLE
Enable / Disable power management.
APMIO_HALTCPU
APMIO_NOTHALTCPU
Control execution of HLT in the kernel context switch routine.
APMIO_GETPWSTATUS
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
performance.
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 request
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
SEE ALSO
apm(8), zzz(8)
AUTHORS
Tatsumi Hosokawa <hosokawa@jp.FreeBSD.org>
BUGS
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.
Debian November 1, 1994 APM(4)