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# 13.5 Device Hints

Contributed by Tom Rhodes.

During initial system startup, the boot loader(8) reads device.hints(5). 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 `show`. Variables set in `/boot/device.hints` can also be overridden. Device hints entered at the boot loader are not permanent and will not be applied on the next reboot.

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

The syntax for `/boot/device.hints` is one variable per line, using the 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

where `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.

Since device drivers may accept or require more hints not listed here, viewing a driver's manual page is recommended. For more information, refer to device.hints(5), kenv(1), loader.conf(5), and loader(8).

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