INTRO(4)                             BSD                              INTRO(4)



NAME
     special files - introduction to special files

DESCRIPTION
     This section describes various special files found in /dev, which is a
     link to `node_data/dev.  With a few exceptions, these files are devices
     or pseudo-devices.

     Domain/OS implements the conventional UNIX I/O functions (read, write,
     open, close, etc.)  by way of managers specific to each device's type.
     Loosely, there are two classes of device special files:

     ⊕  Those whose characteristics are known to the system and, therefore,
        carry no additional information (and hence are 0-length).

     ⊕  Those that contain information needed by the manager in the file
        itself (for example, some files implementing pseudo-ttys contain the
        circular buffers used for holding the characters being read or
        written).

     This classification is strictly informal: the size and contents of the
     device special file are known to and used only by its manager.

     Each device has associated with it a 23-bit major device number and a 9-
     bit minor device number.  For character special devices, there is no
     further interpretation of the major device number; interpretation of the
     minor device number is up to the manager of the particular device type.

     For block special devices, on the other hand, the device number has the
     following structure:

                     Major Number                          Minor Number
|_____________________________________________________|_____________________|
|___________________NODE_ID____________________|__CT__|_CN_|__DN__|__LVN____|
31                                           12 11   9 8  7 6    4 3      0

     Managers of block special devices interpret these fields as follows:

     Major Number   Major device number (23 bits, 9-31), containing the
                    following fields:

                    NODE ID   The Apollo node identifier (20 bits, 12-31)

                    CT        Controller type (3 bits, 9-11), where
                              winchester = 0, floppy = 1, and storage
                              module = 4.

     Minor Number   Minor device number (9 bits, 0-8), containing the
                    following fields:

                    CN        Controller number within controller type (2
                              bits, 7-8)

                    DN        Drive number on this controller (3 bits, 4-6)

                    LVN       Logical volume number on this drive (based on
                              one drive; 4 bits, 0-3)

     `node_data/device_numbers maps major device numbers to device type UIDs
     (unique identifiers) and vice-versa.  Use mkdevno(8) to view and alter
     the contents of `node_data/device_numbers.

     Use crddf(1) to create special devices.  Raw disk device special files
     must have device numbers identical to the corresponding block disk device
     special files.

FILES
     `node_data/device_numbers   map of major device numbers to device type
                                 UIDs

SEE ALSO
     crddf(1), mkdevno(8)