If you wish to make direct access to a device with a device driver,
you can (with sufficient privileges) typically request logical or
physical I/O via the $qio[w] system service. Be aware that incorrect
direct physical access can corrupt any existing on-disk file structure.
For details, see the I/O User's Reference manual.
The DT$_FD1 foreign disk codes can be used for customers that wish
to or need to write their own disk drivers. (This is not typically
done, as one can often use a disk with an existing device driver.)
