Usage: bios_drive:interface(unit,partition)kernel_name options
    bios_drive   0, 1, ...
    interface    fd, wd or da
    unit         0, 1, ...
    partition    a, c, ...
    kernel_name  name of kernel, or ? for list of files in root directory
    options      -a (ask name) -C (cdrom) -c (userconfig) -D (dual consoles)
                 -d (debug early) -g (gdb) -h (serial console) -P (probe kbd)
                 -r (default root) -s (single user) -v (verbose)
Examples:
    1:da(0,a)mykernel  boot `mykernel' on the first SCSI drive when one IDE
                       drive is present
    1:wd(2,a)          boot from the second (secondary master) IDE drive
    1:da(0,a)?         list the files in the root directory on the specified
                       drive/unit/partition, and set the default bios_drive,
                       interface, unit and partition
    -cv                boot with the defaults, then run UserConfig to modify
                       hardware parameters (c), and print verbose messages (v)