Starting and stopping the system

System fails to boot or displays ``NO OS'' message

If the system fails to boot or you see the NO OS message, the blocks containing the partition boot blocks (/etc/hd0boot and /etc/hd1boot) or masterboot block (/etc/masterboot) may have been corrupted. To restore them:

Insert the boot floppy from your emergency boot floppy set and boot from it. When the Boot: prompt appears, enter:

hd(40)unix
Enter single user mode and enter these commands at the root prompt:

instbb hd /dev/hd0a
dparam -w
instbb(ADM) writes the partition boot blocks to the hard disk. dparam(ADM) writes the masterboot code to the masterboot block on the hard disk.

For some disks it may also be necessary to stamp the disk geometry as described in dparam(ADM).