Encountering the .altboot message

After this message:

Press ESC for .altboot

appears, the secondary boot loader reads in the root block of the partition.

This is the second block of a partition (or disk in the case of a floppy) and contains inode entries for /.boot and /.altboot, the two OS images that are available to load from the root directory.

The boot loader locates the second block, using the parameters for heads and sectors/track that were stored as part of the secondary boot loader when it was written to disk by the dinit utility.

The boot loader then loops for 2 seconds waiting for you to press a key.