Provided by: freebsd-manpages_12.2-1_all bug


     apm — APM BIOS interface


     device apm


     This driver is scheduled for removal prior to the release of FreeBSD 13.0.


     apm is an interface to the Intel / Microsoft APM (Advanced Power Management) BIOS on laptop

     apm provides the following power management functions.

           1.   When the system wakes up from suspended mode, apm adjusts the system clock to

           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>.

                            Suspend system.

                            Get power management information.


                            Enable / Disable power management.


                            Control execution of HLT in the kernel context switch routine.

                            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


     apm(8), zzz(8)


     Tatsumi Hosokawa <>


     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

     Disk spin-down, LCD backlight control, and power on demand have not been supported on the
     current version.