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In order to shutdown the computer, you need to use some sort of power management. Either APM or ACPI.



This is the basic sequence of APM commands that must be given in order to shut down a computer. For details on exactly how to implement these steps, see the APM article.

  • Perform an installation check.
  • Check if the APM version is at least 1.1
  • Disconnect any existing APM interface.
  • Connect the Real Mode interface.
  • Tell the APM that your driver supports version 1.1
  • Enable power management for all devices.
  • Set the power state for all devices to "Off" (03h).

If the APM version is 1.0 (or the APM isn't told that your code supports version 1.1, so it will run as 1.0 for legacy purposes) it isn't possible to set the power state for all devices (maybe it's possible to shut each device down individually)


ACPI Shutdown code with good explanation in C

A summary of ACPI shutdown from the above forum post:

The ACPI shutdown is technically a really simple thing all that is needed is a outw(PM1a_CNT, SLP_TYPa | SLP_EN ); and the computer is powered off. The problem lies in the gathering of these values especialy since the SLP_TYPa is in the \_S5 object which is in the DSDT and therefore AML encoded.

Warning: the code above shouldn't be used in production. There are many things the author skips, like calling the _PTS method. A real AML interpreter is required for that, like for example, ACPICA.

On many hardware calling _PTS is required and failing to call it will cause shutdown to fail, or cause hw to freeze in some half-shutdown phase. This phenomenon mostly occurs on laptops, but also occurs on desktops.

Emulator-specific methods

In some cases (such as testing), you may want a poweroff method, but not need it to work on real hardware.

In Bochs, and older versions of QEMU(than 2.0), you can do the following:

outw(0xB004, 0x2000);

In newer versions of QEMU, you can do shutdown with:

outw(0x604, 0x2000);

In Virtualbox, you can do shutdown with:

outw(0x4004, 0x3400);

See Also


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