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# 12.5 Device Hints

Contributed by Tom Rhodes.

> Note: This is a FreeBSD 5.0 and later feature which does not exist in earlier versions.

During initial system startup, the boot loader(8) will read the device.hints(5) file. This file stores kernel boot information known as variables, sometimes referred to as “device hints”. These “device hints” are used by device drivers for device configuration.

Device hints may also be specified at the Stage 3 boot loader prompt. Variables can be added using `set`, removed with `unset`, and viewed with the `show` commands. Variables set in the `/boot/device.hints` file can be overridden here also. Device hints entered at the boot loader are not permanent and will be forgotten on the next reboot.

Once the system is booted, the kenv(1) command can be used to dump all of the variables.

The syntax for the `/boot/device.hints` file is one variable per line, using the standard hash “#” as comment markers. Lines are constructed as follows:

    hint.driver.unit.keyword="value"

The syntax for the Stage 3 boot loader is:

    set hint.driver.unit.keyword=value

`driver` is the device driver name, `unit` is the device driver unit number, and `keyword` is the hint keyword. The keyword may consist of the following options:

- `at`: specifies the bus which the device is attached to.

- `port`: specifies the start address of the I/O to be used.

- `irq`: specifies the interrupt request number to be used.

- `drq`: specifies the DMA channel number.

- `maddr`: specifies the physical memory address occupied by the device.

- `flags`: sets various flag bits for the device.

- `disabled`: if set to `1` the device is disabled.

Device drivers may accept (or require) more hints not listed here, viewing their manual page is recommended. For more information, consult the device.hints(5), kenv(1), loader.conf(5), and loader(8) manual pages.

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