RTEMS C User's Guide
Each call to the I/O manager must provide a device's major and minor numbers as arguments. The major number is the index of the requested driver's entry points in the Device Driver Table, and is used to select a specific device driver. The exact usage of the minor number is driver specific, but is commonly used to distinguish between a number of devices controlled by the same driver.
The data types rtems_device_major_number and
rtems_device_minor_number are used to
manipulate device major and minor numbers, respectively.
RTEMS C User's Guide
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