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     apm — APM BIOS interface


     device apm


     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

                            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

                            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


     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 zzz(8).

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