---
name: "asmc"
section: "4"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["freebsd", "openbsd"]
url: "https://new.carta.tech/asmc/4"
---

# asmc(4)

Apple System Management Controller driver

## NAME

**asmc** – Apple System Management Controller driver

## SYNOPSIS

To compile this driver into the kernel, place the following line in your kernel configuration file:

**device** **asmc**

## DESCRIPTION

The **asmc** driver controls the Apple System Management Controller (SMC for short) found on Intel Apple systems.

With this driver, you can configure your keyboard backlight brightness, check temperatures of several sensors, check the speed of the internal fans and check the status of the Sudden Motion Sensor.

Variables related to the SMC control and inspection are exported via [sysctl(3)](/sysctl/3.md) under the device tree *dev.asmc*.

## KEYBOARD BACKLIGHT

The keyboard brightness on all laptop models, e.g., MacBook Pro, can be controlled by writing a value to the *dev.asmc.%d.light.control* sysctl MIB or with [backlight(8)](/backlight/8.md) utility.

The following sysctl MIBs contains the raw value returned by the left and right light sensors: *dev.asmc.%d.light.left* or *dev.asmc.%d.light.right*.

## TEMPERATURES

The number of temperature sensors and their description varies among systems. You can inspect the temperature sensors on your system by traversing the *dev.asmc.temp* sysctl MIB.

All values are in degrees celsius.

## SYSTEM FANS

The *dev.asmc.fan.%d* sysctl tree contains the leaf nodes *speed*, *safespeed*, *minspeed*, *maxspeed* and *targetspeed*. Each of these leaf nodes represent the current fan speed, the safest minimum fan speed, the minimum speed and the maximum speed respectively.

All values are in RPM.

## POWER MANAGEMENT

The *dev.asmc.%d.auto\_poweron* sysctl controls whether the machine automatically powers on when AC power is restored after an unclean power loss (SMC key “AUPO”). A value of 1 enables automatic power-on; 0 disables it.

## RAW SMC KEY ACCESS

When the kernel is compiled with the ASMC\_DEBUG option, a set of sysctl nodes is provided under *dev.asmc.%d.raw* for reading and writing arbitrary SMC keys by name.

- *dev.asmc.%d.raw.key*

  Set the 4-character SMC key name to access (e.g., “AUPO”). Setting this automatically queries the key's length and type.

- *dev.asmc.%d.raw.value*

  Read or write the key's value as a hex string.

- *dev.asmc.%d.raw.len*

  The auto-detected value length in bytes (read-only).

- *dev.asmc.%d.raw.type*

  The 4-character SMC type string (e.g., “ui8”, “flt”) (read-only).

Example usage:

```
sysctl dev.asmc.0.raw.key=AUPO
sysctl dev.asmc.0.raw.value
sysctl dev.asmc.0.raw.value=01
```

## SUDDEN MOTION SENSOR

The Sudden Motion Sensor (SMS for short) is a device that detects laptop movement and notifies the operating system via an interrupt. The sysctl MIBs present under *dev.asmc.sms* all relate to the SMS.

The most interesting usage of this device is to park the disk heads when the laptop is moved harshly. Configure [devd(8)](/devd/8.md) the following way to automatically park your hard drives using [camcontrol(8)](/camcontrol/8.md):

```
notify 0 {
	match "system"		"ACPI";
	match "subsystem"	"asmc";
	action			"/sbin/camcontrol idle X";
};
```

Do not forget to change *X* in the command example to match the target hard drive, e.g., *ada0*.

Also, please note that parking the disk heads too many times can dramatically reduce your hard drive's life span. Do not rely solely on the SMS to protect your hard drive: good care and common sense can increase your hard drive's life.

## HARDWARE

The **asmc** driver supports the Apple SMC on the following systems:

- iMac

- MacBook

- MacBook Air

- MacBook Pro

- Mac Mini

- Mac Pro

## FILES

- */dev/backlight/asmc0*

  Keyboard [backlight(8)](/backlight/8.md) device node.

## SEE ALSO

[apple\_bce(4)](/apple_bce/4.md), [backlight(8)](/backlight/8.md), [camcontrol(8)](/camcontrol/8.md), [devd(8)](/devd/8.md), [sysctl(8)](/sysctl/8.md)

## HISTORY

The **asmc** driver first appeared in FreeBSD 8.0. The driver was heavily modified in FreeBSD 15.1 to support additional Apple SMC functionality and platforms.

## AUTHORS

Rui Paulo ⟨*rpaulo@FreeBSD.org*⟩ wrote **asmc** as a 2007 Google Summer of Code project.

## BUGS

This driver also supports ARM64 Mac models (in theory), but this has not been tested yet.