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APM(4) Device Drivers Manual (i386) APM(4)


apmAPM 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 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>.
    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.
    standby request
    suspend request
    user suspend request
    critical suspend request
    normal resume
    critical resume
    standby resume
    battery low
    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.

November 1, 1994 Debian